plant growth in relation to endogenous auxin, with special reference to cereal seedlings

Plant Growth in Relation to Endogenous Auxin, with Special Reference to Cereal SeedlingsAuthor(s): C. L. MerSource: New Phytologist, Vol. 68, No. 2 (Apr., 1969), pp. 275-294Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2431346 .

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New Phytol. (I969) 68, 275-294.

PLANT GROWTH IN RELATION TO ENDOGENOUS AUXIN, WITH SPECIAL REFERENCE TO CEREAL

SEEDLINGS

BY C. L. MER

Agricultural Research Council Unit of Plant Physiology, Imperial College of Science and Technology, London

(Received 5 June I968)

INTRODUCTION

The study of auxin physiology was initiated by, and has developed under the influence of, the two precepts 'Ohne Wuchsstoff, kein Wachstum' and 'when there is little auxin there is little growth, when more auxin, more growth' which were first enunciated by Went (1928). These dicta form the basis of the Went-Cholodny theory of tropisms, and as such they have gained universal acceptance as the tenets appropriate for the elucidation of growth, and of irritability, phenomena (Boysen-Jensen, 1937; Went and Thimann, I937; Audus, 1959; van Overbeek, I959; Pilet, I96I; Ball, I463; Heslop-Harrison, I963; Thimann, I964, I966). The way in which the precepts have been employed is too well known to warrant re-statement here; suffice it to say that they permit the forecast of future growth, for example of a geotropic or of a phototropic response, from a knowledge of the present content of auxin.

The inevitable fate of any theory, particularly of a highly original, revolutionary one, is to become encumbered as time passes with an accumulation of inconsistencies and contradictions which ultimately call for a re-evaluation of the initial hypotheses. So far as the auxin theory was concerned the appearance of such discrepancies was not long delayed. van Overbeek (I932) noted that the considerable elongation of dark-grown Lepidium seedlings was associated with the virtual absence of diffusible auxin, whereas much shorter light-grown plants released larger amounts of growth-substance. A similar inverse relationship between auxin and growth was encountered by von Guttenberg and Zetsche (I956, p. Io6); also by Scott and Briggs (I963, p. 654) who remarked that the dearth of active substance in etiolated pea plants upset the long-held generalization that such tissues were rich in auxin. In fact, the generalization was based upon the second dictum rather than on experimental data: if more auxin induces more growth, then ipso facto more growth must signify more auxin. Earlier observations of Laibach and Meyer (1935) and of Fiedler (1936) which apparently demonstrated that growth could occur in the absence of extractable auxin were obviously at variance with the major precept 'without auxin, no growth'. In their important review of auxin physiology Went and Thimann (1937, pp. 62-64 and i92) mentioned these data, but chose to disregard them for no better reason than that they were not in accord with the preferred concepts. Recently, a viewpoint diametrically opposed to the accepted ideas has been ventilated in the denial that aukin content determines growth. Heslop-Harrison (I964, p. 655) after having attempted, without success, to relate floral expression with auxin content replaced it by

275



276 C. L. M ER

some other function ... . like the balance between production and destruction over a period of time, which might be well-nigh impossible to measure'. Dattaray and Mer (I964), using intact, etiolated oat plants were similarly unable to find a correlation between the amount of ether-extractable auxin and either the rate, or increment, of growth. Nor could future growth be predicted from any antecedent content of active substance. Instead, growth was attributed either to the auxin that had been used to produce it and so could not be measured, or to that which could be synthesized in the given context of metabolic circumstances. On this interpretation the assayable, residual auxin was either surplus to requirements if growth had ceased, or was awaiting utilization to an unknown extent in connection with any future growth that might have occurred.

This modified analysis of auxin/growth relationships was suggested by the consistent occurrence of the observation that length and auxin content increased simultaneously with time. Under these circumstances the condition holds that at a later time, 'T2, more

Table i. Some possible interrelationships between growth and auxin supply Rate of Rate of Residual

synthesis consumption Case i Low Low X ,cg Case 2 High High Possibly X ,cg Case 3 High Low Say io X ,cg Case 4 Low High o

auxin was present than at an earlier time, T1, even though some auxin had presumably been utilized to bring about the enlargement observed between T1 and T2. Such a rising content of auxin accompanying growth would be instituted if the rate of production exceeded the rate of consumption. The alternative, that production was unaccompanied by consumption, need not be further discussed as it is not in accord with accepted ideas about auxin participation in growth. Some of the possible combinations of the three parameters, rate of synthesis and of consumption, and residual auxin are set out in Table i.

With these postulates the growth/auxin data of the oat seedling can be explained. For example, observations of different growth rates with the same auxin content, quite inexplicable on the classical hypotheses, call for cases i and 2 together. Growth is controlled by the rates of consumption, low and high, respectively, and not by the same auxin content. Further, the presence of much auxin in slowly growing coleoptiles would exemplify case 3; it is not the high content of auxin that controls growth, but the low capacity for utilization: this would indicate that control resides with some other factor. Lastly case 4, in which growth is regulated by the rate of auxin formation, can account for the discrepancies adduced above which contravened the precept 'without auxin, no growth'. Went and Thimann (I937, p. I62) were on the brink of this analysis for their suggested explanation of Fiedler's observations (I936) is identical with the present case 4. However, they turned away from it by concentrating on detectable auxin. Wright's reaction (I956) to his observation of 'growth, but no auxin' was somewhat different; he postulated that growth was controlled by some other substances. But it is possible that consumption just equalled synthesis; if so, some auxin might be detected if enlargement, i.e. consumption, were to be prevented, perhaps by restricting the water supply.

Such contradictions, while they may stimulate discussion, can also hamper progress. It would appear desirable to understand, if at all possible, why they occur and how to remove them. This is the aim of the present inquiry, which will start by noting that



Plant growth and auxin 277 divergencies always appear between attempts to relate growth to endogenous auxin on the one hand, with, on the other, the precepts derived from experiments in which an active substance, endogenous in origin, was applied externally to test seedlings. For this reason a re-examination has been undertaken of the evidence that gave rise to the idea that auxin and its growth effects are quantitatively related in intact plants, and that auxin serves an integrative function in the cereal seedling.

THE RELATIONSHIP BETWEEN GROWTH AND THE SUPPLY OF AUXIN

There are four propositions which describe the dependence of growth upon the supply of auxin. They are:

(a) The increment of growth in length shows a direct linear relationship to the concentration of growth substance.

(b) The rate of growth shows a direct linear relationship to the concentration of growth substance.

(c) The rate of growth parallels the rate of auxin production. (d) The 'Scott and Briggs' hypothesis; growth is controlled by a specific part of the

auxin present.

16- - -_ -_

14

12-

1)0-

? 8 /

4 - /

2/

I 3 4 5 6 7 8 9 10 Amount of growth substance

'Fig. i. The relationship between the amount of growth substance supplied externally and the resultant angle of bending. Redrawn from Went (1928, Fig. 8).

Proposition (a). This idea, which seems to have been interchanged indiscriminately with proposition (b), arose out of Went's successful development of the Avena bio- assay technique. His original figure (Went, I928, Fig. 8) showing a linear relationship between auxin supply and curvature, has been reproduced here (Fig. i) and, before making the principal contention, two minor features in the data warrant brief mention. First, the amount (Menge) of active substance, rather than its concentration, was related to curvature; the amendment was made at some later date when it was realized that only part of the auxin provided in the agar-block passed into the coleoptile (Thimann and Bonner, I932). Secondly, as the data stand they do not unequivocally establish linearity in response, because the greater part of the line, the 6o% which can be directly measured on the graph lying between the origin and the first observation, is unsupported. If this extrapolation be disregarded, the claim for linearity is substantially weakened; the



278 C. L. MER remnant has a logarithmic form similar to that obtained subsequently by Bentley and Bickle (I952).

The theory of plant growth proposed by Went (I928) appears to have arisen in its entirety from this figure; the shape of the interconnection is therefore of small importance as compared with the use to which it was put. The concept 'Ohne Wuchsstoff, kein Wachstum' simply indicates that the extrapolation can be made to pass through the origin: here, x = o wheny = o, i.e. no auxin, no growth. The second dictum is a direct consequence of the positive relationship between curvature and auxin supply. This, irrespective of its shape, allows one to say 'with more auxin there will be more curvature, or more growth, than with less auxin'.

There is no doubt that the development of this calibration was a great achievement but it must, nevertheless, be recognized that such a calibration is an artificial, empirical device specifically contrived to measure a particular unknown quantity. Generally in bio- assays all factors but the critical one are maintained at a high level; increasing amounts of the limiting factor will then, it is hoped, generate a rising response over a suitable range. With the curvature response established by Stark (I92I), methods were sought of increasing the sensitivity of the test plants to the applied extracts. It was found that two decapitations separated by a 90 minute interval, these operations being performed under 'phototropically inactive' orange-red light, were advantageous. Since the tip was known to release auxin into agar blocks, it was appreciated that the decapitations would deplete, or even reduce to zero, the content of native auxin. The conclusions arrived at should be applied only to this particular result, or to those of other similar calibration exercises.

It is the transfer of these conclusions to the intact plant, which followed almost automatically, that requires closer inspection. To avoid all special provisos the 'linear within limits' restriction was permitted to lapse as may be seen in van Overbeek's description (I959, p. 273) of the phototropic curvature. Also, to make it seem that the auxin status of the intact plant matched that especially brought about in the test seedling for calibration purposes, it was necessary to say that normally in vivo the content of auxin, however much or little it might be, was always growth limiting. Without this equivalence the transfer was impossible, hence the assertion was quite unavoidable; but it is unproven and, because it forges a closed circle of argument, its retention precludes fruitful discussion of auxin/growth relationships. Even though the decline of coleoptile growth after decapitation might be regarded as conclusive evidence for growth control by auxin, it cannot be maintained that a test plant and an intact one are equal in this respect; had they been, pre-treatment would have been unnecessary.

This transfer was no doubt facilitated by the use of the Avena seedling in the assay procedure, and by the simplicity of the operations involved-merely decapitation and removal of the leaves. Had some special, extreme treatment been required to obtain a relationship suitable for calibration purposes, the obvious lack of equivalence between intact and test plants would have inhibited the transfer. Furthermore, when cylinders of coleoptile tissue are used for estimating auxin concentrations in a straight-growth technique a linear relationship has been found between the increment of growth and the logarithm of concentration. In this type of exercise pieces of tissue are floated in the various solutions: the relationship has never been used as a theoretical basis for the analysis of the growth of normal plants. Why, then, should those of the earlier calibration?

This discussion leads to the conclusion that Went's dicta may not be applicable to intact plants. In this event there will no longer be any 'discrepancies', and the data hitherto described in this way may be re-assessed.



Plant growth and auxin 279 A word of justification needs to be interpolated here about the use of Went's data.

Although other less equivocal calibration relationships have been obtained, for example that in Went and Thimann (I937, Fig. 24), Went's original figure was a most important part of the evidence upon which the subject was founded. It has been reproduced in a number of standard texts including Boysen-Jensen (I936, Fig. 2b), Audus (I959, Fig. 3, evidently redrawn) and Pilet (I96I, Fig. gd) and in this way Went's earliest ideas are still being introduced to potential workers on these problems.

Once the transfer had been made from the calibration to the intact plant, the precepts were constantly associated with the latter and together they have become embedded in the literature. In consequence, whenever a general reference is made nowadays either to the 'quantitative relationship between auxin and its growth effects . . .' or to the '. . . critical control of coleoptile growth by auxin. . .' Went's paper of I928 is inadvertently quoted, and the assumption he then made is unconsciously accepted and utilized. For example, while arguing for the intervention of calines in growth Went (I938, p. 73) said on behalf of auxins I... Now one of the most striking properties of auxins is their quantitative action; the more we come to know about them the clearer this stands out. For a whole series of compounds (including indoleacetic, indolepropionic, indolebutyric, indole- valeric, naphthaleneacetic, and anthraceneacetic acids) it has been found that per mol they all cause the same amount of growth, provided that all other conditions are strictly comparable'. The significance of this passage for the present discussion is that by I938 a citation to the establishment of quantitative action was regarded as unnecessary. Also, the comment must refer to a series of calibration exercises, because naphthaleneacetic and anthraceneacetic acids are not naturally occurring. The statement comes, however, in the midst of a discussion about the possible roles of auxins and of calines in the intact pea plant. Thus we see how the ideas have become transfused.

The supposition that the auxin content in a plant must always be limiting introduces a further complication. If the different amounts of auxin distributed variously about the plant limit growth equally, then each tissue must have a correspondingly different sensitivity to endogenous auxin. But if, as it is now suggested, the measurable auxin is surplus to requirements, no such inference is involved. The question of the sensitivity of cells and tissues to native growth substance acting in vivo has yet to be investigated.

Proposition (b). The switch to an interrelationship between auxin content and growth rate was made by Went (I928); the zero growth rate of the mesocotyl (in his Fig. i i) was assutmed to indicate the absence of auxin. More explicitly, Went (I942, pp. 245-246) says '. . . Bonner and Thimann have shown that the decrease in auxin content in the first two hours after decapitation corresponds quantitatively with the amount of growth which has taken place. Considering this quantitative correlation, and the similarity between extractable auxin content and growth rate, it cannot be doubted that the growth rate of the Avena coleoptile is regulated by the amount of extractable auxin in the tissues'.

This particular relationship has not been formally established although qualitative correlations have been noted. Briggs, Tocher and Wilson (I957) found that coleoptiles when 90 hours old displayed their maximum growth rate, and at this time released into agar the 'roughly maximal' amount of auxin. Suitable published data can be used to examine the accuracy of the proposition. Skoog (I937, Fig. 4A) illustrated the increase with time of the curvature of test coleoptiles in relation to the concentration of applied auxin and to the presence or absence of the endosperm. The sequences of observations of the de-seeded plants fell on straight lines during an experimental period of 5 hours, or in some instances up to 61 hours. By measuring a succession of 'x' and 'y' values on an

D N.P.



280 C. L. MER enlarged copy of the figure, the rates of curvature can be estimated with reasonable accuracy; the requisite quotients are set out in Table 2(a). The straight-growth data of Bonner and Foster (1955, Fig. i; sub-optimal concentrations) have been examined likewise, with the results to be seen in Table 2(b).

As the ratios of the concentrations do not equal those of the growth rates, the proposition is not substantiated, by these data at least. Rietsema (I950) also sought, but failed to find, such a relationship; instead a linear correlation was drawn between 'increase in growth-rate' and 'applied concentration of IAA'. This is a much more complex relationship than had been expected, and it is not at all suitable for the analysis of plant growth.

A further tangle may now be unravelled. In a discussion on 'Phototropism', Went wrote (I956, p. 470) 'These investigators (i.e. Bonner and Thimann (I935) and Went (i942)) found that extractable auxin in the lower zones of a decapitated coleoptile decreased in two hours to approximately fifty percent of that found in intact coleoptiles. This means that if extractable auxin were responsible for growth of a coleoptile, the growth rate of a unilaterally illuminated coleoptile could never drop to less than half the

Table 2. The relationship between auxin concentration and growth (a) The correlation with the rate of curvature, estimated from Skoog's

data (I937, Fig. 4A) Curve Auxin Ratio of Mean rate Ratio of

concentration concentration of curvature rates of curvature

I I I8 I.z8 4.9 2 9 o.96 3.7 3 i 3 0.49 I.9 4 I I o.z6 I.0

(b) The correlation with the rate of elongation, estimated from Bonner and Foster's data (I955, Fig. I: sub-optimal auxin concentra-

tions) IAA Ratio of Mean Ratio of

concentrations concentrations growth rate growth rates (X 10-6 M)

5.0 25 o.64 2.4 I.0 5 0.46 I.7 0.2 I 0.27 I.0

normal rate within two hours after illumination'. The strictly linear relationship which was assumed to hold between growth rate and auxin content cannot now be upheld. Consequently, Went's deduction that extractable auxin was not responsible for growth must also be untenable. But, after having been driven to this conclusion, Went acknowledged that it was directly contrary to his earlier opinion (quoted on p. 279). Auxin physiology has thus displayed a kind of 'phasic development': starting with 'growth is controlled by auxin', through an intermediate step 'growth is regulated by extractable auxin', arriving finally at 'growth is not dependent on extractable auxin'. The obvious question-'On what then does growth depend?'-has not yet been answered.

This contretemps has two components: first, the assumption of a linear correlation between auxin supply and growth rate, and secondly, an extrapolation of Bonner and Thimann's conclusion that can scarcely be justified. Their data showed a correlation between the auxin that had been used and the growth that had been made. Now, these are past events to which the present content of auxin is quite unrelated. It is therefore a non sequitur to propose a correlation between present auxin content and future growth



Plant growth and auxin 28i

on the strength of a past interconnection. For, in the last resort, growth might have been finished at the time when the observations were made. This defect in the argument leads one to doubt the conclusions.

Proposition (c). The observations which gave rise to this proposition are illustrated in Went and Thimann (I937, Fig. 27). Parallel rising curves are shown, of 'growth-rate' and of 'auxin production', which reach maxima, the latter slightly after the former, and then decline. The 'rate of growth' curve falls to zero, but the other does not; nevertheless, in descending it remains sensibly parallel to the companion curve. The obvious parallelism is of some importance because it was cited by Jacobs (I959) as a well- established example of 'parallel variation', the first of the criteria which he devised to answer the question 'What substance normally controls a given biological process?' However, Went and Thimann's observations admit of some further conclusions. Since auxin production continued at a perceptible level when growth had stopped, the important principle that elongation ceased for lack of auxin (Went, I928) was not borne out in this instance. Evidently, these data agree with those, derived later, of Dattaray and Mer ('I964). Also, auxin production was not dependent on growth as its formation continued after growth had ceased. Finally, because the maxima of the two curves did not coincide in time, there is a minor element of disagreement with the results of Briggs et al. (I957) and of Briggs (I963).

The correlation becomes suspect when it is appreciated that the output of diffusible auxin was taken as a measure of the 'rate of auxin production' (K. V. Thimann, private communication). Avery and LaRue (I938, pp. I98-I99) also examined the growth of, and the production of auxin by, cut-off coleoptile tips. They were unable to reproduce Went and Thimann's observations, and concluded instead that growth hormone was not a necessity for the growth of the coleoptile and that growth substance need not necessarily be present for growth to occur. This direct denial of 'Ohne Wuchsstoff, kein Wachstum' was countered by Went (I942), who discriminated between extractable and diffusible auxins and their different physiological roles. Growth was to be associated with extractable auxin and not with diffusible; the observations correlated by Avery and LaRue were incompatibles. But such criticism must apply with equal force to the interconnection between growth and diffusible auxin which had been published earlier; the seemingly comprehensible relationship apparent in Fig. 27 of Phytohormones is to be regarded as fortuitous.

This example of 'parallel variation' cannot now be used for the purpose suggested by Jacobs (I959) and the question arises as to what should replace it. Dattaray and Mer's analysis (I964) indicates that a substance might justifiably be regarded as the factor controlling growth if its content remains at a fixed level during a period of rapid elongation. Such a dynamic view of a limiting factor is to be contrasted with the 'static' formula- tion recently discussed by Lockhart (I965). The data of Nitsch et al. (I960), using 'Concord' grapes, provide an appropriate example. In their Fig. ioB, it will be seen that from 6o to icc 'Days after Bloom' the content of auxin remained almost uniformly low, while growth (as shown in Fig. ioA) was proceeding rapidly. It might be argued that the rates of synthesis and of consumption of auxin were just balanced. If so, the classical ideas could apply and auxin should be considered essential for growth. This thesis would be rejected, however, if the data were interpreted to indicate that auxin was neither synthesized nor utilized. At present there is no way of discriminating between these alternatives.

A further observation of considerable moment for an 'auxin theory of growth' is



282 C. L. MER contained in Nitsch et al.'s results (I960, Fig. io). At 48 'Days after Bloom' when the auxin content had reached its maximum value, the enlargement of the grape berries was temporarily halted. During the subsequent IX days, although growth was suspended, the store of auxin vanished. Evidently, the auxin that has disappeared cannot be related to the growth that had taken place, as Bonner and Thimann (I935) suggested. This matter warrants reinvestigation; confirmation of Nitsch et al.'s data would make the analysis of plant growth in terms of auxin yet more difficult.

Proposition (d). This hypothesis is not easy to define explicitly, but Scott and Briggs' (I960) suggest from a study they have made of both diffusible and extractable auxin, that growth is controlled by some freely extractable part of the auxin that moves out of the transport system. In agreement with Nitsch et al.'s data discussed in the previous section, they concluded that the auxin that disappeared need not be related to growth; it might vanish as the result of other entirely independent physiological processes. Apart from the fact that growth is again being correlated with auxin content, the suggestion suffers from a lack of generality because it will apply only to plants grown in the light. From etiolated plants diffusible auxin was unobtainable, hence the analysis became indeterminate.

This discussion of the four propositions relating growth with the supply of auxin has revealed much that is questionable. Some reason has been presented for the opinion that these fundamental matters need to be reconsidered ab initio.

Some relevant peripheral matters may now be dealt with.

Experimental method, and philosophy The first question to pose is 'Why are Went's precepts inapplicable to intact plants?'

The universal acceptance of the implications of Went's experimental results was due, in the last resort, to one particular feature in the method that he used. He obtained an extract from some plants; when it was re-applied to other plants similar to those from which it had come, there ensued a response indistinguishable from the normal one. This procedure has become standard and all our ideas concerning the intervention of chemical substances in growth have originated from experiments in which compounds were applied either to intact plants, or to tissue fragments.

Dore and Williams (I956) have discussed 'the fallacy of the affirmation of the consequent': that if a treatment 'P' elicits a response 'Q', it is fallacious to attribute an observation of 'Q' in the absence of treatment to the intervention of 'P' from some internal source. The discussion by Went (I928) is, in part, an attempt to justify this illogicality. Another example may be found in the work of Leopold (I949) in connection with apical dominance in grasses. He damaged the growing points and, as a replacement, injected an 'auxin' solution (oc-naphthaleneacetic acid). The treated and control plants displayed similar tillering behaviour, from which it was concluded that tillering was normally controlled by auxin produced endogenously in the apical meristem.

Evidence has recently been forthcoming that the effect of an endogenous substance utilized internally miay be quite different from that of the same substance after absorption from an external supply. Andreae and co-workers (see Andreae, I964) have shown that after administration of IAA to pea stem segments it is recovered from them as an IAA- aspartate conjugate. This compound, whose formation might be a de-toxification reaction, is virtually inactive as compared with IAA. It is by no means certain that it



Plant growth and auxin 283 occurs in untreated plants as the usual product of auxin utilization, nor is it yet known if a similar reaction takes place in Avena seedlings after application of IAA. Presumably, the possibility of obtaining active auxin from plants would indicate that the conjugate is present in vivo either not at all, or only in part. Apparently, therefore, the growth response to externally applied IAA is brought about by the free acid and the duration of its activity may depend on the rate of formation of the conjugate. Further examples of this situation have been marshalled by Furano and Green (I963).

Obviously, this experimental method is the only one by which biological activity may be detected and assessed, but the results should be employed with caution to suggest what is ordinarily taking place in the intact plant.

The significance of recent discoveries Auxin was at first looked upon as the final determinant of growth, but Went (1928,

I935) soon appreciated that it might not be; a 'food factor' was introduced, also 'calines' (Went, I929), either as a food factor (Went, I938, p. 58) or as additional hormones (p. 78) which might interact with auxin. With the discovery of other active substances, gibberellins, kinins, inhibitors and an even larger number of synthetic compounds (not necessarily naturally occurring), the variety of possible in vivo reactants in growth phenomena has increased enormously. So much so that final explanations of irritability and correlative phenomena thought to be within our grasp (Went and Thimann, I937, p. I57) have now receded from immediate realization. The resulting change of outlook has been considered by Kefford and Goldacre (I96I): auxin has been removed from its original position as the ultimate growth controller to that of a non-specific 'pre-disposer'. Tech- nically, by their action they have chosen to disregard the Went theory derived from the calibration results, because in that method the effect of auxin was made specific. More- over, some other substance is likely to succeed auxin as the 'key' factor and, unless the fundamental conceptions are changed, a similar aura of confusion will be woven about this new growth determinant. In spite of these developments, auxin itself was still regarded as the controller of the expression of a growth effect, if all other factors were in adequate supply; the precept 'Ohne Wuchsstoff, kein Wachstum' was thus upheld. For example, the execution of a phototropic, or geotropic, curvature is still attributed to a causal imbalance in the content of auxin (Gillespie and Briggs, I96I; Gillespie and Thimann, I963, I964; Thimann, I964, I966).

With the discovery of gibberellin the history of auxin repeated itself, for just as auxin had been implicated in an increasing variety of phenomena, including cambial activity, abscission, apical dominance, flowering, etc. so also was gibberellin (Brian, I959). Although interactions with auxin have been suggested (Brian and Heming, I958; Purves and Hillman, I959; Brian, I96I; Jacobs and Case, I965), work has been carried out on gibberellin alone, in connection with geotropism (Norris and Brotzman, I965) and elongation (Lockhart, I959, i96ia, b). These new data have been interpreted precisely in accordance with the established ideas abo-ut the control of growth. If the word 'gibberellin' in Lockhart's statements about the reversal by gibberellin of the inhibition of growth by light (I959, p. 457, i96ib, p. 5i6) is replaced by the word 'auxin', then Went's original conception is recovered identically. As Lockhart provided no estimates of the content of the endogenous substance in relation to treatment with light, his conclusions about the control of growth by gibberellin are just as conjectural as were those of Went in I928, as regards the control of growth by auxin.

Similar remarks apply to certain work on phytochrome. Briggs and Chon (I966), for



284 C. L. MER

example, started with the expectation that physiological response would be related to the amount of Pfr in the tissues, but they found that it was not.

This brief and highly selective account of the proliferation of knowledge about growth substances seeks to show that the new information has been used for the explanation of growth phenomena, in the framework of the pre-existing concepts. New information of itself cannot mitigate or nullify the effects of inadequate principles now deeply embedded in the literature.

The concept 'Ohne Wuchsstoff, hein Wachstum' It was perhaps unfortunate for the study of auxin physiology that the main precept

was formulated in the negative. Even more so that it should have been employable in the converse sense 'no growth, therefore no auxin', a usage which was decried by Went and Thimann (I937, p. 74). 'Ohne Wuchsstoff, kein Wachstum' is now irreconcilable with experimental observations because it tends to insist upon tangible auxin. It should be awarded an honoured place in the 'Valhalla of Outmoded Ideas', along with 'phlo- giston' and the 'indivisible atom'.

GROWTH INTEGRATION IN THE ETIOLATED OAT SEEDLING

After the isolation of auxin, explanations became possible for more complex phenomena. Went (I928) not only attributed the elongation of the coleoptile to auxin originating in its tip but he also suggested that the remnant of growth substance unused by the coleoptile travelled down to, and controlled the extension of, the mesocotyl; thus the growth of the two structures was thought to be co-ordinated. By coupling this proposal with the idea that a zero growth rate indicated the absence of auxin, it became reasonable to say that the inhibition of the mesocotyl after the tip of the coleoptile had been removed, or after the seedling had been exposed to light, was due simply to the consequent reduction in the supply of auxin. Also, a mobile agency of some kind was needed to account for the inhibition of the mesocotyl of subterranean plants when their coleoptiles emerged through the soil surface; auxin was invoked to fulfil this role. However, without knowledge of auxin contents the thesis was wholly conjectural but the further evidence presented by van Overbeek (I936) was not inconsistent with the explanation. He showed that a heat treatment which reduced mesocotyl growth by about 50?/O similarly diminished the amount of auxin which diffused from cut-off coleoptile tips. This heat-induced inhibition of the mesocotyl could be completely obviated by supplying exogenous auxin (IAA) to make good the loss due to treatment.

With the estimation of auxin contents in Avena and Zea seedlings (heated plants were not investigated) the outlook changed. van Overbeek (I938a) recorded (a) that mesocotyls which had stopped growing nevertheless contained appreciable.amounts of auxin, (b) that these mature tissues were insensitive to exogenous indoleacetic acid, and (c) that after decapitation of the coleoptile more auxin, not less, was present in the mesocotyl. He tried to find the source of this extra growth substance but failed to do so. Synthesis in situ was not considered, it being, presumably, outside the ambit of theoretical possibility. It would have meant the abandonment of the idea that the auxin from the tip of the coleoptile-controlled mesocotyl growth. These observations not only eliminated all sem- blance of coherence from the explanation of growth integration, but they also cast doubts on the data originally used by Went for the construction of the 'auxin theory' of growth; yet they attracted scant attention. Thereafter, apart from an attempt to correlate



Plant growth and auxin 285 dwarfness in Zea with the destruction of endogenous auxin by catalase (van Overbeek, I938b), which was examined in detail by von Abrams (I953) and found wanting, interest in growth integration temporarily lapsed.

It has since been demonstrated in short-term experiments conducted in complete darkness that the elongation of the mesocotyl is independent of the supply of auxin from the coleoptile (Mer, I95I). The growth inhibition attributed to a shortage of auxin after decapitation was due instead to the use of light during handling. Further, when plants were heated, or briefly exposed to light, the content of ether-extractable auxin in the mesocotyl was increased (Dattaray and Mer, I964). Evidently, the inhibitions caused by these treatments could not have been due to a shortage of auxin. In consequence, van Overbeek's procedure of supplying IAA externally to remedy the supposed deficiency of endogenous auxin becomes devoid of rationale and the experimental verification of

70 -

* ,/*

_ / ,~~~~~~~~~~~~~~~/ / ~~~~~~/'

_50 _

I 0 -~~~~~~~~~I

E) _ /

~J30 _ /,

10_ '

2 3 4 5 6 7 Time (days)

Fig. 2. The progress of growth with time of the two members of the plumule of etiolated oat seedlings. , Mesocotyl; ---, coleoptile.

his- ilypothesis loses acceptability. The rise in auxin content after exposure to light reported by Dattaray and Mer (I964) accounts for the similar observation made by van Overbeek (I938a) for his plants would have been illuminated to facilitate decapitation.

In the following alternative account of co-ordinated growth the inverse situation is posited; the elongation of the mesocotyl interferes with that of the coleoptile and leaves. This comes, about because growth of the mesocotyl depends on the formation of cells in the nodal meristem, whereas the coleoptile and leaves are limited by nutrient supply. All substances must traverse the mesocotyl to reach the more distal parts so the nodal meristem, located intermediarily, forms a physiological barrier to free flow while it remains active.

The pattern of growth which has to be accounted for (Fig. 2) shows that the mesocotyl elongates most rapidly first. Both structures grow slowly during the first 48 hours; the mesocotyl then extends rapidly, at a uniform rate, for 2 days, then less rapidly for a further 24 hours until its final length is attained by the fifth day. While the mesocotyl is growing quickly, the coleoptile makes slow progress, but an acceleration in rate occurs



286 C. L. MER

in slight anticipation of the decline on the part of the mesocotyl. The maximum growth rate of the coleoptile (also sensibly constant) occurs from the fourth day onwards and, although not shown in the figure, its growth is finished by the eighth day.

The embryo functions anaerobically for a time after germination starts quite irrespective of the cultural conditions used (Merry and Goddard, I94I). Ethanol consequently accumulates in the tissues and might ultimately prove toxic but for the simultaneous presence there of carbon dioxide; an interaction takes place between these two metabolites which mitigates the deleterious effect of ethanol (Mer, I96I). The initial slow growth of the plumule may be due in part to the formation of ethanol but it most probably results from the paucity of cells that can elongate. This lack of cells persists until the onset of mitosis which occurs some 24-36 hours after germination has begun (Thomson, I954). In this regard particularly, the initial slow growth is of biological importance, because it is under these conditions in darkness that cell division occurs most rapidly. With mitosis instituted and with the products of division also remaining meristematic, the number of dividing cells increases, and so, too, does the rate of cell production. This reaches a maximum value of 2.7 cells/hour on the second day, just when the rapid growth of the mesocotyl is about to begin (Causton and Mer, I966). The declining rate of cell production from then onwards means that fewer and fewer cells are formed whose subsequent elongation can contribute to the over-all length of the mesocotyl. Effectively therefore, the future pattern of growth in darkness and the ultimate dimensions of the mesocotyl are determined when the seedling is 48 hours old.

The cortex of the embryonic mesocotyl is made up of cells, all meristematic, which are about 23 /tm long; they can be regarded as falling into columns each comprising sixteen cells (Thomson, I954). Elongation and maturation of the cells start at the base of the mesocotyl. Here, within the first 5 mm or so, the cells reach a final length of 200-300 ,um. Cells which mature later, and are therefore positioned nearer to the node, attain a final length of about 500 um (Mer and Causton, I963). The wave of growth in the mesocotyl flowing upwards to the node, described by Tucker (I957) is due to the successive extension of these cells. Given cells of a uniform size, the final length of the mesocotyl will be determined by the number of cells in a column, and this is governed by the interplay between the rates of cell extension and of cell production. From their observations of the rate of cell production, Causton and Mer (I966) deduced that a cell loses its capacity for division when it is about I20 /m long. As the cells in sequence from the base of the mesocotyl arrive at, and exceed, this length more rapidly than the remaining cells can divide, so the meristematic zone is continuously encroached upon; it recedes upwards and is finally eliminated when the cells at the top reach the critical length of I20,um. This they do on the fourth day, because in the 'vacuolated' type of meristem present in the node of the mesocotyl the cells grow slowly even though mitoses are taking place; the cessation of division is, therefore, inevitable. Once it has stopped, such further growth as is made by the mesocotyl will be due to the elongation of these uppermost cells from I20o um to 500 ,um. Growth of the mesocotyl then stops for one reason only-the lack of cells that can elongate.

The change of slope shown in the growth curve of the mesocotyl on the fourth day (Fig. 2) is now read as the indication of the cessation of mitosis; the steep slope up to this time results from the successive extensions of a number of cells coupled with a declining rate of cell division; the subsequent lower slope is an expression of cell extension only. How these various happenings combine to produce a uniform over-all growth rate between the second and the fourth day remains to be determined. The final length of the



Plant growth and auxin 287 mesocotyl in this experiment, about 75 mm, is accounted for by the independent observations of I50 cells per column, with a mean cell length of about 500 im.

The importance of nutrient supply for the growth of the coleoptile is suggested by the observation that more elongation takes place after the fourth day, at which time cell division seems to stop (Avery and Burkholder, I936; Thomson, I954; Mer and Causton, i963), than before it. Nitrogenous metabolites are particularly required for cell growth in the coleoptile and leaves because the administration of small amounts of mixed sodium and potassium nitrates to intact seedlings, although scarcely affecting mesocotyl growth, can enhance the elongation of the coleoptile and leaves by about 50?/, (Mer, I959b; Mer et al., i963). The relatively poorer growth of the coleoptiles of plants not supplied with extra nitrates, for example, of the 'control' seedlings which were given tap-water only, can be attributed to the inadequate provision of nitrogenous metabolites from the endosperm. Also, while the nodal meristem is active the new cells that it forms, interposed between the coleoptile and the endosperm, will utilize without stint the nutrients in transit and so tend to deprive the distal members. With the stoppage of cell division on the fourth day this barrier to flow is removed; the coleoptile then attains its maximum growth rate just as that of the mesocotyl is about to decline. The reciprocal growth shown by these two structures can thus be accounted for. Further, the slight increases in rate to be seen in the growth curve of the coleoptile between the second and fourth days can be referred to the declining rate of cell production which obtains during this interval in the mesocotyl.

As regards the coleoptile itself, that is apart from the aspect of co-ordinated growth, it was originally suggested that the auxin formed in the tip was solely responsible for its elongation. In accordance with the precept 'more auxin, more growth; less auxin, less growth', the coleoptile would have had to grow most at the tip and least at the base. The reverse was true; growth was acropetal. A two-stream hypothesis was then devised, in which the auxin moving downwards was held to interact with a counter-flow of nutrients, to form a zone where growth would occur in the presence of all necessary metabolites. As the cells matured, the junction of these opposing streams would rise from base to tip; acropetal growth was thus explicable. Observations of the lengths of cells in the etiolated coleoptile show that they all elongate simultaneously, at different rates depending upon age and on position in the structure (Causton, i965; Mer and Causton, i967). The absence of evidence for a zone of maturation proceeding from the base upwards casts doubt on ther existence of these counterflowing streams of metabolites.

The reactions of the coleoptile and mesocotyl to various growth-modifying factors can now be described in terms of their effects on cell division in the nodal meristem. Exposing 3-day-old seedlings to dim red light for 5 minutes has two effects on the mesocotyl; its growth rate is at once reduced and it displays a shorter final length. The growth of the coleoptile is transiently promoted, however, These responses are due primarily to the inhibition of cell division (Avery, Burkholder and Creighton, I937; Goodwin, I94I; Mer and Causton, i963), and secondarily to the reduction in length of a few hypersensitive cells (Mer and Causton, i963). Likewise, when 3-day-old plants are heated for 3 hours in darkness at 400 C effects have been recorded identical with those brought about by illumination both on the growth of the mesocotyl and coleoptile (Mer, 1951; Dattaray and Mer, i964) and on cell division and extension (Mer and Causton, i967). The growth changes observed thus correspond to those which occur in untreated seedlings on the fourth day, except for the effect on cell length. Hence, the acceleration in the growth rate of the coleoptile after treatment, quite inexplicable in terms of auxin



288 C. L. MER metabolism, is now seen as an expression of the termination of cell division in the nodal meristem.

Two further effects of these treatments may be recorded here for which, as yet, no explanation can be offered. They both cause an enhancement of geotropic sensitivity, evidenced by the subsequent straightness of the coleoptile and upper part of the mesocotyl. The change in the direction of growth produces, a sharp bend in the mesocotyl and at this point the short cells are found (Mer and Causton, I963). Also, the leaves of treated plants are perceptibly yellower than are those of control seedlings; seemingly, synthesis of carotenoid pigments is promoted.

The relationship between the sensitivity of the mesocotyl to a flash of light (estimated by a measurement of length) and the age of the seedling when illuminated, has the form, approximately, of an inverted 'normal distribution'. Minimum length is found when the plants are between 2 and 3 days old at the time of exposure (Goodwin, I94I). This observation can now be explained. Clearly, the mesocotyl will be most sensitive when it contains the largest number of light-sensitive cells, both meristematic and non-meristematic. This will occur shortly after the maximum in the rate of cell production on the second day. Illumination before this time will be less effective, because fewer cells will be present, all of them meristematic; and of these, some few will not be affected because they will not yet have reached that length (about 6o ,um) at which light-sensitivity is first shown. Even earlier exposures will induce less inhibition through an accentuation of these circumstances, reducing to a zero effect when all the cells in the mesocotyl are too short to be sensitive to light. From the second day onwards the number of meristematic cells declines; fewer can therefore be influenced by light. By the third day many non- meristematic cells have become mature, or will exceed the length (about 250 gm) at which light-sensitivity is lost (Mer and Causton, I967). The possibility of inhibiting cell division passes by the fourth day and with further delay in the time of exposure the degree of inhibition will taper away to zero as the final length of the mesocotyl is approached. The shape of the response curve, with maximum sensitivity in the correct place, is thus defined. It may be forecast that the response curve to high temperature will have a similar form.

The need for a mobile chemical agency, auxin, to carry a morphogenetic effect of light from the exposed tip of a coleoptile to the s.ubterranean mesocotyl, can now be dispensed with. Unpublished experiments by the writer have shown that these etiolated seedlings act like glass rods towards light, which, when it impinges at any point along the plant, readily pervades the tissues. The sensitive region at the node (Araki and Hamada, I937) can thus be illuminated directly.

When seedlings are grown in air enriched to 5/00 with carbon dioxide, the growth of the mesocotyl is at first depressed, but it subsequently elongates at a higher rate than do the controls to show a growth promotion (Mer, 1957a; Mer and Causton, I963). The growth of the coleoptile is retarded over the experimental period, but it occasionally gains- equality with that of untreated plants by the time that the final sample is taken. These growth effects are caused by a change in mitotic activity. Apparently, treatment with carbon dioxide prevents cell elongation during the early stages of germination, so bringing about the initial depression of growth. But this inhibition unavoidably keeps the cells shorter than the critical length of I20o m at which mitosis stops and so divisions can take place over a longer period of time in a larger number of cells (Causton and Mer, i966). Ultimately, the impulse for elongation overcomes the restriction imposed by carbon dioxide and the cells extend to attain a length equalling that of the cells in



Plant growth and auxin 289 untreated plants. The extra growth of the mesocotyl induced by carbon dioxide has a unitary explanation; there are more cells. The shortness of the coleoptile is not due to a lack of cells but to shorter ones (Mer and Causton, I963) and this reflects a shortage of nutrients. Two factors contribute to this situation: the larger number of cells in the mesocotyl coupled with a reduction in the quantity of reserves transferred from the endosperm. With this decreased supply goes a simultaneous loss of nitrogenous substances, for the translocation of the two is correlated (Mer et al., I963). The extra cells in the mesocotyl utilize the inflowing metabolites without hindrance to reach a uniform final length; in consequence the mesocotyl sequesters a larger proportion of the reduced supply of nutrients. The coleoptile and leaves are partially starved and so their growth is restricted. Even under these adverse conditions an exposure to light will inhibit cell division in the meristem and the transient growth promotion of the coleoptile is displayed (Mer and Causton, I963).

Growth responses similar to those induced by carbon dioxide are observed when seedlings are treated with low concentrations of ethanol (Mer, I96I), or with a 2% solution of sucrose (Mer and Causton, I967). As the cell behaviour also corresponds, the explanation of the growth effects already given will apply in these instances. Appropriate concentrations of acetaldehyde, acetic acid, mixed sodium and potassium nitrates, mixed sodium and potassium phosphates (Mer, unpublished), and copper-glycine (Mer, I959a) will also evoke similar growth effects but the details of cell behaviour are not yet known. There would be much complexity if each of these various substances influenced growth by acting on a different metabolic pathway. It would be simpler if they all acted like carbon dioxide, i.e. to prevent elongation of the meristematic cells during the early stages of germination; a single explanation would then suffice. The integrated growth phenomena under review would be explicable without mention of the word 'auxin'.

This account inevitably prompts some questions about van Overbeek's demonstration (I936) that the inhibitory effect of high temperature (400 C), which reduced mesocotyl length from 48.5 mm to 20.7 mm, could be completely obviated by the subsequent application of IAA to the coleoptile, in that mesocotyl length was returned to 48.2 mm. As it is now known that heat treatment inhibits cell division and stops elongation of some few sensitive cells, the first question is 'Was cell division re-started?' Exogenous IAA when- applied externally to plants is known to affect cambial activity (Wareing, I958), but it seems unlikely that a renewed capacity for mitosis would have been conferred upon a deceased meristem. However, the occurrence of cell division may not be relevant in this particular context because the shortness of the untreated mesocotyls (48.5 mm) indicates to the writer that the plants might have been exposed to light prior to heat treatment; cell division would thereby have been inhibited. If so, the treatment with IAA could only have resuscitated a capacity for elongation in cells which had been deprived of it by heating. Further questions now arise 'Why did these cells react to IAA administered externally?' and 'Why were they insensitive to the copious supply of endogenous auxin?' Answers to these queries can only come from future investigations.

The one experimental technique that has contributed more than any other to the elucidation of the growth phenomena described here has been the rigorous exclusion of light while the seedlings were handled. Of necessity, therefore, each sample of plants was used only once and the variability encountered was dealt with by statistical methods. The 'phototropically inactive' orange-red light customarily used in a former era for growth substance investigations had tp be avoided because of its high potency as an inhibitor of mesocotyl growth (Weintraub and McAlister, I942; Weintraub and Price,



290 C. L. MER

I947). In some laboratories it has been superseded by 'morphogenetically inactive' green light (Schneider, I94I; Nitsch and Nitsch, I956; Briggs, I963; Ng and Audus, I964). This opinion about green light is unwarranted (Mer, I966), yet it has persisted in the face of all the evidence to the contrary (Schneider, I94I; Goodwin, I94I; Weintraub and McAlister, I942; Huisinga, I964). To take one recent example, the assertion by Ng and Audus (I964) concerning the 'inactivity' of their green light is not supported by their data. For the delineation of the growth curve of the mesocotyl and coleoptile of intact seedlings, Ng (I96I) measured the same group of plants on successive occasions under green light. He recorded a mean final length for the mesocotyls of 28-33 mm; also, he mentioned his failure to observe a growth acceleration by the coleoptile after an experimental dose of red light.

The shortness of the mesocotyl, as compared with usual values of 65-67 mm, immedi- ately indicates an effect of light; the absence of an acceleration of growth on the part of the coleoptile confirms this view. This response by the coleoptile had passed unnoticed when the plants were first exposed to green light to measure them and, as it is a secondary effect-a consequence of the cessation of mitosis-it would not have occurred for a second time. All treatment effects, therefore, are confounded with those of exposure to green light, and the data can contribute nothing to the resolution of growth integration phenomena. For work with Avena sativa at least, darkness is essential.

Is MOBILE AUXIN OF PHYSIOLOGICAL SIGNIFICANCE?

The involvement of transportable auxin with a number of phenomena, particularly tropisms, is so universally acknowledged that to raise the question at all may appear unrewarding. However, the issue is forced by the implications of the data of cell growth accumulated by Causton (i965). He found that the cortical cells in the mesocotyl were about 23-50 gm when formed and they grew to a final length of about 500 ,tm. Appar- ently, at formation they possessed an in-built, almost uniform, de-limited 'impulse for growth', which might therefore be said to stop when the impulse had been expended. This supposition will be familiar; it was a feature of Went's first theory that growth stopped for lack of auxin. There are now so many examples of the converse, that growth has ceased with abundant auxin present, that auxin can no longer be regarded as the critical factor for cell growth.

The idea of an initial quantum of growth potential per cell is supported to a small extent by the observation that no way has yet been found of increasing the length of a cell beyond what is 'normal', apart from a very small, but nevertheless significant, effect of treatment with ethanol (Mer and Causton, i967). It was expected that much longer cells than usual would have been found in the mesocotyls of seedlings grown on a medium containing 200 sucrose. This nutrient has two effects relevant in this context; it increased not only the content of free sugars (Mer et al., i963), so adding to the osmotic potential for growth, but also that of auxin (Dattaray and Mer, i964). Each acting independently might have caused an increase in cell length, which would have been enhanced had they acted additively. They did not, however, because cell length was unchanged (Mer and Causton, i967). Seemingly, the only response that a cell can make to an applied treatment is to refrain somewhat from growing; the treatment would then be described as 'inhibitory'. The effect of a flash of light on the elongation of the mesocotyl may be introduced attthis juncture. In this part of a 3-day old seedling, the cells can be regarded as being arranged in columns; at the top the cells will be small and meristematic; lower down they



Plant growth and auxin 291

will be non-meristematic, but still capable of elongation; nearer the scutellum they will be mature. If seedlings comprising such mixed cell populations are illuminated for 5 minutes any residual capacity for growth that the cells retain will express itself after the plants have been replaced in darkness: different growth potentialities are, in fact, observed. Some, estimated to be IOO-I20 ,pm long, lying near the lower boundary of the meristem are exceptionally sensitive to light; they attain a length of 250 pm instead of about 500, m. Others, somewhat shorter and longer than Ioo-I20o m and situated, respectively, above and below these sensitive ones, are less affected by light because they grow longer than 250 pm. Hence, in such mature mesocotyls one finds a few short cells sandwiched between longer ones.

Why are these intermediary cells so short? Either auxin was flowing downwards from the upper elongating cells or it was not. With the first alternative a loss of sensitivity to auxin is also required, for otherwise a supposed deficiency resulting from illumination could have been made good from the stream of growth substance passing through the cells from above. Preference for the second alternative demands either the destruction of part of an original quantum of auxin or, as this seems unlikely, once more a loss of sensitivity. This now emerges as the simplest single hypothesis to account for the observations. If movement of auxin does occur it need have no special physiological significance. Furthermore, any 'blanket' theory of auxin action, such as '. .. light destroys auxin . . .' is no longer admissible because, depending on their state of maturity when exposed, cells are affected to differing degrees; many not at all.

This postulate, that a change of sensitivity to endogenous auxin occurs upon illumination, may now be invoked to resolve a query outstanding from the previous section of this review. By the time growth is over, the coleoptiles of illuminated plants are always shorter than those of untreated seedlings. As plenty of auxin is present at the time, a loss of reactivity to it may serve as an explanation.

Some comments are called for about the evidence in favour of transportable auxin. Many experimental results show that the removal of the tip of a coleoptile is followed by a loss of responsiveness to geotropic and phototropic stimuli and also by a reduced ability to elongate. But, from the earliest times, the plants which were used to obtain this sort of information were exposed to light in order to perform the decapitations, as well as to suppress the growth of the mesocotyl. These extraneous exposures to light will have had some effect on the sensitivity of the cells to endogenous auxin and certainly, too, on other metabolic activities. Hence the consequences of removing the tip will have been confounded with these other effects of light. A method must be found for securing information about the elongation of a coleoptile after decapitation, without exposing the plant to light at any stage. With this information at hand, a less equivocal assessment of the role of the tip will become possible.

Dolk (I926, p. II I5) recorded that when decapitated coleoptiles were stimulated geotropically their response was different from that of a normal seedling. They began bending from the base, and the curvature progressed upwards to the tip. Coleoptiles which had been decapitated twice showed a similar response which he attributed to some residual geotropic sensibility of the basal cells. Evidently a reaction was possible in the absence of the tip and of. the flow of auxin issuing therefrom. Could a similar response to light have been overlooked?

Support for a flow hypothesis comes from observations of the progress of a phototropic curvature. After having been stimulated by a unilateral beam of light, a coleoptile begins to bend at the tip the lower part meanwhile remaining vertical. The position of the bend



292 C. L. MER

then appears to move downwards; the upper part leaning towards the light becomes longer and longer as the lower vertical part becomes progressively shorter. The basipetal movement of the position of the bend has been regarded as the external indication of the internal flow of the auxin front. Is the interpretation necessarily true? Consider, for instance, that a set of coordinated still photographs can convey the appearance of movement when viewed in sequence at the appropriate speed. The motion perceived is an illusion. The appearance of a bend moving downwards may also be an illusion; it might result from an integrated sequence of individual effects taking place in columns of cells which, because they are of different physiological ages, will have correspondingly different capacities for response. The need to postulate a moving front of auxin would not then arise.

This attempt to explain a few of the more obvious contradictions in the study of auxin physiology will have indicated the need to re-evaluate the fundamental postulates.

Apparently, too, many data may be considered suspect because of the possible consequences of exposure to light during the performance of other experimental manipulations. Such effects were overlooked while formulating theoretical explanations of the observations made.

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Croissance Vjgdtale, p. 559. Gif-sur-Yvette. ARAKI, T. & HAMADA, H. (I937). Lokalisation der lichtempfindlichkeit von Keimorganen bei Avena sativa.

Bot. Mag., Tokyo, 5I, 498. (Summary seen in Yap. J'. Bot. (I937), p. 9.) AUDUS, L. J. (I959). Plant Growth Substances. Leonard Hill, London. AVERY, G. S. & BURKHOLDER, P. R. (I936). Polarized growth and cell studies on the Avena coleoptile,

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first internode and coleoptile of Avena in relation to light. Bot. Gaz., 99, I25. AVERY, G. S. & LARUE, C. D. (I938). Growth and geotropic responses of excised Avena coleoptiles. Bot.

Gaz., Ioo, i86. BALL, N. G. (I963). Plant tropisms. In: Vistas in Botany, Vol. 3 (Ed. by W. B. Turrill), p. 228. Pergamon

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McGraw-Hill, New York. BRIAN, P. W. (1959). Effects of gibberellins on plant growth and development. Biol. Rev., 34, 37. BRIAN, P. W. (I96I). Interactions between gibberellins and auxins. In: Recent Advances in Botany, p. IO99.

Academic Press, London and New York. BRIAN, P. W. & HEMING, H. H. (1958). Complementary action of gibberellic acid and auxins in pea

internode extension. Ann. Bot., 22, I. BRIGGS, W. R. (I963). Red light, auxin relationships, and phototropic responses of corn and oat coleoptiles.

Am. 5'. Bot., 50, i96. BRIGGS, W. R. & CHON, H. P. (I966). The physiological versus the spectro-photometric status of phyto-

chrome in corn coleoptiles. PI. Physiol., Lancaster, 41, II59. BRIGGS, W. R., TOCHER, R. D. & WILSON, J. F. (I957). Phototropic auxin redistribution in corn coleoptiles.

Science, N.Y., 126, 2IO. CAUSTON, D. R. (I965). Cell division and extension in Avena seedlings as affected by carbon dioxide and other

external factors. M.Sc. thesis, University of London. CAUSTON, D. R. & MER, C. L. (I966). Analytical studies of the growth of the etiolated seedling of Avena

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DATTARAY, P. & MmE, C. L. (I964). Auxin metabolism and the growth of etiolated oat seedlings. In: Regulateurs Naturels de la Croissance V6gdtale, p. 475. Gif-sur-Yvette.

DOLK, H. E. (I926). Concerning the sensibility of decapitated coleoptiles of Avena sativa for light and gravitation. Proc. K. ned. Akad. Wet., 29, I I I3.



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plant growth in relation to endogenous auxin, with special reference to cereal seedlings

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