C O R R E L A T I O N B E T W E E N F O R M , S T R U C T U R E A N D H A B I T I N S O M E L I A N A S

BY S. BHAMBIE [Department of Biological Sciences, B.I.T.S., Pilani (Raft)]

Received June 29, 1971

(Communicated by Prof. V. Puri, F.A.SC.)

ABSTRACT

1. The present paper deals with the gross anatomy and certain mechanical properties of the stem of sixteen lianas with anomalous secondary growth in comparison to that of an erect plant.

2. An analysis developed from mechanics considerations shows clearly that liana stems have adopted themselves to some well-known engineering principles in developing their internal structure to counteract the stresses and strains caused by the natural external forces of wind, rain, light, etc.

3. Apart flora the size of the pith, the shape of the pith (circular, oval) and its eccentricity with respect to the xylem cylinder also affect the flexibility.

4. The order of increase of flexibility as predicted on theoretical considerations agrees well with the order obtained from the experimental results. Further, a close relation between flexibility and water content of these plants exists.

~. A detailed study of three varieties of Bougainvillea lends support to the viewpoint stressed recently by Esan and Cheadle (1969) that complimentary cambial strips develop just adjacent to the secondary phloem and the cambium is bi-directional in this plant.

INTRODUCTION

LIANAS are woody climbers which inhabit mostly tropical rain forests. Some of them are good ornamentals too. They usually have thin, long, strong, pliable stems varying in shape from slender smooth to massive, knotted, twisted, flattened and corrugated types. Their internal structure is quite aberrant as compared to that of straight erect stems as they do little to support themselves and are usually much twisted. The aberrant internal structure is mainly due to anomalous secondary growth which appears to 246

Correlation Between Form Structure and Habit in Some Lianas 247

be responsible for the presence of a special type of xylem cylinder. In the present study a number of woody climbers have been worked out to ascertain whether structural configurations of the xylem cylinder throw any light on the flexibility and strength of these plants.

Structural configurations of woody lianas.--Out of the seventeen species studied here, the material of Tinospora eordifolia Willd., Cocculus hirsutus (L.) Diels. (Menispermaceae), Capparis decidua (Forsk.) Edgew. (Cappari- daceae), Vitis vinifera L. (Vitaceae), Quisqualis indica L. (Combretaceae), Ipomoea pentaphylla Jacq., Ipomoea pes-capre L. R. Br. (Ipomoea biloba), Ipomoea sp. (Convolvulaceae), Pyrostegia venusta (Ker. Gawel.) Miers. (Bignonia venusta), Campasis grandiflora Seem. Teeoma grandiflora (Bignonia- ceae), Petrea volubilis Jacq. (Verbenaceae), Bougainvillea speetabilis Willd. (Nyctaginaceae) and Antigonon leptopus Hook. and Am. (Polygonaceae) was collected from Pilani (Raj.). Materials of Leptadenia reticulata Wt. and Arn. (Asclepiadaceae), Thunbergia grandiflora Roxb. (Acanthaceae), Gnetum ula L. (Gnetaceae) and Capparia sepiaris L. (Capparidaceae) were obtained from Meerut (U.P.), Pachmarhi (M.P.), Pathanamthitta (Kerala) and Delhi respectively. All the species were fixed in F.A.A. Hand or wood microtome sections were prepared and customary methods of dehydra- tion and mounting were followed after staining with Safranin and light green.

In a normal erect plant, e.g., Capparis decidua the cambium produces xylem tissue centripetally and phloem tissue centrifugally with the constant increase of the axis. The xylem forms a continuous cylinder traversed by narrow rays with phloem and other tissues at its periphery (Fig. 1). The structure of C. sepiaria is similar to that of C. decidua except for a well- developed pith cylinder in C. sepiaria. The structure produced by the normal cambium is the same in Petrea, Quisqualis, Campasis and Ipomoea. In Petrea the pith is usually oval in shape while in Quisqualis and Campasis it is eccentrically placed. In Campasis additional arcs of cambia, develop- ing on the margin of the pith, give rise to xylem and phloem in an inverse order leading to the formation of an inner cylinder of xylem just adjacent to the normal one (Figs. 10, 11). In Ipomoea successive growth rings in some species (I. pentaphylla and I. pes-caprae) form extra patches of xylem and phloem in the cortex (Fig. 9). Interxylary phloem is found in I. pes-caprae, though intraxylary phloem is usually present in all the species of Ipomoea.

In Leptadenia and Thunbergia also a cambium develops and functions normally in the beginning (Fig. 2). Later on at places it cuts off a few

248 S. BHAMalE

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FIGS. 1-12. Diagrammatic representation of xylem cylinders as observed in t.s. of Capparis, Leptadenia, Pyrostegia, Bougainvillea, Tinospora, Cocculus, Antigonon, Ipomoea, Campasis and Gnetum respectively, to calculate flexibility theoretically.

(a = radius of pith cylinder; b = radius of xylem cylinder ; r = radius of phloem patches ; c = width of rectangular phloem wedges ; d = depth of rectangular phloem wedges ; s = pitch of concentric xylem rings; t = thickness of xylem ring; A ~ width of the wedge at the inner periphery; B = width of wedge at the outer periphery; D = radial thickness of the wedge IsR = axis of the wedge passing through its c.g.; I n = axis perpendicular to the axis I~a and passing through the c.g. of the wedge; I~z and Ivv = mutually perpendicular axis of the cylindrical cross-section.)

Correlation Between Form Structure and Habit in Some Lianas 249

undifferentiated parenchymatous cells centripetally. These patches of thin- walled cells become embedded in the xylem and subsequently differentiate into phloem islands or interxylary phloem. Leptadenia has intraxylary phloem too. The xylem cylinder of Leptadenia and Thunbergia is, therefore, interpolated at places by interxylary phloem. In Pyrostegia the cambium cuts off more phloem than xylem at four places thus producing rectangular wedges of thin-walled cells in the cylinder of the xylem (Fig. 3).

A normal cambium develops in Tinospora, Vitis, Cocculus and Gnetum, However, it cuts off ray cells in the interfasicular region. Xylem is therefore, split up into small or big lamellae by the broad medullary rays (Figs. 6, 7, 12). The structure recalls that of the Aristolochiaceae where the xylem cylinder is composed of a number of wedge-shaped vascular bundles separated by radial bands of large interfasicular rays. In Coeeulus succes- sive complete, and in Gnetum complete or partial rings of cambia develop ,~orming bundles repeating the structure of a young stem.

In Bougainvillea many complete rings of cambia arise in close succes- sion. The boundary between xylem cylinder and the adjacent lignified tissue is not clearly defined; so the phloem sometimes appears to be in the form of islands (Figs. 4, 5). However, there are a number of rings of xylem alternating with those of phloem and prosenchyma. The condition in Antigonon recalls that of Coeculus and Bougainvillea. First two or three cambia form wedge-shaped vascular bundles arranged in rings while those developed later give rise to concentric rings of xylem and phloem (Fig. 8).

In brief the structural complexities in Leptadenia, Thunbergia and Pyrc- stegia are due to anomalous activity of the cambium. The structural aber- rations of Campasis, Ipomoea, Cocculus, Gnetum, Bougainvillea and Antigonon can be related to anomalous origin of cambium rings either complete or partial. Xylem cylinders can be conveniently brought under the following categories: (1) continuous xylem cylinder; (2) xylem cylinder having patches of interxylary phloem; (3) xylem cylinder having four rectangular wedges symmetrically placed at the periphery; (4) multiple concentric xylem cylinders interspaced with phloem; (5) xylem cylinder itself composed of radial wedges and (6) any other modifications due to combination of more than one of the above-mentioned five categories (Figs. 1-12).

Photomicrographs of a few important types have been taken to obtain the area of pith and xylem cylinders, their configurations and proportions. Table I shows the area of pith, xylem cylinder and other green tissues in the xylem cylinders.

B 4

250 S. BHA~VraI~

TABLE |

Proportions of xylem cylinders and their configurations

Name

Capparis

Z¢ptadcnia

~'e~ania

BouKaim, illea

Tinas:ara

Diam. of Diam. xylem includ-[ Area of of pith ing green I pith

cm* tissue and pith em z c m

3 .6

1.8

4 .6

9 . 4

2 .6

12"0

17.0

11.0

28.4

11.6

10.73

2.54

16.61

69.36

5.21

Area of tylem exclud-

ing green tissue

c m 2

102.20

194.08

70.21

Area of green tissue

¢ m 2

No. of ~ings Total or medullary, area rays or

[ rectang.ular cm2 j wedges

112.93

226.86

9 4 . 9 8

291.23

60.22

. °

30.24

8 .16

69.24

34" 20

429.83

105.63

° .

4 wedges; c - d = 2 : 3

9 rings

16 medullary rays

* The dimensions are taken f rom the photomicrographs of the sections.

Mathematical modelling for rigidity.--An attempt has been made to develop mathematical models for predicting the flexural rigidity of some of the woody lianas discussed in the preceding paragraphs.

The following simplifying assumptions have been made for the present analysis: (i) the value of modulus of elasticity of xylem for all lianas under discussion is the same; (ii) the rigidity of the cross-section is mainly due to xylem portion only and the contribution of pith, cortex and green tissue (interxylary phloem, wedges of phloem or broad medullary rays) in the xylem towards rigidity is negligible; (iii) the form of pith and xylem cylinders has been geometrically idealized.

Based on the structural observations discussed earlier, the various structures under study have been grouped into five categories for the purposr of analysis: (a) hollow xylem cylinder (Fig. 1); (b) hollow xylem cylinder interpolated with thin tissue concentrated at the mean radius (Fig. 2); (c) hollow xylem cylinder with four rectangular wedges symmetrically placed at the outer periphery (Fig. 3); (d) multiple concentric xylem cylinders interspaced with thin-walled tissue (Figs. 4, 5), and (e) hollow xylem com- posed of radial wedges of xylem, interspaced with thin-walled tissue (Fig. 6).

From the elementary theory of bending the flexural rigidity, J, expressed as the ratio of bending moment to the curvature, is given by the product of

Correla t ion Be tween F o r m S t ruc ture and Hab i t in S o m e Lianas 251

the moment of inertia of area of cross-section and the modulus of elasticity, i.e., J = EI, where E = modulus of elasticity of xylem and I = M.I. of cross-section.

The expression for the moment of inertia of each cross-section has been developed as given below which is directly proport ional to the rigidity of the cross-section.

Case L- -Le t a = radius of pi th;

7"r I ----~4 • (b 4 - #) .

Case / / . - -Le t

Ah = total area of thin tissue.

Ah = n (~rr2),

where

b = outer radius of cylinder (Fig. 1).

r = average radius of each patch;

n = number of patches (Fig. 2).

I = ~ , (b 4 - a 4) - - 2 "

Case I l L - - L e t

c = width of rectangular wedge;

d = depth of wedge (Fig. 8).

1 2 - - 2cd ( b . I : 4 ( b ~ - a ' ) - 6 c d ( c + d ' ) - - d ) 2

Case IV. - -Le t

s : average pitch of continuous xylem cylinder.

t : thickness of xylem cylinder (Figs. 4, 5).

: - - n - - - - 3 r r tb ~ s n - - I mrtb s ~rts 3 ( n ~ _ l ) 2 ( n ~ l )

+ ½ ~rtbs 2 n (n + 1) (2n + 1).

252 S. BHAMBIE

Case V.--Let

D = (b -- a);

A = width of wedge at inner radius;

B = width of wedge at outer radius (Fig. 7).

Moment of inertia of cross-section of each wedge about its radial centre line (Fig. 11) is given by:

IRR = i2 DAn q- D 2 72 - q- 36 '

Moment of inertia about the line perpendicular of RR and passing through its C.G. is given by

I~N (B~ + 4AB -k A s) D 3 = 36 (A q- B)

For number of wedges equal to 16 the moment of inertia of the total cross- section is given by

I -~ 8 (I~R ÷ INs).

Experimental determination of flexural rigidity.--In order to observe bending in some of the mature twigs, an experiment was set up. The twigs selected were of equal length and of equal diameters approximately. The twigs were simply supported at both ends and the load was applied in steps of 200 gm centrally. The deflection for each twig at the load point was noted for increasing and decreasing loads. The average change in deflec- tion for this step was taken for calculation of rigidity J.

Table II gives the data of bending tests and the calculated values of flexural rigidity.

Fahn (1967) emphasized that pliability depends on the water content of a plant to a certain extent and it increases with an increase in the water content. The water content in some of the twigs of equal diameter and length has also been determined by taking fresh and dried weight with the help of an electrically operated balance. Table III shows the data and a comparative amount of the percentage water content in different materials.

Correlation Between Form Structure and Habit in Some Lianas

T A B L E I I

Bending of twigs Normal i zed rigidity = EI /b ' = 16EI/d 4 = WIa/38d 4 Inc rementa l load = 0 .20 kg. Suppor t ed length o f twig = 13 .0 era.

253

Material Diam. Averase of twig deflection El/b 4 cm 8 cm

Diam. Average of twig deflection EI/b ~

cm 8 em Mean Ellb 4

Ca~parir

Leptadenia

Bou gaiuvillea

Tinospora

. . 0 .83

. . 0 .73

. . 0.65

. . 0 .96

0.052 6-0 x 108

0.082 7 . 5 x 1 0 a

0.103 8 . 1 x 1 0 ~

0.158 l . l x l 0 a

0.61 0.094 11 .4× 10 s

0.615 0.092 I I . 0 × 1 0 s

0-755 0.097 4 . 5 × 1 0 3

0 .89 0.273 0 . 9 × 1 0 ~

8.7x I0 s

9.3XI0 3

6.3XI0 3

I.OxlO ~

TABLE III

Water content of various twigs

Name

Cap#aris ..

Leptadenia . .

Tiwos~ora ..

l~ortlaga

Cocculu:

Bignania

Bougainvillea . •

Fresh w t .

gm

1.0938

0.8966

1.0415

0.9312

0.9445

0.3405

1.32200

Dry wt,

gm

0 . 5 4 4 5

0.2523

0.2269

0.2228

0.3932

0.14925

0.33990

Water content present

gm

0.5493

0.6443

0.8146

0.7214

0.5513

0.1912

0.98210

Diamtter

c m

0.5333

0.4367

0"4833

0.5233

0.4733

0"37

0"55

Length

ClI

5.1

5 .2

5"0

5 .2

5 .2

3.05

4 .9

Volume (~ r2 I )

CEIl 8

1 • 140

O. 7793

0 .9175

1.118

0.9152

0"304

1.104

Density of fresh

wood g/c.c.

0.960

1.15

1.135

0.834

1.03

0.628

0.8485

Percentage wator content

(by wt,)

50 .2

71-7

78.3

77.5

58.5

56.14

74.29

R E S U L T S A N D D I S C U S S I O N

The distribution and orientation of mechanical tissue is an interesting problem as it reveals how nicely lianas have adopted to some well-known engineering principles to adjust themselves against many forces of stress and strain. An analysis developed from mechanics considerations for the rigidity of these different structures agrees closely with those observed in nature.

254 S. BHAMBIE

A comparison of theoretical and experimental results of flexibility of some of the lianas is presented in Table IV. It may be noted from Table IV that the order of increase of flexibility as predicted on theoretical considera- tions agrees well with the order obtained from the experimental results" Further a close co-relation between flexibility and the water content exists as Tinospora, the most pliable one, has the maximum amount of water.

If the amount of twist and bending is to be considered in the five cases mentioned, we shall have to consider the distribution of hard tissues in the cross-section with respect to the central pith core. The xylem in Capparis is a cylinder composed of cells with strong bonds (thick-walled rays) and gives the largest moment of inertia of the cross-section and thus shows the maximum rigidity though a relatively larger size of pith in C. sepiaria makes it slightly less rigid in comparison to C. decidua, In Leptadenia the xylem cylinder is taken to be interspersed with small patches reduces the moment of inertia of the cross-section resulting in potential reduction in rigidity. In Pyrostegia the presence of four rectangular wedges of phloem equally spaced at the outer periphery of the xylem cylinder and a relatively large size of the pith reduces further the moment of inertia of the cross-section resulting in further reduction in rigidity and making it more flexible than Leptadenia.

TABLE IV

Theoretical and experimental results of flexibility

Name

Ca~aris

Lepw.d~'a

Bi gnaHia

11oxgainz, il/ea

Tinospora

Radius o1 pith a - cm

1.8

0"9

2.3

~.7

1.3

Outer radius

of xylem b-cm

e.O

8.5

5.5

11.7

5.8

Particular data

Average green tissue, i e. , interxylary phloem---- 30.24 cm~

Width of rectangular wedge c--2 cm- depth of rectangular wedge dfffi3cm

Average pith of xy cylinder s=0.778 cm, average thickuess of xy cylinder t=0.062 cm, No. of cylinders n--9

Interspace width between two wedges=O.776 cm, No. of wedges n = t 6

Calculated (I/~')

0.785

0.720

0.662

0-532

0.038

Ratio o f theoretical flexibility 1/El

1

1 .0 9 : 1

1-19:1

1 .48 :1

~0.6 ;1

Experi- mental ~atio of

flexibility

1

I : l

2 .9 : 1"

1 - 4 : 1

i 9 : 1

* This reading appears to be wrong possibly due to the paucity of fresh material.

Percent- age

water content

50.2

71'7

56-14

74.29

78.3

Correlation Between Form Structure and Habit in Some Lianas 255

The cross-section of Bougainvillea consists of concrete rings of xylem inter- spaced with phloem. This structure gives lesser moment of inertia than Leptadenia stem and makes it more flexible. In Tinospora the xylem cylinder consists of a number of wedge-shaped vascular bundles separated by radial bands of large interfasicular rays and in addition there are two pericycle patches on the outer periphery of each wedge. The discrepancy between the theoretically calculated and experimentally determined flexibility ratio of Tinospora appears to be due to the fact that the wedges are not completely free to bend about their own axes (as assumed) because of the bond provided by the medullary rays to the neighbouring xylem wedges, thus resulting in lower value of flexibility than predicted by the theoretical model in which this effect has been neglected. The moment of inertia of the xylem cylinder for this configuration is the least of all the lianas studied, making it most flexible. This shows that structure in Capparis has the largest moment of inertia and hence has maximum stiffness whereas Tinospora has the minimum, is therefore, most pliable or flexible. Table IV clearly indicates the relative flexibility in different woody lianas.

In comparison to Capparis, other plants are more pliable. The plia- bility appears to be due to the occurrence of oval or eccentric pith, inter- xylary phloem, interspaced broad, medullary rays, complete broken or partial broken cylinders of xylem, etc. Further in Quisqualis, Petrea and Campasis the pith is eccentrically placed proving the occurrence of compression wood on one side and tension wood on the other side--an important character of pliable structme. Moment of inertia in such cases is also theoretically less than a plant where pith is centrally placed. This is put forward on the basis of the values of moment of inertia of the structules, when they show circular, eccentric or oval pith in the xylem cylinder (Figs. 10, 11). The calculated values are as under: Ixx = 12"4in. * for circular pith; Ixz----12"3in. 4 for oval pith; Ixx = 11-8 in. 4 for eccentric pith. Extra hard xylem cylinder of Campasis can be related to the development of roots at each node which help in climbing.

It would not be out of place here to point out in brief that a few recent papers by Balfour (1965), Philipson and Ward (1966) and Esau and Cheadle (1969) have given two different viewpoints to explain secondary growth in Centrospermae. According to Philipson and his associates the cambium is more or less unidirectional in this group while Esau and Cheadle (1969) considered it to be bidirectional. The present observations on three varieties of Bougahzvillea support to the view of Esau and Cheadle (1969) as com- plimentary arcs of cambia replace the first formed cambium and function

256 S. BHAMBIE

normally. These newly developed cambial strips have led Balfour (1965) to consider them as the outer part of a self-perpetuating zone. She has probably overlooked the transformation of first formed cambium ceils. These two processes, i.e., cessation of earlier formed cambium cells and the differentiation of new complimentary strips of cambia occur one after the other in close succession.

ACKNOWLEDGEMENTS

The author expresses his deep sense of gratitude to Professor V. Puri (Meerut University, Meerut) for suggesting the problem and keen interest throughout the progress of this work; to Professor T. A. Davis (Indian Statistical Institute, Calcutta) for going through the manuscript and giving some valuable suggestions; to Professors A. K. Dattagupta and S. K. Pillai for facilities and encouragement; and to Dr. C.S. Sharma and Mr. K . C . Gupta of Mechanical Engineering Department of this Institute whose keen interest in the problem has made this work possible.

Balfour, E, E.

Esau, K. and Cheadle, V . I . . .

Falm, A. ..

Philipson, W. R. and Ward. J. M.

REFERENCES

"Anomalous secondary thickening in Chenopodiaceae, Nycta- ginaceae and Amaranthaceae," Phytomorphology, 1965, 12, 110--43.

"Secondary growth in Bougainvillea," Ann. Bot., •969, 33, 807-20.

Plant Anatomy, Oxford, 1967.

"The ontogeny of the vascular cambium in the stem of seed plants," BioL Rev., 1966, 40, 534-79.

3513/72. Published by B. S. Venkatachar, Editor, "Proceedings of the Indian Academy of Sciences", Bangalore and printed by V. J. F. Jesudason at the

B~mgalore Press, Bangalore-18.