Wood Science and Technology Vol. 2 (1968) p. 279-291

FOREST PRODUCTS LABORATORY (Madison, W i s . , 53705) F o r e s t Service , U.S. DEPARTMENT O F AGRICULTURE

Approve d Technical Art ic le

Factors Affecting Permeability and Pit Aspiration in Coniferous Sapwood

By G. L. COMSTOCK, Madison, Wis., and W. A. CÔTÉ JR., Syracuse, N. Y.

Summary The influence of drying methods on the permeability of red pine and eastern hemlock

sapwood was investigated. Permeability was found to be reduced by normal drying procedures to only a small percentage of the green permeability. The reduction was more severe a t higher drying temperatures; less severe but still very large a t - 18° C. Pit aspiration was shown to be responsible for the reduction. Replacing the sap with surfactant solutions and organic liquids and evaporating them revealed that pit aspiration occurred with surfactant solutions having surface tension values of less than 20 dynes/cm and did not occur with organic liquids having surface tension values as high as 44 dynes/cm. It is suggested that a critical factor in pit aspiration is the adhesion of the torus to the pit border, and the failure of the organic liquids to cause pit aspiration is due to their inability to promote adhesion between the torus and pit border.

Zusammenfassung Der Einfluß verschiedener Trocknungsverfahren auf die Durchlässigkeit des Splintholzes

bei Kiefer und Eastern Hemlock wurde untersucht. Es zeigte sich, daß bei Anwendung üblicher Trocknungsverfahren die Durchlässigkeit gegenüber jener des frischen Holzes sehr deutlich vermindert war. Höhere Trocknungstemperaturen setzten die Durchlässigkeit weit stärker herab; bei - 18° C war die Verminderung etwas weniger stark, jedoch noch immer beträchtlich. Als Ursache für die Durchlassigkeitsverminderung wurde der Tüpfelverschluß gefunden. Beim Austausch des Zellsaftes gegen oberflächenaktive Lösungen bzw. gegen organische Flüssigkeiten und bei nachfolgender Verdampfung derselben ergab sich, daß die oberflächenaktiven Lösungen mit Oberflächenspannungswerten von höchstens 20 dyn/cm einen Tüpfelverschluß erzeugten, die organischen Lösungen mit Oberflächenspannungswerten bis zu 44 dyn/cm jedoch nicht. Es wird postuliert, daß beim Tüpfelverschluß das Haften des Torus am Tüpfelrand das Kriterium bildet; die Unfähigkeit bestimmter organischer Lösungen, einen Tüpfelverschluß zu bilden, ist darin zu sehen, daß sie die Adhäsion zwischen dem Torus und dem Tüpfelrand nicht zu fördern vermögen.

Introduction The permeability of wood is a measure of the ease with which fluids flow

through it. Longitudinal permeability of coniferous woods is controlled almost exclusively by the bordered pits. In green sapwood these pits are quite permeable and permit easy passage of fluids and small suspended particles. During drying, however, these pits frequently become aspirated, resulting in a marked reduction in permeability. The research reported here is aimed a t determining quantitatively the extent t o which permeability is reduced by drying as influenced by drying conditions and the properties of the liquid evaporated from the wood. The influ­ence of surface tension of the evaporating liquid is given particular attention, because it has previously been regarded as the most important factor in pit aspiration.

Pit Aspiration Most important conifers, in particular those belonging to the Pinaceae family,

possess a bordered pit structure with pit membrane characterized by a centralized

280 G. L. COMSTOCK and W. A. CÔTÉ, JR.

thickened disk, the torus, and a supporting membrane, or margo, consisting of strands of cellulose microfibrils [LIESE, 1965]. I n the green condition, most of the sapwood pit membranes are centrally located and quite permeable. Drying the wood generally causes the torus to be displaced and come into contact with one of the pit borders. This phenomenon is referred to as pit aspiration and the forces which cause it to occur were discussed in detail by HART and THOMAS in 1967.

In 1933,PHILLIPS made what appears to be the first comprehensive study of pit aspiration. He found that drying sapwood caused a gradual increase in the number of aspirated pits with loss of moisture down to the vicinity of the fiber saturation point. At this point, virtually all the springwood pits became aspirated; whereas about one-third of the summerwood pits remained unaspirated. He ascribed the greater tendency of summerwood pits to resist aspiration to the greater rigidity of the summerwood pit membrane. LIESE and BAUCH in 1967 reported observing the same phenomena. BRAMHALL’S 1967 permeability studies indicate that there is little change in summerwood permeability upon drying, but spring-wood permeability can vary as much as 30 times, depending upon the drying method.

GRIFFIN in 1919 and 1924, PHILLIPS in 1933, ERICKSON and C R A W F O R D in 1959, and LIESE and BAUCH in 1967 observed that pit aspiration does not occur in coniferous sapwood when the water is replaced by alcohol prior to drying. The prevention of aspiration is invariably ascribed to the lower surface tension of alcohol compared to water. ERICKSON and CRAWFORD observed that when sap­wood was air dried the permeability was reduced by drying to only 1 . . . 3 percent of its original value in the green condition; whereas drying by solvent exchange with alcohol, acetone, or alcohol-benzene followed by evaporation of the solvent prevented pit aspiration and maintained the permeability at its original level. LIESE and BAUCH dried several conifers by solvent exchange, using alcohol-water and acetone-water mixtures. They observed that for solvent concentrations in excess of 75 percent ethanol or 80 percent acetone, pit aspiration was incomplete. These concentrations correspond to a surface tension of about 26 dynes/cm in the pure liquids, and LIESE and BAUCH concluded that a surface tension of 26 dynes/ cm is required to cause pit aspiration in springwood.

The concept of the authors regarding pit aspiration is that three factors are involved, any one or combination of which may control aspiration. These are:

(1) Surface tension forces tending to pull the torus into contact with the pit border.

( 2 ) Rigidity or stiffness of the pit membrane which results in a force opposing the surface tension forces exerted by the evaporating liquid.

(3)Adhesion of the torus to the pit border when they are brought into contact.

The first factor, surface tension forces of the evaporating liquid, is primarily dependent on the surface tension of the liquid and the sizes and shapes of the pit aperture, chamber, torus, and pores in the margo. I n 1967, HART and THOMAS discussed in detail the forces which may develop due to the presence of air-liquid menisci in a pit. They concluded that the forces associated with a meniscus be­tween the torus and pit border increase with loss of liquid as the torus is drawn closer to the pit border.

Coniferous Sapwood Permeability and Pit Aspiration 281

The relative rigidity of the pit membrane compared to the surface tension forces will determine whether the torus is displaced sufficiently from the central position to come into contact with the pit, border. The stiffer and more rigid the membrane, the greater will be the force required to aspirate the pit. Summerwood pits tend to aspirate less than springwood pits, evidently because summerwood pits are usually smaller and have a thicker, more rigid membrane. Another factor which may affect, the rigidity of the pit membranes, when drying from non­aqueous liquids, is the degree of swelling of the liquid. It is well known that most strength properties of wood, including the modulus of elasticity, increase with loss of moisture below the fiber saturation point). Strength of wood swollen with liquids other than water would be expected to change in similar fashion with the degree of swelling of the wood. For example, ERICKSON and REES observed in 1940 that the crushing strength of wood decreased with increasing degree of swelling caused by the organic liquid in which the wood was soaked. It is antici­pated, therefore, that the stiffness of the pit membrane may be increased by re­placing water with liquids of lower swelling power.

The third factor involves adhesive forces between the torus and the pit border. Assuming that the surface tension forces are sufficient to overcome the stiffness of the pit membrane and thus bring the torus into contact with the pit border, the torus will stay in this position only if there are adhesive forces between it and the pit border. This particular aspect has apparently been ignored in the past and is worthy of consideration. Whether an intermediate substance, such as a resin, is involved as an adhesive or whether there is direct bonding of the surface of the torus to the pit border, is unknown. The type of forces involved is likewise un­known.

Permeability Measurements The purpose of these experiments was to determine the extent to which the

permeability of coniferous sapwood is changed by various drying methods and to determine whether the changes observed are related to aspiration of the bordered pits. Permeability measurements prior to drying were made with water. Measure­ments after drying were made with nitrogen gas. That comparisons of the measure­ments obtained by these methods are valid has been demonstrated by COMSTOCK in 1967 and 1968. He showed that the permeability values obtained with gases and liquids are essentially identical if viscosity is included in the permeability equation and a correction is made for slip flow of the gas. That gas permeability increases with loss of moisture in the hygroscopic range because shrinkage of the wood causes the intertracheid pores to become larger was also shown. This effect was found to be very small in solvent-dried wood and somewhat larger in air-dried wood.

Water permeability was determined using the technique described earlier [COMSTOCK 1965]. Permeability values were calculated using DARCY’S law.

(1)

where k = permeability (Darcys) = viscosity (centipoise) Q = flow rate (cm3/sec) A = flow area (cm2) L = specimen length (cm) P = pressure drop (atm)

2 3 Wood Science and Technology, Vol. 2

282 G. L. COMSTOCK and W. A. CÔTÉ JR.

The superficial gas permeability, kg, was calculated according to DARCY’S law for gases, which takes into account the compressibility of the gas.

where Pis the absolute pressure at which Q is measure and P is the mean absolute pressure in the specimen.

Slip flow was then corrected for by use of the following expression [COMSTOCK

1967]:

(3)

where = mean free path of the gas = radius of the intertracheid pores

The procedure used for gas permeability measurements was to measure k, at mean pressures of one and three atmospheres and to extrapolate the plot of k, vs. 1/P . . . 1/P equal to zero. The value of k obtained in this way is the slip-corrected permeability. The apparatus used for gas permeability measurements is similar to that described by COMSTOCK in 1968.

Influence of Temperature and Rate of Drying on Permeability This experiment was conducted to determine the extent to which the reduction

in permeability on drying depends on the drying conditions employed. Five drying temperatures were used: -18°,20°, 60°, 100°, and 140° C. At the three intermediate temperatures, two rates of drying were used. Only slow drying was carried out at -18°C and only rapid drying at 140° C.

Drying at -18°C was accomplished by placing the specimens in a desiccator over phosphorus pentoxide and placing the desiccator in a room controlled at -18° C. Drying at 140° C was accomplished by putting the specimens between two platens heated to 140° C and bringing the platens into contact with the wood. All other drying was done in a humidity- and temperature-controlled cabinet. Rapid drying was achieved by placing the specimens directly into the air stream in the cabinet. Slow drying was achieved by placing the specimens in a desiccator over P2O5 and placing the desiccator in the cabinet at the desired temperature. Since no circulation was provided in the desiccator, drying was much slower than in the open air stream.

Specimens were taken from three eastern hemlock trees, and two specimens from each tree were dried at each set of conditions. The specimens used in this experiment were approximately 1.3 cm in diameter by 2 cm along the grain. Specimens from a given tree were all taken from the same growth rings to minimize the influence of within-tree variation on the results.

Water permeability of the green sapwood of several specimens was determined prior to drying so that the reduction due to drying could be estimated. The average permeability before drying was found to be 2.78 Darcys.

The average permeability values after drying are plotted graphically as a function of drying temperature in Fig. 1. Specimens dried at -18°C had perme-

Coniferous Sapwood Permeability and Pit Aspiration 283

ability values considerably higher than those dried at the higher temperatures, but they were still only about 5 percent as permeable as they were before drying.

Above 27° C there is a grad­ual trend toward decreasing permeability with increasing drying temperature. Rate of drying had no effect on per­meability. This is contrary to the findings of ERICKSON and CRAWFORD in 1959, which showed that slow drying at room temperature resulted in higher permeability, whereas drying at 100° C had the same effect as drying rapidly at room temperature.

The results of this experi­ment indicate that pit aspira­tion is most severe in specimens dried at high temperatures, but it appears to be reasonably complete in specimens dried at -18°C, since the permeability of specimens dried at this tem­perature is only about one-twentieth as large as it was be­fore drying. It was expected Fig. 1. Slip-corrected nitrogen permeability of dry specimens

as a function of drying temperature and rate of drying.that pit aspiration would not Permeability before drying averaged 2.78 Darcys. occur in specimens dried at - 18° C, because freezing of the free water in the wood should eliminate the surface tension forces which cause aspiration. In 1967, THOMAS observed that vacuum freeze drying did prevent aspiration from occurring in pine, but the drying temperature was probably lower. It is speculated that sufficient liquid phase was present at -18°C to exert the attractive force to cause pit aspiration.

The more severe aspiration at higher temperatures is apparently due to the greater plasticity of the wood at this temperature rather than surface tension, since surface tension decreases with increasing temperature.

Influence of Surface Tension and Other Properties on Pit Aspiration To isolate some of the factors involved in pit aspiration, the sap in a series

of sapwood permeability specimens was exchanged for liquids having different surface tension and swelling properties. Aqueous surfactant solutions and organic liquids were used to vary these properties.

Two experiments were conducted using this exchange procedure. The first was done using eastern hemlock samples from the three trees used in the drying condition experiment. Two specimens from each tree were assigned randomly to each of the exchange treatments. The treatments are given in Table 1. The 23*

284 G. L. COMSTOCK and W. A. CÔTÉ, JR.

Table 1. Exchange Treatments given Samples Prior to Drying

0.1 % fluorocarbon surfactant (cationic) 0.1 % fluorocarbon surfactant (nonionic)

code letter Treatment

0.1 % organo-silicone surfactant (nonionic) 0.1 % organo-silicone surfactant (cationic)

Ethanol water1

Ethanol

Methanol acetone pentane1

Methanol acetone toluene1

Controls - air-dried immediately

Arrow indicates succession of treatments on the same specimens.

surfactants used are powerful surface tension depressors, which lower the surface tension well below the value of 26 dynes/cm suggested by LIESE and BAUCH in 1967 as a lower limit which causes aspiration. The other treatments were aimed a t determining the effect on permeability of drying from nonaqueous liquids of varying surface tension.

Table 2. Surface Tension of Several Exchange Solutions and the Permeability of Eastern Hemlock Sapwood after Drying from these

Solutions1

-

-19.4 23.2 23.5 28.2 ----

2.78

.053

.071

.114

.098

.094

.088 3.01 4.44 5.10

Treatment

None -

R 2 - air-dried A - FC-134 B - FC-170 C - L-77 C - L-79 K1 - Water K2 - Ethanol N - Toluene L - Pentane

(water permeability)

Initial2

dynes per cm

-

-18.7 21.8 22.8 24.3 72.0 24.0 29.6 18.4

Surface tension Final3 Permeability4

Darcys

Measurements of permeability were made with nitrogen gas after drying wood over P2O5. Permeability values are corrected for gas slip.

Value obtained from tensiometer measurements on liquids before specimens were introduced.

Value obtained from tensiometer measurements on liquids after specimens were removed.

Average of six or more specimens.

The results of this experiment are shown in Table 2. The surface tension values listed were determined on the solutions in which the specimens were im­mersed using a ring tensiometer. Air drying is seen to reduce permeability to about one-fiftieth of the permeability before drying. The effect of drying after

Coniferous Sapwood Permeabllity and Pit Aspiration 285

exchanging the sap for surfactant solutions is seen to be nearly like air drying directly from the green condition, even though the surface tension of these solu­tions was as low as 19.4 dynes/cm. On the other hand, drying from any of the organic liquids maintained the high permeability of the green sapwood and may have increased it somewhat in the case of the pentane and toluene treatments. Treatment K1 in which the procedure was to replace sap with ethanol and then replace the ethanol with water resulted in a permeability reduction on drying similar to the air-dried controls.

A second experiment was conducted using a large number of liquids, mostly organic, with a wide range of surface tension and swelling properties to see if the important properties controlling pit aspiration could be isolated. In this experi­ment sapwood of red pine and eastern hemlock was used. All specimens were closely matched and two from each species were randomly assigned to each treatment. The first step in all treatments was complete exchange of the water for acetone, a dehydration step. The acetone was then exchanged for the liquids shown in Table 3. Old solution was discarded and new added three times for each exchange step to ensure essentially complete exchange. After exchange was complete, the liquids were allowed to evaporate from the wood by simply air drying.

Table 3. Permeability of Red Pine and Eastern Hemlock Sapwood before Drying and after Evaporation of Liquids of Varying Surface Tension and Swelling

Properties

Treatment

None -

Air dried Pentane Ethylether Ethanol Methanol Acetone 1, 1, 1Trichloroethane Chloroform Toluene Benzene Cellosolve P-chlorotoluene Dioxane Furfural Water

(water permeability)

Surface tension

dynes per cm

-

-

17.8 18.4 23.3 23.5 24.8 26.5 28.4 28.8 29.0 29.6 32.1 33.8 44.4 62.5

Swelling relative to water

Red pine1

% of water

-

-

3 3

81 94 63

6 6 5 4

92 3

10 68

100

Eastern hemlock1

% of water

-

-

2 4

90 97 77 11 10 8 8

98 4 7

56 100

Permeability

Red pine1

Darcys

5.30

.24 6.47 6.12 6.54 6.88 5.99 6.28 5.83 6.02 6.26 6.85 5.88 6.14 6.17

.76

Eastern hemlock1

Darcys

5.30

.016 5.48 6.68 6.94 5.69 6.40 6.38 6.40 6.68 6.04 6.92 6.86 6.85 6.67

.024

Each value is the average for two specimens.

Surface tension values were determined on the pure liquids and on the liquids after the specimens were removed. There was little change except for water, which had a reduced surface tension after being exposed to the wood specimens. The surface tension values determined on the liquids after the specimens were removed are shown in Table 3.

286 G. L. COMSTOCK and W. A. CÔTÉ, JR.

The swelling by the different liquids was determined experimentally using specimens about 2 cm square by 5 mm along the grain. These were ovendried, measured, and immersed in the desired organic liquid and the change in dimension after 6 clays of immersion was taken as the swelling. Radial and tangential swelling were measured on two specimens of each species for each liquid. The average of the radial and tangential values is expressed as the percentage of the swelling in water in Table 3. Some swelling occurred in organic liquids which were cited by STAMM in 1964 as nonswelling, such as benzene. This could have been due to trace amounts of water in the liquids or adsorption of atmospheric moisture during the measurements. The values in general, however, agree well with STAMM’S values, with the exception of dioxane, for which STAMM reports a value of 62 percent as opposed to 10 and 7 percent found here.

Results of permeability measurements described earlier are also shown in Table 3. It is interesting that in no case involving solvent drying is there evidence of pit aspiration even though surface tension values as high as 44 dynes/cm were observed. The only cases where permeability was reduced were those of drying directly from the green condition and drying after going from sap to acetone and back to water. The latter drying procedure gave somewhat higher permeability than air drying, but still reduced permeability well below the values for the green wood. Red pine was reduced to about 15 percent of its original permeability and eastern hemlock was reduced to less than 1 percent of its original permeability. Permeability values after drying from any of the organic liquids were similar and slightly higher than the values measured with water before drying. The slightly higher values after solvent drying may be due to removal of extractives or perhaps deaspiration of some pits by the solvents. There does not appear to be any trend in permeability with either surface tension or swelling of the liquids, so it seems unlikely that the higher permeability is produced by evaporation of the solvent from the wood. The increase produced by solvent drying is small compared to the decrease caused by evaporation of water from the wood and mill not be considered further here.

Electron Microscopic Examination of Pits in Hemlock Specimens of eastern hemlock dried from several of the different liquids were

examined to see whether changes in the permeability were consistent with visual changes in the bordered pits. Replicas were prepared from split radial surfaces using the direct carbon replica technique described by CÔTÉ KORAN, and DAY in 1964.

The results of these investigations agreed with those from the permeability studies. Pits in air-dried specimens were almost invariably tightly aspirated as shown in Fig, 2. Tight aspiration is inferred when the imprint of the pit aperture on the torus is clearly visible, indicating that the torus is sealed tightly against the pit border.

Examination of samples dried from surfactant solution C revealed that the pits are aspirated, but not tightly, since the aperture imprint was missing. Fig. 3 is typical of the pits found in specimens examined. The less severe aspiration is manifested also in many supporting strands in the margo being elevated above the back border. The less severe aspiration is evidently responsible for the slightly higher permeability compared to the air-dried controls.

Coniferous Sapwood Permeability and Pit Aspiration 287

Fig 2. Electron micrograph of a tightly aspirated pit in air-dried eastern hemlock sapwood.Magn.: 5 800:1.

Fig. 3. Electron micrograph of a n eastern hemlock sapwood pit dried after the water had been replaced by surfactant solution C. Magn.: 5 600: 1.

Fig 4. Electron micrograph of an unaspirated eastern hemlock sapwood pit dried by acetone exchange.Back border was torn away during replication, revealing the fibrillar structure of the margo.

Magn.: 4 200:1

Fig. 5. Electron micriograph of two unaspirated pits in eastern hemlock sapwood dried by ethanol exchange. Torus extensions which are charateristic of hemlock are clearly visible. The dark band

around the torus is the outline of the replica of part of the pit border behind the torus. Magn.: 3 150:1.

Coniferous Sapwood Permeability and Pit Aspiration 289

Fig. 6. Electron micrograph of an unaspirated pit in eastern hemlock sapwood dried by solvent exchange using benzene as the evaporating liquid. The light circular area on the torus is the area of the pit aperture behind the torus. The dark band occurred because the entire pit border behind the torus was

replicated and remained intact. Magn.: 4 700:1.

Specimens dried after solvent-exchanging water for ethanol, acetone, benzene, and ethyl ether were examined and the invariable result was that the majority of the pits were unaspirated, which corresponds to the permeability findings. Typi­cal electron micrographs are shown in Figs. 4, 5, and 6. Notice first that the tex­ture of the margo is clearly visible in these unaspirated pits and the margo is seen to have a very high porosity. This corresponds to the 1966/67 findings of THOMAS and NICHOLAS on solvent-dried southern pines. Although different pits often exhibited greatly different margo densities, no consistent differences were observed between samples dried from the different organic liquids. This also agrees with the permeability results.

Direct carbon replicas of unaspirated pits exhihit some peculiar traits not found in replicas of aspirated pits. For example, the dark band which appears around the edge of the torus in Figs. 5 and 6 is due to the deposition of a layer of carbon and some metal on the pit border behind the torus. Where the replica of this border occurs behind the torus there is a darker appearance because two layers of carbon must be penetrated by the electron beam, one being the replica of the torus, the other the replica of the back border. In Fig. 6 the entire back border is intact and the lighter area in the center of the torus corresponds to the pit aperture.

Frequently, in replicating unaspirated pits, the back border is torn away during the process of replication because of its loose attachment to the annulus. When this occurs, a replica similar to that shown in Fig. 4 results, showing the

290 G. L. COMSTOCK and W. A. CÔTÉ, JR.

torus, margo, and annulus with the back border missing. The margo texture is of course clearly visible in replicas of this type and the extreme porosity of the margo is evident.

Discussion of Results It is clear from the electron microscopic studies that changes in permeability

during drying are directly related to the condition of the bordered pits. When aspiration occurs, permeability is low; if aspiration is prevented, permeability is maintained at a high level.

It is obvious from the results shown in Tables 2 and 3 that we cannot speak glibly about the role of surface tension in pit aspiration as though it were the only important' factor. The evidence clearly indicates that pit aspiration can occur using aqueous surfactant solutions with surface tension values of less than 20 dynes/cm as the evaporating liquid. On the other hand, pit aspiration did not occur when drying from organic liquids with surface tension values as high as 44 dynes/cm.

It seems that either the influence of these liquids on the rigidity of the pit membrane or on the adhesion of the torus to the pit border may be the determining factor. The experiments involving organic liquid displacement followed by water displacement showed that the effect of the organic liquid was not that of any permanent fixation of the pit membrane. LIESE and BAUCH also found this in 1967.

The influence of the various liquids on the rigidity of the pit membrane should be in nearly direct proportion to the swelling of the wood by the liquid. If this factor were of major importance it should have manifested itself in a reduced permeability with liquids like cellosolve and furfural which have both a high swelling and high surface tension. The fact that permeability remained high after drying from those liquids suggests that swelling was not the controlling factor.

By a process of elimination, it thus appears that a key factor involved in the mechanism of pit aspiration is the adhesion of the torus to the pit border. Even though the surface tension is presumably sufficiently large to bring the torus into contact with the pit border, adhesive forces between the torus and pit border apparently do not develop when the organic liquids are evaporated from the wood. This is analogous to the greatly reduced strength in paper when formed from nonaqueous liquids as reported by BROUGHTON and WANG in 1955. The role of water in the adhesion process appears to be an overriding factor which provides the molecular forces necessary for adhesion to occur. The exact nature of the adhesive forces is not known, but it is clear that substances soluble in organic solvents are not acting as an adhesive, since pit aspiration occurs in solvent-extracted sapwood when water is evaporated. It appears, therefore, that there is direct bonding of the surface of the torus to the pit border, presumably by some type of Van der Waals-forces. From the chemical nature of wood and the impor­tance of water, it would seem that these adhesive forces may be hydrogen bonding forces, but this is speculative at this stage.

The 1967 results of LIESE and BAUCH with varying concentrations of water in acetone and in ethanol can be explained on the basis of varying adhesion of the torus to the pit border, depending on the concentration of water present at the

Coniferous Sapwood Permeability and Pit Aspiration 291

final water stage of pit aspiration. The exact amount of water required is uncer­tain, because the concentration at the time of pit aspiration can be quite different, than the initial concentration, due to different evaporation rates of the water and the organic solvent.

References BRAMHALL, G. : Longitudinal Permeability Within Douglas-fir (Pseudotsuga menziesii (Mirb.)

Franco) Growth Increments. MS Thesis, University of British Columbia (1967). BROUGHTON, G., and J. P. WANG: The Mechanical Properties of Paper. Part III. TAPPI

Vol. 38 (1955) No. 7, p. 412/415. COMSTOCK, G. L. : Longitudinal Permeability of Green Eastern Hemlock. For. Prod. J. Vol. 15

(1965) No. 10, p. 441/449. -: Longitudinal Permeability of Wood to Gases and Nonswelling Liquids. For. Prod. J.

Vol. 17 (1967) No. 10, p. 41/46. -: Physical and Structural Aspects of the Longitudinal Permeability of Wood. Ph. D. Thesis,

N.Y.S.U. College of Forestry, Syracuse (1968). CÔTÉ W. A, JR., Z. KORAN, and A. C. DAY: Replica Techniques for Electron Microscopy of

Wood and Paper. TAPPI Vol. 38 (1964) No. 8, p. 477/484. ERICKSON, H. D., and R. J. CRAWFORD: The Effects of Several Seasoning Methods on the

Permeability of Wood to Liquids. Amer. Wood Pres. Assoc. Proc. (1959) p. 210/220. -, and L. W. REES: Effect of Several Chemicals on the Swelling and the Crushing Strength

of Wood. J. Agr. Res. Vol. 60 (1940) No. 9 p. 593/603. GRIFFIN, G. J.: Bordered Pits in Douglas-fir: A Study of the Position of the Torus in Moun­

tain and Lowland Specimens in Relation to Creosote Penetration. J. Forestry Vol. 17 (1919) No. 7, pp. 813/822.

-: Further Note on the Position of the Tori in Bordered Pits in Relation to Penetration of Preservatives. J. Forestry Vol. 22 (1924) No. 6, p. 82/83.

HART, C. A., and R. J. THOMAS: Mechanism of Bordered Pit Aspiration as Caused by Capillarity. For. Prod. J. Vol. 17 (1967) No. 11, p. 61/68.

LIESE, W.: The Fine Structure of Bordered Pits in Softwoods. In CÔTÉ, W. A., JR. (Ed.): Cellular Ultrastructure of Woody Plants. Syracuse Univ. Press. (1965) p. 271/290.

-, and J. BAUCH: On the Closure of Bordered Pits in Conifers. Wood Science and Technology Vol. 1 (1967) No. 1, p. 1/13.

NICHOLAS, D. D., and R. J. THOMAS: Ultrastructure of Bordered Pit Membrane in Loblolly Pine. For. Prod. J. Vol. 18 (1968) No. 1, p. 57/59.

PHILLIPS, E. W. J.: Movement of Pit Membrane in Coniferous Woods with Special Reference to Preservative Treatment. Forestry Vol. 7 (1933) p. 109/120.

STAMM, A. J.: Wood and Cellulsoe Science. New York 1964: Ronald Press. THOMAS, R. J. : The Development and Ultrastructure of the Pits of Two Southern Yellow

Pine Species. DF Thesis, Duke University (1967). -: The Structure of the Pit Membranes in Longleaf Pine: An Electron Microscope Study.

Amer. Wood Pres. Assoc. Proc. (1967). -, and D. D. NICHOLAS: Pit Membrane Structure in Loblolly Pine as Influenced by Solvent

Exchange Drying. For. Prod. J. Vol. 16 (1966) No. 3, p. 53/56.

(Received May 10, 1968)

W. A. CÔTÉ JR., Professor of Wood Technology, New York State University College of Forestry at Syracuse, N. Y., USA.

GILBERT L. COMSTOCK, Associate Wood Scientist, U. S. Forest Service, Forest Products Laboratory, Madison, Wis., USA.

PURCHASED BY THE FOREST PRODUCTS LABORATORY, U. S. DEPARTMENT OF AGRICULTURE, FOR, OFFICIAL USE.