PROGRESSIVE EFFECTS OF POLYPORUS VERSICOLOR ON THE PHYSICAL AND CHEMICAL PROPERTIES OF RED GUM SAPWOOD
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TECHNICAL BULLETIN NO. 527 SEPTEMBER 1936
PROGRESSIVE EFFECTS OF
POLYPORUS VERSICOLOR ON THE
PHYSICAL AND CHEMICAL
PROPERTIES OF RED
GUM SAPWOOD
By
THEODORE C. SCHEFFER
Associate Pathologist
Division of Forest Pathology
Bureau of Plant Industry
^^^
UNITED STATES DEPARTMENT OF AGRICULTURE, WASHINGTON, D. C.
Forsaleby the Superintendent of Documents, Wathington. DiC. ------ p«/.« in ,TECHNICAL BULLETIN NO. 527 SEPTEMBER 1936
UNITED STATES DEPARTMENT OF AGRICULTURE
WASHINGTON, D. C.
PROGRESSIVE EFFECTS OF POLYPORUS VERSI-
COLOR ON THE PHYSICAL AND CHEMICAL
PROPERTIES OF RED GUM SAPWOOD^
By THEODORE C. SOHEIFFEB^
Associate pathologist, Division of Forest Pathology,^ Bureau of Plant Industry
CONTENTS
Page Page
Introduction 1 Effects of Polyporus versicolor upon physical
Methods 2 properties—Continued.
Determining effects upon structure and Moisture-absorbing and moisture-retain-
physical properties 5 ing properties 23
Determining effects upon chemical prop- Effects of Polyporus versicolor upon chemical
erties 6 properties 33
Effects of Polyporus versicolor upon physical Composition 33
properties 7 Calorific value 39
Microscopic structure 7 Effect of Polyporus versicolor upon relation of
Color 8 strength to chemical properties 40
Dimensions 9 Summary 42
Specific gravity 10 Literature cited 43
Strength 11
INTRODUCTION
That the character of wood decay is highly variable is at once
evident from the diversity in external appearance of rotted material.
For this reason, decays of wood have persistently been relegated to
types based on macroscopic physical differences. The terms brown
rot, white rot, spongy rot, pocket rot, and the like are familiar in
this connection. It was early realized that these visible differences
must be regarded as partial manifestations of even more varied and
extensive alterations in the physical and chemical composition of the
attacked wood, and, spurred by the economic and scientific impor-
tance attached, investigators since the time of Eobert Hartig have
endeavored to establish the nature of these fundamental changes.
The practical significance of studies on the effects of decay usually
is not immediately apparent. The very fact, however, that decay
will always be associated with wood utilization in its broader aspects
makes any contribution to the understanding of this most important
iThis paper was submitted to the University of Wisconsin in partial fulfillment of
the requirements for the degree of doctor of philosophy.
2 The author v^ishes to express appreciation of the generous cooperation throughout
the study and of the helpful suggestions received from C. A. Richards, of the Division
of Forest Pathology, and from various members of the Forest Products Laboratory,
especially L. F. Hawley, G. J. Ritter, and T. R. C. Wilson.
3 In cooperation with the Forest Products Laboratory, Madison, Wis.
12 TECHNICAL BULLETIN 52 7, U. S. DEPT. OF AGRICULTURE
form of wood deterioration of potential economic value. For sus-
tained advancement toward the goal of practical application, pre-
liminary investigations must necessarily confine themselves to funda-
mentals from which may be obtained comparative data regarding
specific fungi and wood species. Rapid advances in wood physics
and chemistry will demand revision of early conclusions; neverthe-
less, knowledge and technique in these natural sciences have already
reached a stage where their application to the study of wood decay
has proved to be definitely fruitful.
Standard strength tests have been spasmodically applied in decay
studies, beginning with the work of Von Schrenk {41)^ in 1899.
These have served primarily to show that alterations in strength may
be detected before other physical changes resulting from decay.
With the advent of proximate methods of chemical analysis, espe-
cially as devised by Schorger in this country for use on wood, chemi-
cal investigations of decayed wood have assumed considerable im-
portance. The relatively recent contributions of Hawley, Fleck, and
Eichards (16), Wiertelak (44)^ and Campbell (^, ^, 6) indicate that
the principal differences between different decay fungi as regards
their effects on the wood constituents may lie in the differential con-
sumption of lignin and cellulose, the order of attack upon the pento-
sans, and the extent to which the alkali solubility of the wood is
changed. Differences in alkali solubility furnish the most constant
and outstanding means of chemically distinguishing between rots of
the brown and white types.
As far as is known, correlation studies comparing physical and
chemical changes in wood undergoing decay have been touched upon
only by Cartwright and associated workers (7), who determined
changes in modulus of rupture and alkali solubility of the« same test
material. The progressive effects of decay have not received as much
attention as they possibly deserve, most of the studies being limited
to one or two stages of decay. Only analyses of progressive changes
are readily adaptable to comparison between one study and another,
for it is rare that two independent studies deal with stages of decay
close enough together to justify considering them, to be the same.
The present study was designed primarily to obtain further infor-
mation concerning the nature of decay of the white rot type, as
induced by Polyporm^ versicolor (L.) Fr., emphasizing the relation,
between changes in the strength, specific gravity, and chemical com-
position of the wood and the progressive effects of this decay from
the earliest stages capable of significant analysis to the more ad-
vanced stages where the wood no longer retains utility value. Sec-
ondary aims were to determine the effects of the decay upon the color,
dimensions, microscopic structure, moisture-absorbing, and moisture-
retaining properties, and the calorific value of the wood ; and to cor-
relate these effects wherever possible, both with each other and with
findings in regard to the strength and chemical changes.
METHODS
Since the phases of this study are more or less diverse, the specific
procedure for each is presented separately. The general preliminary
* Italic numbers in parentheses refer to Literature Cited, p. 43.EFFECTS OF POLYPORUS VERSICOLOR 3
treatment can be advantageously considered at this point. A single,
known species of wood-attacking fungus was employed. Polyporus
versicolor ^ was chosen because of its prevalence on hardwoods and
the typical white rot which it produces. Moreover, it penetrates the
wood rapidly and uniformly, and seems to be able to grow well over
a considerable range of temperature and moisture—all excellent
qualifications where strength determinations are to be made. On
account of its commercial importance and its high susceptibility to
attack^ by P. versicolor^ the sapwood of red gum {Liquidmribmr
styracifiua L.) was selected as the species of wood to be studied.
Carefully matched wood specimens were used throughout. The
manner of preparing and matching them is indicated in figure 1. A
freshly cut, even-growth log, averaging 26 rings per inch, was sawed
FIGURE l._Method of sawing and marking test pieces ; a and & designate two of the
radially positioned segments from which test units were sawed.
into^34. by %- by 10-inch straight-grained test sticks and %- by
%- by 3-mch moisture samples. These were labeled by letter accord-
ing to the segment from which they were taken and by numbers
referring to the section of the log to which they originally belonged
and to their relative positions longitudinally, radially, and tangen-
tially in the segment. Thus the designation of test piece lA 2-2
would signify that the piece had been taken from section no 1
segment A, longitudinal unit no. 2, and radial-tangential position
no. 2. Withm the same annual rings, then, this stick would be
matched longitudinally by the control lA 1^2 and tangentially by the
control lA 2-1, thus providing a double and very close check. Pieces
having their relative positions in the segment designated by two
even numbers were arbitrarily selected for inoculation, whereas those
having one even and one odd number for the segment allocation were
reserved for controls. Sticks having both position numbers odd
were to be inoculated with a brown rot fungus. It was found how-
ever that the brown rot organisms tried decayed the sticks so un-
evenly that subsequent physical and chemical analyses would have
been of little value.
Immediately following sawing, the sticks were weighed and meas-
ured and their ends coated with collodion, to prevent excessive end
decay. Ihe controls and those which could not be promptly inocu-
lated were wrapped in waxed paper and placed in refrigeration.
IngÄid'^Äi M^rÄ^^C Wis'" "'• ''"' ''""'^''^ '''^ ^ sporophore found grow-4 TECHOTCAL BULLETIN 5 2 7, U. S. DEPT. OF AGRICULTURE
Autoclaving was avoided. However, in view of the fact that the
interiors of the pieces were free from infection, a short immersion
of 3 to 5 seconds in boiling water was given as a partial surface
sterilization. To increase the effectiveness of such a mild treatment,
înoculation was made very heavy by uniformly distributing three
1-cm' portions of mycelial mat, cut from Petri-dish cultures, on each
side of the sticks. This procedure might not be suitable with slower
growing organisms than Poly form versicolor^ but in this case it
proved satisfactory. In all but a few instances the planted fungus
was able to cover the block within a week, and apparently completely
suppressed occasional mold growths.
Cultures were incubated in sterile, large-bore test tubes (pi. 1, ^),
capped with cotton and waxed paper, which were placed horizontally
in racks within a special humidity room maintained at the Forest
Products Laboratory. Temperature and relative humidity were held
approximately constant at 80° F. and 90 percent, respectively.
Under these conditions decay was rapidly promoted, and the loss
of moisture from the test pieces so retarded that it was unnecessary
to add water over the entire incubation period. ,
The horizontal alinement of the test sticks during incubation
should be emphasized as more than a casual point in handling. Pre-
vious decay studies at the laboratory have shown that, when m a
vertical position, sticks of 10 inches or more in length will have,
within a few weeks, sufficient moisture differences between the top
and lower ends to produce an uneven distribution of decay with
accompanying irregularities in the strength analyses.
It is noted from the foregoing methods that every precaution was
taken to avoid, insofar as possible, extraneous influences that might
affect the physical, chemical, or biological properties of the wood,
having especially in mind unnatural conditions incurred by prelimi-
nary heat sterilization, oven drying, and air seasoning; by reduction
to sawdust; and by promoting decay through placing the wood m
direct or indirect contact with an artificial nutrient medium.
Certain of these points of procedure are of disputed importance ;
nevertheless, they were incorporated because they seemed to be m
various degrees necessary to the correct determination of effects as
they might occur in nature. For example. Chapman {8) has recently
shown that the biological effect on wood of relatively mild heat
treatments ordinarily used for sterilization purposes may be pro-
nounced. It is conceivable that in such an event the normal order
of selection of wood components by the attacking fungus may be
appreciably altered, as may also be the resultant character of strength
reduction. Potter {SI) and Hawley, Fleck, and Kichards (16),
moreover, were able to show that normal sterilization procedures
may produce slight chemical changes which are readily detectable.
Working with temperatures as low as 220° F., Pillow {30) found
that ash wood showed a tendency toward brashness and a decrease
in toughness. Seasoning of wood prior to testing its strength ren-
ders it liable to minute checking; moreover, the influence of dif-
ferences in moisture content of wood reported as air-dry is not readily
taken into account. It is common knowledge to most wood patholo-
gists that sawdust is for some reason a different product biologically
than the original wood; unnatural air and moisture relations are
perhaps responsible for a large measure of the differences encoun-Teehnieal Bulletin 527. U. S. Department of Agriculture
'- ' i 1
Í':.- .
*
■•
U
Test sticks. A, Test-tube culture of Polyporus versicolor on one of the experimental sticks of red gum
sapwood. B-D, Air-dry sap gum test sticks. B and D were decayed for equal lengths of time by
Lenzitcs trabea and P. versicolor, respectively. C, Shows sound controls.EFFECTS OF POLYPORUS VERSICOLOR 5
tered, but work of Eobinson {S7) indicates that when wood fibers
are ruptured, as would be the case with sawdust, the physical-
chemical relationship of their components is markedly altered. This
is corroborated by findings of Dadswell« and others in the course
of analyses of sawdust of various degrees of fineness. Eegarding
the rotting of blocks over a nutrient medium, it would seem logical
to raise the question of the effect of this secondary source of food
upon the order and rate of decomposition of the wood constituents.
That there may be a differential selection of carbon from several
sources present was early shown by the experiments of Pfeffer {28).
DETERMINING EFFECTS UPON STRUCTURE AND PHYSICAL PROPERTIES
Microscopic structure and the following physical properties were
considered: Color; dimensions; specific gravity; strength in bend-
ing, compression parallel to grain, and hardness ; and moisture-ab-
sorbing and moisture-retaining characteristics. The dimensions of
the test sticks were determined by micrometer measurements; direct
comparisons were confined to the green volumes, since none of the
material was dried until the strength tests had been completed.
Specific gravity was always based on volumes when green and
weights when oven-dry. Each of the standard strength tests was
conducted in accordance with requirements of the American Society
for Testing Materials {1). For the static-bending tests an Olsen
machine reading to 1-pound loadings was used ; for the compression
and hardness tests Eiehle machines, reading to loadings of 10 pounds
and 1 pound, respectively, were used. Bending specimens were
tested on an 8-inch span with load at the center. They were so posi-
tioned that the load was applied to the tangential face. An Ames
dial, coupled with a special assembly for handling sticks below
standard test size, served to indicate deflections. Hardness was
determined by the ball-penetration method used at the Forest Prod-
ucts Laboratory, pressure being applied on the tangential face
nearest the bark.
The static-bending tests were run first, using the entire 10-inch
specimens. A 3-inch length was then taken from each 10-inch piece,
beginning 1 inch from an end, to serve as material for the compres-
sion-parallel-to-grain tests. This was permissible, since the strain
encountered in bending is concentrated at the center of the sticks
and hence would not affect the crushing resistance of the shorter
pieces, taken at least 1 inch from this point. The remaining por-
tion of the original 10-inch piece was used for the hardness test. A
closer comparison of the results of all three tests was thus provided
because virtually the same test material was used throughout.
^ The relative moisture-absorbing and moisture-retaining proper-
ties of the sound and decayed wood were analyzed under four dif-
ferent conditions: (1) With the pieces at equilibrium moisture con-
tent in atmospheres of various relative humidities; (2) with the
pieces under changing conditions of relative humidity; (3) with the
pieces submerged in water ; and (4) with the pieces drying from the
water-soaked condition. Desired relative humidities were attained
by the use of saturated solutions of the proper salts within airtight
xy^èi?^Z^^^^\ COUNCIL FOR SCIENTIFIC AND INDUSTRIAL RESEARCH, DIVISION OF FOREST
PRODUCTS, ANNUAL REPORT FOR 1930-31. 17 pp. Melbourne. 1931. [Mimeographed ]
See section of wood chemistry, pp. 12-14. L^T^inicugiaputiu.j6 TECHNICAL BULLETIN 5 2 7, U. S. DEPT, OF AGRICULTURE
chambers, or by special humidity rooms maintained at the labora-
tory. To hasten absorption toward the end of the submersion test,
a period of reduced pressure was provided, followed by atmospheric
pressure again. A filter pump was used to accomplish this.
DETERMINING EFFECTS UPON CHEMICAL PROPERTIES
Proximate chemical analyses of some of the wood constituents
most frequently investigated, together with incidental determinations
of changes in the acidity and calorific value of the wood, over
several stages of the decay were made.
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MODULUS or RUPTURE EXPRESSED AS A PERCENTAGE OF THE CONTROL I^ALUE
FIGURE 2.—Diagram which served as a basis for the selection of test specimens to be
used in the chemical analyses. Allocation of specimens entering into decay samples
A, B, C, D, and J is designated.
The chemical analyses were made on material furnished by speci-
mens used in the strength tests, thus providing for an exact compar-
ison between the chemical and physical changes. In order that the
changes could be based on wood most truly representative of aver-
age conditions in respect to the accompanying physical ^ changes,
only those test pieces that were approximately representative of all
three of the properties—modulus of rupture, maximum crushing
strength, and specific gravity—were selected. Since the series of
analyses required more wood than could be obtained from one piece,
several specimens, taken at approximately the same stage of decay,
were used to make up each sample. To facilitate this selection, a
graphic summary of the specific gravity and strength values was
resorted to, as explained by figure 2.
The test pieces thus chosen for their different degrees of sound-
ness were reduced to sawdust and the individual lots mixed to formEFFECTS OF POLYPORUS VERSICOLOR
the composite samples representing wood with physical properties
averaging as shown in table 1.
TABLE 1.—Average physical properties of composite samples
Values expressed as percentage Values expressed as percentage
of original sound wood value of original sound wood value
Sample Maxi- Sample
Specific Modulus mum Specific Modulus Maxi-
mum
gravity of rup- crushing gravity of rup- crushing
ture strength ture strength
X (control) 100 100 100 0 90 77 81
A 95 88 90 D 86 74 77
B.__ 94 82 86 J,. 68 43 48
Only that portion of each sample which passed through a screen
containing 30 meshes to the inch was retained for analysis. No
grinding was necessary, since the amount of material not passing
this screen was always small. A rough screen analysis of heavily
decayed, slightly decayed, and sound wood showed the samples to
contain approximately the same proportions of particle sizes. These
precautions were important, since Dadswell ^ has shown that the use
of selected screen sizes may amount to a retention of unnatural pro^
portions of the wood constituents; also, Mahood and Cable (23)
found that particle size would have a considerable effect on the analy-
ses, especially in those determinations where surface contact comes
into play.
Following screening, the sawdust samples were allowed to dry in
the laboratory to a constant, uniform moisture content and then were
placed in airtight containers. Throughout the analyses, moisture
determinations were made on duplicate samples from each of the
containers, thus avoiding the need for oven drying the actual samples
to be tested. Whether or not this precaution was justified is disputed ;
nevertheless, it has been shown by O'Dwyer (^^), Campbell and
Booth (^), and others that oven drying may result in perceptible
chemical changes within wood.
The technique employed in the proximate analyses was that
adopted at the laboratory. Cellulose determinations were made by
the Cross and Bevan method as described by Schorger (S9) and later
revised by Eitter {S2) ; lignin was determined in accordance with
Eitter, Seborg, and Mitchell's (SS) modification of the sulphuric acid
method of Ost and Wilkening (27) ; pentosans were determined by
the furfural-phloroglucinol method of Schorger (40) ; and stable
cellulose was computed from the hydrolysis number, determined
according to the procedure described by Hawley and Fleck (Í5).
Hydrogen-ion concentration and total acidity were established on
the filtrates containing the hot- and cold-water extractives, the for-
mer being determined colorimetrically.
The calorific value of the samples was determined by means of a
Burgess-Parr oxygen-bomb calorimeter of the adiabatic type.
EFFECTS OF POLYPORUS VERSICOLOR UPON PHYSICAL PROPERTIES
MICROSCOPIC STRUCTURE
The vessels were the first wood elements occupied by the fungus
and served as the principal channels for its longitudinal spread.8 TECHNICAL BULLETIIT 52 7, U. S. DEPT. OF AGRICULTURE
After decay was well established these were found to be literally
choked with mycelium (pi. 2, J.), the hyphae passing from them
into adjacent wood fibers. Eadial and tangential spread seemed to
be accomplished primarily by passage of the hyphae through the
pits; bore holes were rarely observed. Although both Bayliss (2)
and Hubert (17) reported that Polystictus versicolor typically pro-
duces bore holes, their observations seem to have been made on more
severely decayed material than that dealt with here. As previously
noted, material for this study was not decayed beyond a point where
it would be unsuitable for quantitative strength tests.
With the exception of a negligible number of bore holes no attack
on the middle lamella was apparent except that on the pit mem-
branes. From microscopic observations no tendency for the bond
between individual cells to become weakened could be detected.
Two striking effects upon the microscopic structure were thinning
of the cell walls (pi. 2, B and C) and enlargements of the pit aper-
tures. The walls of the fibers were uniformly reduced in thickness,
changing the normally^ angular shape of the lumens to one roughly
circular. The lenticutar apertures of the fiber pits were further
opened to a broad oval shape while the simple pits of the wood rays
were merely enlarged (pi. 2, -á).
Bayliss {2) seems to have been the only one describing in detail
the effects of this fungus on wood structure. Since her work dealt
with more advanced decay, however, it cannot be held strictly com-
parable with the present investigation. It is corroborative in respect
to the rapid thinning of the cell walls and the early attack on the
vessels, thus making it possible to conclude that the larger portion
of the losses in wood substance resulting from such decay can be
accounted for by a progressive consumption of the cell walls, from
the lumen outward, rather than by a consumption of the walls by
means of numerous bore holes. Polyporm versicolor has been held
to utilize the lignin preferentially and in this event might be ex-
pected to remove the middle lamella lignin to an extent that would
allow the fibers to become easily separated. The apparently normal
attachments between the decayed wood fibers, together with the
absence of a general attack on the middle lamella, indicate that this
is not the case. This çorint is further corroborated by the result
of chemical-analyses, which will be considered later.
An important feature in the attack of this organism is its apparent
marked ability to produce enzymes capable of affecting the wood sub-
stance not in direct contact with the hyphae. This was most evi-
dent from the considerable enlargement of the pits through which
hyphae passed and from the even, regular manner in which the cell
walls were "dissolved" away.
COLOR
The effect of Polyporus versicolor upon the color of the wood was
pronounced. As soon as the surface mycelium had completely cov-
ered an area, the underlying pinkish-brown^ sapwood was always
bleached to a decided white—hence, the term "white rot." Test
sticks, after 10 days of incubation, were completely penetrated by
the fungus, and the loss of color had kept pace with the spread of
the hyphae.riiotomicruuraphs of sound wood and ot wood decaved by Polyporus versicolor. A, Cioss-section stowing compacting of hyphae within tt« vessels, inlarged pits may b« n«liri
in the ray cells. B, Cross-section of decayed wo'od, sliowing a marlicd reduction in cell wall thickness and attendant enlargement of the lumens. C, Lross-seelion GC soanel
gum sapwood showing normal cell-wall tiiickness.EFFECTS OF POLYPORUS VERSICOLOR 9
The bleaching effect, for a short time, became more intense as
decay progressed; and, peculiarly enough, this was more in evidence
atter the wood had been reduced to sawdust form. With the aid of
a hand lens it was then possible to distinguish flecks of color in mate-
rial least decayed, indicating that the remnants of color, although
uniformly distributed, were concentrated in small particles of wood
rather than homogeneously intermixed. After approximately 10
percent of the specific gravity had been lost, it was difficult to detect
further color changes. Plate 1, B to Z>, gives an idea of the differ-
^ces between the color of sound gum sapwood and wood decayed by
Foli/porus versicolor and by a brown rot fungus, Len^ites trabea
(Pers.) Fr.,^ respectively.
The exact manner in which the color was removed was not sought
J^^i 1 ^ ' however, that the pigment was partially soluble in hot
and cold water, 1-percent sodium hydroxide solution, and alcohol;
and insoluble m benzene or xylenol. The greatest solubility occurred
m solutions of sodium hydroxide, although it was impossible to
extract the color completely. By comparing filtrates containing
extracts of sound and decayed wood, it was found that the amount of
colored substance taken from the decayed wood was comparatively
small, as would be expected. Increasing the acidity of filtrates con-
taining this pigment, or boiling of the wood in acid solutions, caused
no change to take place in the natural color.
Microscopical examination of numerous cross sections of sound
wood disclosed that the coloration of the sapwood is largely asso-
ciated with materials contained in the lumens of the ray cells with
a smaller amount uniformly present in the cell walls in general.
Concurrent with the early disappearance of color it was found that
the substances contained within the wood rays were also removed
leaving the ray cells free of other than colorless inclusions (compare
pi. 2, B and O). v r
From this evidence it may be concluded (1) that the elimination
o± the natural color followed closely the proximity of the fungus
hyphae and was not due to the removal of lignin, as has often been
surmised; (2) that destruction of the color, although not confined
to the region of hyphal contact, did not extend far enough in ad-
vance of this to remove completely the color until a certain con-
centration of the hyphae had been reached; and (3) that, since the
decay increases the hydrogen-ion concentration of the wood (as will
be brought out later), the bleaching was not due to an indicator
phenomenon, resulting from the increased acidity produced by decav,
but was m all probability attributable to a hydrolysis or oxidation
preliminary to or accompanying the actual utilization of the chro-
mogenic compound or compounds concerned.
DIMENSIONS
No changes in the dimensions of the wet test sticks could be de-
tected, even after 70 days of incubation, at which time rou^hlv 30
percent of the original dry weight of the wood had commonly
been lost. Upon drying there was no uneven shrinkage, visible
checking, or collapse of surface cells—all so typical of the ordinary
diveisi types^^oÄ^"" *''''^''' ^^^ '^""^^^ incidental, to allow contrasting of two such
66996°—36 210 TECHNICAL BULLETIN 52 7, U. S. DEPT. OF AGRICULTURE
brown rots; nor was the degree of shrinkage of the decayed speci-
mens différent from that of the controls. This may be seen by ±ur-
ther reference to plate 1, B and Z>, which brings out m a striking
manner these diflFerences between the two types of decay.
That Polyporus versicolor did not alter appreciably the dimensions
or the normal shrinkage qualities of the wood is probably due to its
extremely uniform penetration, and the manner of its attack where-
by the cell walls were thinned proportionately, leaving the wood
with a cross-sectional appearance not unlike that of a sound, less
dense species. General observations indicate that this may be tairly
typical of others of the hardwood rots of the "white ' type. Col-
lapsing of cells while still wet, irregular shrinkage, and transverse
breaking of the fibers is typical of brown rots such as those caused
by Lemites trahea. The extreme contrast in appearance between
the two kinds of rot must be accounted for by the outstanding
differences in the manner in which the wood is altered physically,
as well as the difference in chemical effects.
SPECIFIC GRAVITY
Seduction of the specific gravity of the test specimens was per-
ceptible in all cases after 13 days of incubation (the shortest period
considered). Graphic analyses of specific-gravity changes m this
material based on length of incubation indicate that it would ordi-
narily be impracticable to attempt to detect specific-gravity losses
earlier than 8 or 10 days subsequent to inoculation (fig. 3). Atter
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:EFFECTS OF POLYPORUS VERSICOLOR H
the incidence of weight reduction, however, the losses were pro-
nounced, m proportion to the length of incubation, and amounted
to approximately 30 percent after about 70 days. The effect of
decay on this property is taken up in more detail in connection with
considerations of strength changes.
STRENGTH
The results of the strength tests are presented graphically in
iigures 4 to 12, wherein, with the exception of figure 10, the strength
values of each stick tested are presented as percentages of the con-
trol values. Specific gravity, which serves as the basis for com-
parison, is presented in the same manner. In this way relative devi-
ations of the respective properties from their sound-wood values are
immediately evident and offer a means of ready comparison.
Referring to figure 4, it may be seen that modulus of rupture
was rapidly reduced from the first. The trend in the lowering of
this strength value as decay progressed was definite, the deviations
±rom the averages appearing to be no greater than would be encoun-
tered with sound wood over the same range of specific gravities.
After the initial rapid drop in this particular strength property, the
rate of reduction appears to have been quite uniform. At all times
the drop m strength was proportionately greater than the accom-
panying drop m specific gravity. Thus approximately 10 percent
ot the original modulus of rupture was lost while the concurrent
drop m specific gravity amounted to but 2 percent; and beyond this
point, as far as recorded, the modulus of rupture was decreased at
rates predominately between 35 and 50 percent greater than those for
specific gravity.
Fiber stress at the proportional limit was likewise markedly low-
ered by decay but with relatively large differences among the test
pieces m this respect. Thus, as seen from figure 5, the early trends
m strength reduction, with reference to rate of specific-gravity loss
were obscure and hence differed considerably in this respect from'
the early changes in modulus of rupture. This scattering of strength
values and comparative indefiniteness of trends, especially in the
early periods of decay, appear to be characteristics restricted to
those strength tests that depend upon a determination of the pro-
portional limit or the stiffness of the material. However, the gen-
eral trend over the entire period was uniform to the extent that
losses m specific gravity definitely were accompanied by relatively
greater losses in fiber stress.
Modulus of elasticity (fig. 6), although the broad trend was defi-
nite, like fiber stress at the proportional limit, showed considerable
variation m strength losses among the individual test pieces, espe-
cially m the earlier stages of decay. Peculiarly enough, some of
the test specimens showed higher modulus-of-elasticity values, after
a certain amount of decay had taken place, than did the controls.
1 hese instances may have been in large measure due to the limitations
oí the test, but they point suspiciously to the possibility that such
occurrences may be frequent wherein the modulus of elasticity is
not appreciably reduced after a moderate amount of decay. On
the whole, reduction of modulus of elasticity took place at a greater
rate than reduction of specific gravity.12 TECHNICAL BULLETm 5 2 7, U. S. DEPT. OF AGRICULTURE
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5P£C/nc GPAV/TY £J14 TECHNICAL BULLETIN 527, U. S. DEPT. OF AGRICULTtJEE
M 60 es 10 15 00 OS 9» SS
SPECIFIC GMViry EXPRESSED AS A PERCENTAGE OF THE
CONTROL VALUE
FiGü«E 6.-Relatlon between modulus of elasticity and specific gravity of decayed test
pieces.EFFECTS OF POLYPORUS VERSICOLOR 15
The amount of work or energy absorbed by the specimen up to
the proportional limit was more variable than any of the other
strength tests dealt with in this study (fig. 7). The general in-
fluence of the^ decay in reducing work to this point was marked,
but for any given specific gravity the associated work value often
differed by as much as 20 to 30 percent. Here, as with modulus
of elasticity, the cases were frequent in which well-established decay
apparently had^ but little effect. Here again the general trend of
reduction of this strength evaluation was substantially more rapid
than that for specific gravity.
The effect of decay in reducing the work required to subject the
pieces to a maximum load was well defined. The initial changes
were extremely rapid. Eef erence to figure 8 will show that roughly
only 70 percent of the work originally required to affect this loading
remained after the specific gravity had been reduced between 1 and
2 percent j in other words, 30 percent of this work requirement had
been eliminated by the time it had become possible to measure readily
specific-gravity losses. These extreme changes stopped more or less
abruptly, however, and for specific gravities below 97 percent the
effect of decay was somewhat moderated and approximately uniform.
The total work involved in carrying the loading to the maximum
and then on to a point at which the load had become reduced to
one-half the maximum was not affected by decay a great deal dif-
ferently than the work to maximum load. The difference consists
only of the work required to carry the loads on the sound sticks
past the maximum and on to half maximum. The initial, extremely
rapid reduction was continued somewhat longer and the changes
toward the later stages of decay may not have been so rapid as with
work to maximum load (fig. 9).
Figure 10 is presented as supplementary to the graphic study just
dealt with. The work values are represented by the areas under the
curves. The curves shown in this figure set out more clearly the
general differences in the ability of the sound and decayed test sticks
to withstand slow center loading. The principal differences will be
seen to lie in the maximum loading and the amounts of work ex-
pended.^ Invariably the decayed sticks, no matter how strong other-
wise, failed m tension with a brash break, resulting in complete loss
of ability to withstand further loading. On the other hand, the
sound wood failed under splintering tension, requiring various
amounts of work to carry the load on to one-half the maximum
value. Another feature brought out by the curves in figure 10 is
that although the proportional limit may be considerably lowered
as a result of decay, the deflections under loadings below this point
are not much greater than those of sound wood under the same load-
ings. This in considerable measure explains the relatively large
diversity and tardiness among individual test specimens with respect
to reductions in modulus of elasticity and work to the proportional
limit in the early stages of decay.
The resistance of the pieces to crushing under loads applied paral-
lel to the gram compares closely with the resistance to breaking
under center loading perpendicular to the grain (modulus of
rupture). This is clearly brought out by figure 11. which closely
resembles figure 4.16 TECHNICAL BULLETIN 5 2 7, U. S. DEPT. OE AGRICULTURE
100
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APPROXIMATE STRENGTH-SPECIFIC GRAVITY
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ôPCC/r/C GRAVITY EXPRC5SED A5 A PERCENTAGE OE THE
CONTROL VALUE
FIGURE 7.—^Relation between work to proportional limit and specific gravity of decayed
test pieces.EFFECTS OF POLYPOKUS VERSICOLOR 17
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SPECiriC GRAVITY EXPRESSED AS A PERCENTAGE OF THE
CONTROL VALUE
FiGUEE 8.—Relation between work to maximum load and specific gravity of decayed
test pieces.
66996°—36 318 TECHNICAL BULLETIN 5 2 7, V. S. DEPT. OF AGRICULTURE
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oEFFECTS OF POLYPORUS VERSICOLOB 19
Outstanding in the results of the hardness tests was the closeness
to which reductions in this strength property followed reductions
of some of the others, especially those indicated by the maximum
crushing strength and the modulus of rupture. In spite of the
fewer determinations made, the trend (fig. 12) in the effect of the
decay on the hardness of the pieces was very well defined. This test
gave values for individual specimens that deviated but little from
the general trend.
Specific gravity was
chosen as being better
suited than time of
incubation as the rela-
tive basis for compar-
ing alterations of the
strength and chemical
properties of the wood.
This study is concerned
only in a general way
with the effects of de-
cay as they depend ^
upon time. Obviously "^
such a relationship "^
would vary according
to cultural conditions.
Of particular interest
are the changes in
strength and chemical
composition in relation
to concurrent losses in
wood substance —rela-
tions which, under ordi-
nary circumstances, can
probably be regarded as O.Z 0.3 0.5
essentially constant. DEFLECTION QNCHES)
Nevertheless, it is al- FIGURE lO.—Typical stress-strain diagrams for a decayed
ways of interest to Ív!*-f^^^^ Javmg 82.4 percent of its original specific
1 1 ^""^^ ^'^^ ^^ gravity, and for the sound, matched control.
know what periods have
been involved in the production of different degrees of decav ; for
this purpose figure 3 is included.
Figure 3 further emphasizes the more rapid reduction in strength
values over those of specific gravity. In this respect it is interesting
to note that with few exceptions, at any given period of incuba-
tion, the smallest percentage losses in strength are in the neio-h-
borhood of the greatest losses in specific gravity. Assuming that
important decay was initiated by the fifth day after inoculation it
IS evident that the most rapid losses in modulus oí rupture occurred
very early. This might be expected since the initial attack pri-
marily affected the surface layers and hence weakened the v/ood
elements contributing most to bending strength. After the thir-
teenth or fifteenth day (beginning of strength analyses) the gen-
eral trend of modulus-of-rupture losses was fairly regular.
Since specific-gravity values essentially bear a straight-line rela-
tion to time, a general idea of the other time relations can be derived20 TECHNICAL BULLETIN 5 2 7, U. S. DEPT. OF AGRICULTURE
directly from the specific gravity-strength curves just considered.
For instance, comparison of figures 3 and 4 will show that the
100
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oEFFECTS OF POLYPORUS VERSICOLOR 21
than time as a basis for comparing the effect of decay on various
strength properties.
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ôPEcinc cRAv/ry EXPRESSED AS A PERCENTAGE or THE
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FIGURE 12.—Relation between hardness (tangential face) and specific gravity of decayed
test pieces.
It was particularly hoped that the strength analyses would bring
out whether relations between the strength and specific gravity of
decayed wood differed from those existing for sound wood of the
same species. It has been the general experience of timber physicists22 TECHNICAL BULLETIN 5 2 7, tJ. S. DEPT. OF AGRICULTURE
that within the same species the strength of wood containing decay is
less than that of sound wood of the same specific gravity, although
there seems to be no record of special attempts to substantiate this.
Kauffman and Kerber {18), Longyear (21), and Colley {9) were
able to show that considerable loss in strength may occur before any
appreciable reduction in specific gravity takes place ; and the present
data, especially those of modulus of rupture, end compression, and
certain of the work values, agree with theirs closely.
The dotted curves shown in figures 4 to 9, 11, and 12 are derived
from a consideration of the work of Newlin and Wilson {25) and
of strength-specific gravity relations that have been studied for a
few individual species and groups of closely related species. They
represent approximately what would be expected for average
strength-gravity relations if data from sound gum sap wood speci-
mens with a considerable range in specific gravity were available.
Comparing these hypothetical curves to the series of points repre-
senting tests of decayed {Polyporus versicolor) wood, it is indicated
that in all properties considered this wood is lower than would be
expected of sound wood of the same specific gravity. The deficiency
is apparent throughout the full range with a tendency to be more
marked the lower the specific gravity, that is, the more advanced the
decay. The deficiency is also greater in shock resistance, as measured
by the work values plotted in figures 8 and 9, than in other properties.
It is evident from the trend of the plotted points in figures 4 to 9,
11, and 12 that strength values approach zero well in advance of the
point of complete wood consumption as measured by loss in specific
gravity.
From the graphic analysis of strength changes, it is easily possible
to evaluate the various strength properties as to their suitability
as indicators of incipient decay, or of the extent of decay, caused by
Polyporus versioolor. Thus fiber stress at the proportional limit
and modulus of rupture are measures of the stress in the outermost
fibers of the test piece at two different and well-defined loadings,
but it iö obvious that the weakening of these fibers is more quickly
apparent under maximum loading than under such loading as will
still allow complete recovery from the strain. Therefore, of the two
tests, that of modulus of rupture would be much more suitable for
judging the progress of decay. This conclusion is strengthened by
the lesser deviation from the average of modulus-of-rupture values.
For the same reasons maximum crushing-strength and hardness
determinations appear to be about on a par with that of modulus
of rupture for this purpose. The use of the hardness test would have
to be accompanied with more caution, however, since variability in
the degree of decay over or throughout the wood would present
greater possibilities for error than with the other tests because of
the more limited amount of wood involved in the test. No doubt
the close correlation between the hardness and other tests in the
present series was due to the very uniform decay which the test
pieces had undergone.
The work analyses with the exception of that to the proportional
limit present themselves as being the most valuable indicators of
the presence of decay. The considerable variability shown m the
intermediate stages of decay reduces their value as indicators of theEFFECTS OF PÔLYPÔRUS VERSICOLOR 23
extent of decay, however. As already discussed, work requirements
were reduced rapidly anÄ far before appreciable reductions in specific
gravity occurred. Work indicates the toughness of the wood, or it^
ability to absorb shock, and where toughness alone is to be evaluated
it is preferable to use the more rapid impact-bending test. The
brash tension failures were secondary indications of lack of tough-
ness, showing in even the least decayed pieces, and could be relied
upon to indicate qualitatively the presence of decay. The least
desirable tests for decay seem to be that indicating modulus of
elasticity and that of work to proportional limit, both because of
the great variability encountered and because of the tardiness shown
in responding to decay.
It is possible that modulus-of-elasticity tests might be useful
where it is desired to follow the advance of decay in the same test
pieces. More explicitly, the modulus of elasticity of a piece could
be determined while the wood was sound; then, subsequent decay
progress might be followed by repeating the test on the same piece.
In this way the natural variability of this determination would be
eliminated by using the initial test on the sound wood as the control.
It would, of course, be mandatory that the loads always be kept
safely below the proportional limit.
Unfortunately, suitable brown rot could not be produced under
the methods used for this study. The chief obstacle in the way of
testing brown rots was the inability to obtain uniform decay through
the test sticks. It seems to be typical of many brown rot fungi to
attack localized areas vigorously rather than to penetrate the wood
uniformly as did Polyporm versicolor. Quantitative strength tests
would be of little value under such circumstances.
MOISTURE-ABSORBING AND MOISTURE-RETAINING PROPERTIES
In determining the effect of PolyporuS' versioolor upon the mois-
ture-absorbing and moisture-retaining qualities of the wood, it was
necessary, as already stated, to subject the wood to water in both the
liquid and vapor forms. The results of these tests are presented in
tables 2 to 6.
After the test specimens had been allowed to air-dry and then
absorb moisture to equilibrium under atmospheres of different rela-
tive humidities, it was shown that the equilibrium moisture-content
values of the decayed material were on the whole slightly less than
those of the sound controls (table 2). The actual differences in
moisture content, however, did not appear to be in any particular
relationship to the extent to which the sticks had decayed or to the
relative humidity involved, although under a more refined pro-
cedure it might be expected that certain variations in this respect
would be apparent.
Whether or not it is a general characteristic of all types of decay
to lower the equilibrium moisture-holding capacity of wood, as found
m these tests, is impossible to say with the present paucity of infor-
mation on the subject. Komarov {19) found that with the exception
of 1 species of wood out of 16 studied, the hygroscopicity of sawdust
of decayed woods was 1 to 1.5 percent lower than that of sound wood.
The data of Eose and Lisse {38)^ concerning brown rots of Douglas
fir, indicate that the decayed wood which they observed may have24 TECHNICAL BULLETIN 5 2 7, U. S. DEPT. OF AGRICULTUEE
been slightly less hygroscopic than the sound wood. Further than
these instances nothing was encountered whieh would aid in answer-
ing this question.
TABLE 2.—Differences in equilihrium in moisture content of decayed and sound
control test sticks in atmospheres of different relative Tmmidities ^
Specific Difference in moisture contained Specific Difference in moisture contained
gravity of by decayed test sticks and by gravity of by decayed test sticks and by
decayed the matched controls at— decayed the matched controls at—
sticks ex- sticks ex-
pressed as pressed as
a percent- percent 60 percent 88 percent a percent- 56 percent 60 percent 88 percent
age of the 56relative relative relative age of the relative relative relative
control humidity humidity humidity control humidity humidity humidity
value value
Percent Percent Percent Percent Percent Percent
63 +0.63 +0.30 79 +0.67 +1.35
64 +1.63 +.66 80 +0.50
65_.-._._. +1.09 82 +.41
66 +1.37 +.59 83 3+.07 3+.32
68 -.02 +.93 -.39 86 2 +.10 +.69
69 2+.33 2+.38 87 +.26
70 2+.97 2+.56 88 +.08
71 +.40 89 +.41 +.06
72 +.68 90 -.19
73 2+.40 2+.40 91 +1.24
74 +.85 92 +.50 .00
75.- +.92 +1.37 94 +.08
76 +.41 +.69 +1. 59 95 +.69
77 +.42 +.10 +1.01 97 +.03
78 +.74 99 +.04
1 Data to be interpreted as qualitative only, since humidities fluctuated and weighing bottles were not
used. Plus (+) sign indicates difference in favor of control; minus (—) sign indicates difference in favor
of decayed stick.
2 Average for 2 specimens.
3 Average for 3 specimens.
The reason for the differences in equilibrium moisture contents
found in the present study is not clear. It is known that the
fiber-saturation point of certain wood species, or specimens within
the same species, may differ considerably. Clearly this must be due
to differences in either the chemical composition of the wood or the
physical condition of the wood constituents. It will be shown sub-
sequently that the relative proportions of the various principal
chemical components of the wood were but little altered by decay,
thus making it difficult to explain the changes in hygroscopicity
on a basis of differences in chemical composition as analyzed. In
view of the fact that differences in the tests herein described were
apparent after very little change in specific gravity had occurred,
it would seem that alteration of the hygroscopic properties of orig-
inally existing constituents had been responsible for at least part
of the reduction in moisture-holding capacity. Or it is faintly pos-
sible that a contributing factor may have been the early, rapid re-
moval of the cold-water-soluble components of the wood, as described
later ; this would be true, of course, only in the event that the hygro-
scopic properties of these components were appreciably higher than
those of lignin and cellulose.
From table 3 it may be seen that the differences in the rates of
water absorption or loss by the decayed test pieces as they were
transferred from atmospheres of one relative humidity to another
were variable according to whether equal masses or equal volumes
of the test stick were used as bases of comparison. Thus, considering
equal masses of wood, the rate of absorption and loss of water wasEFFECTS OF POLYPORUS VERSICOLOR 25
nearly always greater for the decayed sticks, whereas, considering
equal volumes of wood, the reverse was true. Unquestionably, dif-
ferences in surface area were responsible for most of the former
differential exchange of water, and differences in the total absorp-
tive capacity of the wood were responsible in the latter instance.
Considering equal masses of sound and decayed wood, under the
same changes in exterior conditions, the effect of the extent of decay
on the absorption and loss of atmospheric moisture did not seem
to be consistent. Differences in moisture losses from equal masses
of sound and decayed wood definitely increased with decrease in
specific gravity. Considering moisture changes in equal volumes
of the specimens, differences in both absorption and evaporation in
favor of the controls increased definitely and consistently with
increased decay.
TABLE 3.—Moisture Changes in sound and decayed test sticks at constant tem-
perature, due to changes in surrounding relative humidity^
Change per 100 g oven- Change per green vol-
dry wood 2 ume occupied by 100
g oven-dry control 3
Specific
gravity ex- Gain 5 days Loss 1 day Gain 5 days Loss 1 day
pressed as a after trans- after trans- after trans- after trans-
Items compared fer from an fer from an fer from an fer from an
percentage
of the con- atmosphere atmosphere atmosphere atmosphere
of 56 per- of 90 per- of 56 per- of 90 per-
trol value
cent to 90 cent to 60 cent to 90 cent to 60
percent percent percent percent
relative relative relative relative
humidity humidity humidity humidity
Grams Grams Grams Grams
Matched control 100 13.40 6.90 13.40 6.90
Decayed stick 95 13.53 7.25 12.85 6.89
Difference ._ _ _ Ö -.13 -.35 -f.55 +.01
Matched control 100 13.47 6.76 13.47 6.76
Decayed stick 94 13.59 7.14 12.77 6.71
Difference. 6 -.12 -.38 +.70 +.05
Matched control 100 13.07 6.81 13.07
Decayed stick 6.81
92 13.35 7.04 12.28 6.48
Difference 8 -.28 -.23 +.79 +.33
Matched control 100 12.35 6.82 12.35 6.82
Decayed stick 91 13.17 7.00 11.98 6.37
Difference 9 -.82 -.18 +.37 +.^^5
11 '*, , ,
Matched control 100 13.33 7.02 13.33 7.02
Decayed stick 82 13.37 7.63 10.96 6.26
Difference 18 -.04 -.61 +2.37 +.76
Matched control 100 13.20 6.81 13.20
Decayed stick 6.81
80 14.04 7.71 11.23 6.17
Difference . 20 -.84 -.90 +1.97 +.64
Matched control 100 13.71 6.97 13.71
Decayed stick. 6.97
77 14.50 7.50 11.16 5.78
Difference 23 -.79 -.53 +2.55 +1.19
At the start of each experiment the moisture contents of the sticks were in equilibrium with relative
humidities existing prior to transfer (that is, 56 and 90 percent); subsequent moisture contents, upon which
are based the values shown in the gain and loss columns, were substantially below the equilibrium points
^^¡\ff^^' changing at the time the sticks were removed for drying and weighing
Values correspond to differences in percentage moisture content, based on equal weights of wood or on
equal volumes of wood, as indicated. The present method of presentation is used in order that actual
moisture changes m equal weights of wood may be compared with corresponding changes in equal
volumes of wood; this comparison is made through the control-stick values, which are the same on either26 TECHNICAL BULLETIN 52 7, U. â. DËPT. OF AGRICULTURE
TaUe 3. Moisture changes in sound and decayed test sticks at constOMt tem-
perature, due to changes in surrounding relat-ive humidity—Cont.
Change per green vol-
Change per 100 g oven- ume occupied by 100
dry wood g oven-dry control
Specific Gain 5 days Loss 1 day Gain 5 days Loss 1 day
gravity ex- after trans- after trans- after trans- after trans
pressed as a fer from an fer from an fer from an fer from an
Items compared percentage atmosphere atmosphere atmosphere atmosphere
of the con- of 56 per- of 90 per- of 56 per- of 90 per-
trol value cent to 90 cent to 60 cent to 90 cent to 60
percent percent percent percent
relative relative relative relative
humidity humidity humidity humidity
Grams Grams Grams Grams
100 13.78 7.17 13.78 7.17
76 14.23 7.95 10.81 6.04
Decayed stick
24 -.45 -.78 +2.97 +L13
Difference
100 13.40 6.49 13.40 6.49
IVTatohpd control -
74 14.16 7.26 10.48 5.37
Decayed stick
26 -.76 -.77 +2.92 +1.12
Difference
100 13.69 7.55 13.69 7.55
72 14.38 8.49 10.35 6.11
Decayed stick
28 -.69 -.94 +3.34 +1.44
Difference
100 13.42 7.10 13.42 7.10
Decayed stick 68 13.65 8.05 9.28 5.47
Difference - 32 -.23 -.95 +4.14 +1.63
100 13.52 6.84 13.52 6.84
Decayed stick 65 13.80 7.86 8.97 5.11
Difference -- - 35 -.28 -1.02 +4.55 +1.73
That a certain amount of water loss can take place before the dif-
ferences in total absorptive capacity become noticeably effective was
shown by a supplementary study in which the water losses were meas-
ured only over the first hour. The results are summarized in table
4. Here, with equal volumes of sound and decayed sticks, the loss
of water from the decayed specimens was definitely greater. With
prolonged exposure to a condition favoring the loss of water, the
results would no doubt have rapidly approached those covered by
table 3, wherein the greater original quantity of water in the sound
blocks made it possible for these to surpass the decayed blocks in the
amount of water given off during a day.
It would seem from the data just presented that the decayed wood
was fundamentally more susceptible to moisture changes than the
sound wood, but, as already suggested, where equal volumes are con-
cerned this advantage is soon overbalanced by the greater mass of
the sound wood and attendant prolongation of the time required to
saturate its fibers and check absorption or, in the case of drying,
time required to produce an equilibrium vapor pressure between the
surface cells and the external atmosphere and check evaporation.
The mechanism and factors affecting the absorption of water
vapor by wood are not so well understood as those pertaining to
liquid water. Although partially different forces from those in the
case of the movement of liquid water are involved, it is generally
held that the limiting factors are associated with the pits. Pidgeon
and Maass (29) have shown that movement of water vapor through
the cell walls themselves is relatively unimportant. For these rea-You can also read
