The periodicity and distribution of radial growth in trees and their relation to the development of "annual" rings

THE PERIODICITY AND DISTRIBUTION OF RADIAL
GROWTH IN TREES AND THEIR RELATION
TO THE DEVELOPMENT OF
“ANNUAL” RINGS.

BY

J. G. GROSSENBACHER.

REPRINTED FROM THE TBANSACTIONS OF THE WISCONSIN ACADEMY OF
Sorencess, Anns, AND Lerters, Vor.” XVIII, Part I.

x

Issued October, 1915.

— QKe45
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THE PERIODICITY AND DISTRIBUTION OF RADIAL
GROWTH IN TREES AND THEIR RELATION TO
THE DEVELOPMENT OF “ANNUAL” RINGS.

J. G. GROSSENBACHER,

INTRODUCTION.

The study of the so-called ‘‘annual’’ rings in trees has re-
ceived the attention of numerous investigators during past years
and still claims the interest of many. Research along that line,
however, is not as active as formerly apparently owing to the
general prevalence of the idea that the causes of ring formation
are beyond our ability to fathom at present; although it is gen-
erally conceded that an environment resulting in discontinuous
radial growth is somehow responsible for their occurrence.

In studying crown-rot of fruit treest I found that radial
growth and especially its distribution on trees during late sum-
mer seemed to have a relation to the occurrence of the disease.
A number of more or less incidental remarks had been noted in
the literature concerning irregularities in the time of commence-
ment and closing of cambial activity, but the irregularities oc-
curring in fruit trees during late summer and fall were found
so marked that the literature was more carefully examined. The
number of significant papers on the subject proved so large and
the conclusions drawn so varied and contradictory that it seemed
desirable to discuss radial growth and the factors thought to
determine its distribution in a separate paper before writing up

1Crown-rot, Arsenical poisoning and winter-injury. N. Y. State Agrl.
Expt. Sta. Tech. Bul. 12:367-411. 1909.

Crown-rot of fruit trees: field studies. N. Y. State Agrl. Expt. Sta.
Tech. Bul. 23: 1-59. 1912.

1—S. A

2 Wisconsin Academy of Sciences, Arts, and Letters.

the results obtained from a histological study of the early stages
of crown-rot.

The purpose of this paper, then, is to summarize in some de-
tail most of the important hypotheses and investigations dealing
with the matter included in the title, to compare them with one
another and to bring out their relation to the writer’s observa-
tions. Thus collecting the widely scattered ideas and summariz-
ing the records of research along this line, it is hoped will stim-
ulate a wider interest in the causes of periodic growth in trees
and encourage and lead to their reconsideration from a more
modern or quantitative standpoint. In the main the aim is to
restate the questions raised by the investigators, although some-
times in a modified form. <A restudy of the structural and
tension changes accompanying periodic growth may also lead to
an investigation of the enzymes active during radial growth and
to the effect which adverse changes of environment have
upon them while in an active condition. In any ease studies of
this type will throw more light on the relation of a varying en-
vironment to vegetative and reproductive processes in woody
plants and thereby increase the knowledge necessary for a com-
prehensive investigation of their diseases. Most of the diseases
of trees which are of much economic importance and of most
scientific interest begin in the bark, and their origin seems to
have a definite relation to such radial growth and consequent
bark tensions, the normal adjustment of which is interfered with
by subsequent changes in the environment. Studies of that
kind will also help to clarify and perhaps correct some misappre-
hension that may exist regarding the relation of mycology and
physiology to plant pathology.

SEASONAL PERIODICITY OF GROWTH.

It is generally held that seasonal periodicity or the alterna-
tion of one or more growing and resting periods during the year
is a more or less unalterable inheritance of perennial plants of
temperate zones, but Klebs starting with his extensive investi-
gations on the artificial control of periodicity in algae and fungi,”
has reached a very different conclusion regarding the periodic

2 Klebs, G. Willktirliche Entwickelungsinderungen be Pilanzen. pp. 166.
Jena, 1903.

Grossenbacher—Radial Growth in Trees. 3

habit cf such plants. He maintains that the periodic or discon-
tinuous habit of vegetative activity in plants is due to an alter-
nation of favorable and unfavorable seasons of the year or to a
periodicity of the climate, and that it, therefore, may be made
continuous by modifying the environment. From his experi-
mental work he concludes that dormancy is due to a reduction in
one or more of the factors essential for growth, such.as tempera-
ture, moisture and mineral nutrients, below the required amount;
and that when such conditions occur the further manufacture
and accumulation of organic foods inhibits the action of the
enzymes necessary for growth. A timely increase in the limit-
ing factor is said to either prevent or terminate a period of
dormancy in most cases. The reduction in the supply of mineral
foods was found to be a very important factor in inducing dor-
mancy and, therefore, raising the temperature and increasing the
supply of water and mineral foods was often found to force
plants into growth. Berthold* also concluded that a reduction
in the supply of nutrient salts is the chief factor inducing a ces-
sation of terminal growth. This same conclusion was more re-
cently drawn by Lakon® who caused the buds of various decidu-
ous trees and shrubs to open when cuttings were placed in
Knop’s solution. Klebs thinks that in many cases the individ-
ual periodicity of the different branches and twigs of a tropical
plant are due to differences in transpiration and mineral nutrient
supply of such structures. It is thought probable that there
may be a periodicity in the supply of mineral nutrients in the
tropics which at times becomes a limiting factor inducing par-
tial dormancy. On the other hand Smith® maintained that
elongation growth of various Ceylon plants is controlled chiefly
by the temperature and water supply; sometimes one and then
the other or perhaps both acting together as the limiting factors.

In his interesting study of the second growth occurring on

*Klebs, G. Uber die Rhythmik in der Entwicklung der Pflanzen.

Sitzungsber. Heidelber. Akad. Wiss. Math. Naturw. Klass. 23. 1911.
pp. 84. :

“Berthold, G. D. W. Untersuchungen zur Physiologie der pflanzli-
chen Organization. 2:1381-257. 1904.

5Lakon, G. Die Beeinflussung der Winterruhe der Holzgewachse
durch die Néhrsalze.: Ein neues Friihtreibenverfahren. Zeit. Bot.
4:561-82. 1912.

*Smith, A.M. On the application of the theory of the limiting factors
to measurements and observations of growth in Ceylon. Ann. Roy. Bot.
Gard. Peradeniya. 3:303-75. 1906.

4 Wisconsin Academy of Sciences, Arts, and Letters.

trees Spdth’ comes to still another conclusion. According to
him the occurrence of the June elongation-growth which makes
its appearance fairly regularly on vigorous young trees of oak and
beech, is not determined by the environment but follows the
close of the spring elongation period after a fairly definite in-
terval of time, and may even develo pduring a drought or while
conditions are extremely unfavorable for growth. In Quercus
the resting period between the spring and June growth was
from 30 to 40 days and in Fagus from 15 to 20 days. It is said
to last 9 to 16 days in the former and 13 to 24 days in the lat-
ter. The length of these second shoots is thought to depend
chiefly upon the amount of available water and is usually but
not always less than that of spring shoots. The species with the
June-elongation habit have a short but very active spring-
growth period as compared with those not having the June
growth. It was found impossible to prevent the June elonga-
tion growth by reducing the food and water supply and by low-
ering the temperature, nor could it be made fo continue beyond
the ordinary period by supplying heat, moisture and food con-
ditions favorable for growth.

Spith also found that the second growth is made up of three
types. In one kind the axillary buds of an elongating shoot devel-
op into branches before they are fully formed. This happens in
Salix, Populus, Taxus, Buxus, Prunus, Pyrus, and is called syllep-
tic growth. In the second type known as June growth (Johannes-
trieb) the buds are fully formed before they open after the ter-
mination of the spring elongation growth. The third type is
called proleptic growth and is said to develop at any time during
summer from buds which normally would not have opened until
the following spring but which open early owing to wound or
some other strong environmental stimulus.

Neither sylleptie nor June elongation-growth was said to have
a zonation effect upon radial growth while the production of
proleptic shoots practically always resulted in more or less dis-
tinct zonation of the radial growth. This was shown during
their development by the wood cells produced being wider than
those differentiated just before the new shoots appeared.

$path, H. L. Der Johannistrieb. Ein Beitrag zur Kenntniss der
Periodizitat und Jahresringbildung sommergriiner Holzgewichse. Ber-

lin, 1912. pp. 91.

Grossenbacher—Radial Growth ‘in Trees. 5

Tue BEGINNING AND DuRATION OF RADIAL GROWTH.

Observations and statements regarding the commencement of
cambial activity or radial growth in spring are many but no
positive conclusion can as yet be drawn as to just where on any
particular species of tree it will begin one season after another.
In fact it seems to differ considerably for individuals of the same
species.

According to Strasburger® in pines as well as in Picea, as
many as five layers of tracheids had been formed from the cam-
bium in one-year shoots and considerable elongation growth had
occurred by the first of May, while at the bases and on the
trunks of eight-year old branches the cambium was still inactive.
In case of Robinia Pseudacacia and some other species, how-
ever, radial growth was found to begin first on the trunk. But >
in general cambial activity is said to begin in one-year shoots
just back of the unfolding buds and to proceed downward to the
larger branches and trunks on which it usually begins uniform-
ly and at about the same time from top to bottom. He found
that the cambium gives rise chiefly or almost exclusive to wood
cells® in spring, and as the vegetative season advances, the pro-
duction of phloem increases while that of wood cells decreases.
In trees of our zone wood formation is said to cease by mid-Au-
gust while that of the phloem continues practically up to the
end of the vegetative season. Wood cells are therefore usually
matured before winter but phloem cells sometimes enter the
dormant season in an immature condition.

Pfeffer’? also says that ‘‘the secondary growth of xylem in
trees begins and ends soonér than that of the phloem.”’

Hartig™ states that although no growth had occurred on April
15 on any of the sixteen-year-old trees under observation, by
May 5 the new radial growth in oak was about equal on all parts
of the trunk but that none had occurred underground; while
in maple, though the buds were farther advanced than in oak,
the growth as yet was confined chiefly to the one-year shoots.

®Strasburger, E. Ueber den Bau und die Verrichtungen der Leitung-
sbahnen in den Pflanzen. Histologische Beitrige 3:494. 1891.

*1.¢. p. 282.

10 Pfeffer-Ewart. The Physiology of Plants. 2nd revised Ed. 2:207.
1903.

11 Hartig, Th. Beitrage zur physiologischen Forst-Botanik. Aligem.
Forst-u. Jagd-Zeit. 1857: 281-96. 1857.

\

,

6 Wisconsin Academy of Sciences, Arts, and Letters.

On pine and larch the greatest growth had occurred at the base
of the trunks. By August ‘19 radial growth had ceased on
above-ground parts of broad-leaved trees, only a small amount
had occurred on the lateral roots and none on the fibrous roots.
In conifers radial growth was not entirely completed on aérial
parts and the roots were in about the same condition as those of
broad-leaved trees. In oak and maple radial growth on the
fibrous roots began about the 1st of August, in pine about the 1st
of September, in larch about mid-September. Hastings’? found
that radial growth started first back of opening terminal buds
in broad-leaved trees and proceeded basad. By the time the
five to six-year branches were producing new wood radial growth
had become general all over the trees. In case of pine radial
growth commenced on the two to three-year old portions of
branches and apparently before the buds opened. It was
thought that perhaps growth started on two-year branches
in pine because leaves are retained two years, for it was noted
that in the hemlock, where the leaves are retained six to seven
years, radial growth seemed to have started first on six-year-old
branches, while in the bald sypress radial growth started first
just back of the opening terminal buds as in broad-leaved trees.
On the other hand Knudson* reports that radial growth begins
on young trees of the American larch in the fourth to six-year-
old branches. He holds that the cambium first gives rise to
phloem cells in spring and that wood cells are developed later
though his counts show only a few cells. The branches showing
the first radial growth were found in the middle region of the
tree. Here growth began at the apexes while in the trunk
xylem formation is said to start near the middle. Darkened
bark, owing to its heat absorbing qualities, is thought to induce
early growth.

According to Goff** spring growth begins in many plants on
their roots. From his examinations in late March he reports
that the roots of Ribes vulgare had elongated as much as 7.5 em.
(3 inches) before aérial growth had begun. Of the following

"Hastings, G. T. When increase in thickness begins in our trees,
Plant World. 3:118-16. 1900. Sec. 12:585-86. 1900.

% Knudson, L. Observations on the inception, season and duration
of cambium development in the American larch. Bul. Torr. Bot. Club.
40:271-93. 1913.

144 Goff, E. S. The resumption of root growth in spring. Wisc. Agri.
Expt. Sta. Ann. Rpt. 15:220-28. 1898.

Grossenbacher—Radial Growth in Trees. 7

species he says that root growth had also ‘‘started more or less
in advance of the buds:’’ Picea excelsa, P. alba, P. pungens,
Pseudotsuga Douglasii, Abies concolor, Thuja occidentalis, Pinus
sylvestris, Tsuga canadensis, Tamaria amurensis, Acer sacchari-
num, Pyrus Malus, P. Communis, Prunus cerasus, P. virginiana,
Betula alba, Morus alba, Cornus stolonifer, Eleagnus hortensis,
Ribes nigrum and R. oxyacanthoides. When these observations
are compared with those of von Mohl*® who found that, though
radial growth in conifers has practically ceased by winter and
that in deciduous trees it usually has not, it seems likely that Goff
overlooked the possibility that portions he held to be new spring
growth may have been very late growth of the preceding fall.
Hartig'® found that the roots of various forest and fruit trees
had ceased radial growth in January, as judged by the thickness
of the new ring and by the presence of starch in all of the ray
cells of the cambial region. Russow’? made similar observations
in regard to both forest and fruit trees. Hartig notes an excep-
tion in the case of a species of willow where radial growth of the
roots had not been completed as shown by the thinness of the
ring as well as by the absence of starch in the ray cells of the
cambial region. Resa?® also made some observations which sup-
port Goff in some cases at least. He found that the roots of
Picea and Fagus ceased growth in November and recommenced
in February and March, while in case of Aesculus Hippocastan-
um and Tilia root growth ceased in October and recommenced
in December or later. In Alnus glutinosa root growth began in
October and continued practically through the winter except
when the ground was frozen. Root growth began in late May in
Acer campestre and in June in Quercus Robur. It is not usually
considered that such enormous variations occur in the root
growth of our trees and shrubs and for want of more detailed
information it seems necessary to admit that at least in some

1% yon Mohl, H. Hinige anatomische und physiologische Bemerkun-
gen iiber das Holz der Baumwurzeln. Bot. Zeit. 20:225-30; 233-39;
268-78; 281-87; 289-95; 313-19; 321-27. 1862.

18 Hartig, Th. Ueber die Zeit des Zuwachses der Baume. Bot.
Zeit. 21:288-89. 1863.

17Russow, E. ther den Inhalt der parenchymentischen Elemente der
Rinde vor und wébhrend des Knospenaustriebes und Beginns der Cam-
biumthatigkeit in Stamm und Wurzel der einheimischen Lignosen.
Stizungsber. Naturforscher-Ges. 6:386-88. 1884.

18 Resa, F. Ueber die Periode der Wurzelbildung. Inaug. Dissert.
Bonn. 1877. pp. 37.

8 Wisconsin Academy of Sciences, Arts, and Letters.

cases root growth may precede the growth of aérial parts of trees
in spring.

Schwarz?® found that radial growth may start in spring in
various parts of trees depending on the environment. In case of
a much shaded or overtopped tree it was found that radial
growth had begun at the base, while half way up the trunk the
cambium was still dormant. In another instance 43% of the
ring had formed at the base of a tree by July 27, while 5.5 m. up
the trunk no growth had yet occurred. These irregularities are
held not to be attributable to differences in temperature occurring
at the different regions. Mechanical stimuli to be discussed
later are held to be the instigators and distributors of radial
growth.

THE RELATION OF FOOD DISTRIBUTION AND THE PRESENCE OF
ELONGATING STRUCTURES TO THE OCCURRENCE OF RADIAL
GROWTH.

It is of interest to know definitely what relation exists between
the occurrence of radial growth and elongation growth or
whether both are simply dependent upon the presence of certain
unknown amounts of elaborated and inorganic foods in connec-
tion with the enzymes that may be involved in food transforma-
tions and growth. The experiments of Jost?° indicate that a cas-
ual relation exists between radial growth and some phases of elon-
gation growth or the presence of unfolding buds, since on the re-
moval of the buds from seedling beans radial growth practically
ceased although elongation might continue. Starch was present in
abundance and increased after the operation yet cambial activ-
ity remained in abeyance. All the elongation buds were re-
moved from several years growth of branches of Pinus Laricio
on March 8 while the dwarf branches and their leaves were al-
lowed to stay. The dwarf branches which were nearly terminal
then developed elongation buds. By the end of May but few
tracheids had developed in the decapitated branches while in
normal branches a new layer of about twelve tracheids was pres-
ent, and they had become lignified. A month later the mutilated

1° Schwarz, F. Physiologische Untersuchungen tiber Dickenwachstum
und Holzqualitat von Pinus silvestris. Berlin. 1899. pp. 371.

»Jost, L. Ueber Dickenwachstum und Jahresringbildung. Bot.
Zeit. 49:485-95; 501-10; 525-31; 541-47; 557-63; 573-79; 589-96;
605-11; 625-30. 1891.

ee

Grossenbacher—Radial Growth in Trees. 9

branches had a layer of tracheids not to exceed five or six while
a branch from which all dwarf-branches or assimilating leaves
had been removed on March 8 but on which the terminal buds:
had been left, had developed a layer of eighteen to twenty
tracheids.

In another experiment Jost removed buds from branches in
early May. When examined in fall it was found that at a cer-
tain point or line in the year’s growth the radial diameter of the
tracheids was suddenly reduced and then increased again, thus
indicating the time when the buds were removed. The doubl-
ing effect on the wood ring resulting from the removal of the
leaves at a certain time, has since been investigated by Kiihne as
noted below.

In a later paper Jost?! reports some further experiments along
this line. Defoliated pine branches were found to undergo nor-
mal radial growth provided the terminal buds are not removed,
though they may be kept in the dark; while when the last
grown leaves and the terminal buds were removed very little or
no radial growth occurred. Practically the same results were
obtained following a similar experiment with Rhododendron.
Holes were bored into the trunks of various trees in late Sep-
tember and covered to prevent evaporation. By mid-October
callus formation had occurred in all but Tilia, even though gen-
eral growth had ceased. That is, it appears that although
cambial activity is usually started by leaf or shoot elongation
wounding may also induce it, and that not the availability of
food but a distal connection with some growing leaf-structures or
buds is necessary for the occurrence of radial growth. This
same phenomonon is also indicated by the results of an experi-
ment wlth Periploca. Although this plant has bicollateral
bundles, removing a girdle of bark: prevented radial growth on
the basad side of the girdle. Nordlinger?? had noted that in
case of most trees from which the branches are removed in win-
ter practically no radial growth occurred during the following
vegetative season although in some instances slight growth re-
sulted-

21 Jost, L. Ueber Beziehungen zwischen der Blattentwicklung und
der Gefassbildung in der Pflanze. Bot. Zeit. 51:89-138. 1893.

2Nordinger, H. Der Holzring als Grundlage des Baumké6rpers.
Stuttgart. 1871. pp. 47.

23 Vochting, H. Zur experimentellen Anatomie. Nachrichten Kgl.
Ges. Wiss. Gottingen. 1902:278-83. 1902.

10 Wisconsin Academy of Sciences, Arts, and Letters.

Véchting®* also found that decapitating herbaceous plants re-
sulted in the cessation of radial growth of the stele though in-
crease in diameter may result from the growth of the pith and
cortical parenchyma. After such decapitated plants were
budded cambial activity was resumed.

Reiche” also notes regarding trees of Chili that radial growth
begins after the buds burst and that it does not occur unless bud
development precedes it.

The more detailed experiments by Lutz * also give support
to Jost’s conclusions regarding the relation of growing leaves or
buds to radial growth, and they show besides that other things
being equal the distribution of food may also be a determining
factor in the occurrence of radial growth. All the buds and
leaves of six to ten-year old trees of Fagus silvatica and some of
Pinus silvestris five to seventeen years of age were removed at
intervals from spring through the summer and the amounts of
reserve starch and growth were determined. The buds were re-
moved on March 20 from a Fagus which was about a meter high.
Branches were examined for the distribution of starch and for
radial growth on June 15, July 1, 15 and 30, August 10 and 20,
on the 10th of September, October and November, as well as De-
cember 5 and 23. The adventitious buds were removed but con-
tinued to reappear, some large ones being removed on October
10. Only minor fluctuations in the starch content of the pith
rays, wood and bark of the branches were noted through the sum-
mer with an almost entire disappearance of starch in December.
The branches remained healthy-looking but no radial growth re-
sulted. Similar trees were defoliated on May 20, June 15,
July 1, 15 and 30, and August 28 respectively, and also freed of
buds at intervals during the remainder of the growing season.
Branches of these trees were also examined on the above dates.
In the tree defoliated on May 20 no starch was found in the
branches aside from traces which occurred in the pith and
broad rays during midsummer, and even that had disappeared
by August 20. Only a small amount of radial growth took place
which had all occurred by July. On October 30 the stem or
trunk was found to contain considerable starch at the ground or

24 Reiche, K. Zur Kentniss der Lebensthatigkeit einiger chilenischen
Holzgewachse. Jahrb. Wiss. Bot. 30:81-115. 1897.

*Tutz, K. G. Beitrige zur Physiologie der Holzgewiachse. Beit-
rige Wiss. Bot. 1:1-8. 1897.

Grossenbacher—Radial Growth in Trees, 11

crown in the pith, rays, and bark yet no radial growth had oc-
curred at that point, while, 20 em. above ground where no starch
was present, about 4% of the normal amount of radial growth
had occurred. The thickness of the new growth in the trunk
increased upward until at 75 em. above ground a maximum of
80% of the normal amount had occurred although no starch was:
present there. A little starch was present in the main root near
the crown but none occurred in the laterals and no radial growth
had oecurred in them.

Corresponding results were also obtained with the other trees.
The starch content and radial growth were found to have in-
creased in each case, until, in the tree defoliated on August 28,
the amounts of both starch and growth were normal. It should
be noted, however, in cases where defoliation induced much re-
duction of food and growth of the trunks, that a radial growth
maximum usually occurred about 75 to 80 cm. above ground,
such as given above in detail. The year’s growth of full-leaved,
young trees was found to be in excess of that occurring in pre-
ceding years and their starch content was very high throughout
the summer.

Five young trees of Pinus silvestris were used in similar ex-
periments; one being defoliated on each of the following dates:
March 20, May 20, June 15, July 1, and August 30. The buds
which had been left on the tree defoliated March 20 had burst by
May 20, although the needles had not reached full size. On
July 1 and 30 some more buds had burst and begun to develop
needles. On June 15 small amounts of starch were present in
the branches. On August 20 no starch was present and only
from 4 to 20% of normal radial growth was found. On Octo-
ber 10, when the tree was taken out traces of starch were still
present in the base or crown of the trunk but none occurred in
the roots. The roots had died and their bark had become loose
and infested with nematodes. Brown spots occurred on the
bark of the stem and the twigs were being eaten by insects. The
new growth was very irregularly distributed over the stem.
Around the circumference just above the ground growth varied
from none to 8% of the normal thickness and from this point
upward the variations were equally as marked.

In the tree defoliated on May 20 no starch was found during
the summer, yet from 10 to 60% of the normal increase in thick-

12 Wisconsin Academy of Sciences, Arts, and Letters.

ness had occurred. In the remaining three trees traces of starch
were present which soon disappeared. The radial growth
ranged from 25% to normal. The tree defoliated June 15 was.
dead by October and the one defoliated in August by the follow-
ing May.

The stems of the first four and of some untreated young pines.
were cut in 15 to 30 em. pieces in October and by December the
bark on the treated-tree pieces was found to have loosened espe-
cially where considerable radial growth had occurred. The bark
had split and was shrunken both in length and circumference;
while that on the pieces from untreated trees adhered firmly to
the wood. In December pieces were also cut from the branches
of the last treated tree, the bark of which had lost its turgidity
after the operation but regained it again. A discolored circle
was found in the cambial region. Groups of undifferentiated
wood cells had been ruptured or broken down and were discol-
ored.

In his researches on the reserve food of trees du Sablon”®
found that the carbohydrate content underwent farily definite
seasonal changes which apparently occurred irrespective of the
weather. On March 17 the roots of pear trees contained much
more sugar and very much more starch than the stems and the
total carbohydrate content of roots was also higher. In stems
of chestnut trees the carbohydrate content reached a maximum
in October and a minimum in May, while in roots the maximum
came in September and the minimum in May. In case of quince
the maximum in both root and stem was found in January with
a minimum in stems in May and in roots in June. In peach the
minimum in both root and stem came in May and the maximum
in the stem in July and in roots in November. In willow both
stem and roots were found to have a minimum of carbohydrates
in April and a maximum in October, but both the maximum and
minimum were more extreme in the roots. In the case of rasp-
berry bushes the roots had a minimum in April and a maximum
in October, while in the biennial stems a high carbohydrate con-
tent was maintained during the first summer with a maximum in
October, followed by a slight depression and subsequently a les-
ser maximum in the second April. Afterwards a fairly constant

26 du Sablon, Leclerc. Recherches physiologiques sur les matiéres de
reserves des arbres. Rev. Gen. Bot. 16:339-68; 386-401. 1904.

Grossenbacher—Radial Growth in Trees. 13

decrease in the carbohydrates occurred until the end of the
stem’s life.
_ The observations by Fabricius’’ on the distribution of food in
large spruce trees throughout the entire year seems also to throw
some light on the possible relation this may have to the inception
of radial growth in spring. ‘The first tree was cut in February.
It was 25 m. high, had 68 growth rings at the base and its low-
est branches were 14 m. above the ground. The bark of the
stem 30 em. above ground had considerable starch in the medul-
lary rays, and less in the parenchyma. The older phelloderm
and ray cells contained less starch than the younger ones. Prac-
tically the same distribution of starch obtained in the entire bark
of the trunk up to the first branches. From the branches up-
ward the starch gradually increased to a maximum at 21 m. and
diminished again near the distal tip where but little was present.
The twenty-five outer rings in the lower part of tne trunk had
live, starch-bearing wood-rays and gum-canal cells and only the
outer half of the youngest wood-ring contained no starch. Fif-
teen meters above ground where the stem had 36 rings only 19
‘contained live cells and at 18 m. about a tenth of the ray cells
were alive and starch bearing in the innermost of the 21 rings.
In the one-year shoot only about half of the pith contained starch.
‘The distribution of the fats was similar to that of the starch but
it was much less in amount except in the youngest twigs. The
decrease of reserve food near the distal portions was thought to
‘be due to a loss through respiration during winter.

The starch content of small roots was slight but usually in-
creased with their diameter up tol to 2mm. An excentric root
having 55 rings on one side and 37 on the other contained starch
in the outer 20 rings of the thicker side and in the outer 15 of the
thinner. Only the roots over 2.5 em. in diameter contained fats.
In some cases excentric roots were found to have a difference of
as much as 50 growth rings between the broad and narrow sides,
yet the cambium on the thinner side was normal, although it was
evident that it often remained inactive during several years.
‘The relative amounts of starch stored on the different sides of an
excentrie root was proportional to the amount of growth on any
‘side.

*7 Fabricius, L. Untersuchungen tiber den Starke-und Fettgehalt der
‘Fichte auf der oberbayerischen Hochebene. Naturw. Zeit. Land-u.
Forstw. '3:137-76. 1905.

14 Wisconsin Academy of Sciences, Arts, and Letters.

A tree with 82 rings at its base and 22 m. high was cut in
March. The bark was fairly rich in starch from the ground up.
The 32 outer rings of wood contained starch. At the first
branches 12 m. above ground, where the stem had thirty rings,
only the fifteen outer rings were alive and starch bearing. Ata
height of 18 m. eleven or twelve of the fourteen rings present
contained starch. Considerable starch occurred in the wood at
the tree’s base and decreased rapidly upward to a minimum
about 3 m. above ground, above which it gradually increased
again to a maximum just below the branches. From this point
upward a decrease occurred which reached a second minimum
18 m. above ground, and was followed by a second increase up-
ward to a maximum at the point where the stem had but six
wood rings. No fats could be found in the bark and very little
in the wood. Apparently fats had been changed to starch.
More starch was present in the small branches of this tree than
of the one cut in February. Both the wood and bark of the
roots contained considerable starch except the youngest phloem
cells which were devoid of it. In excentric roots the starch dis-
tribution was similar to that found in the former tree.

Another tree which was much like the one cut in March as to
size, age, etc., was cut in late April. Its bark was rich in starch
with the exception of the phloem about 8 m. above ground where
none occurred. The reduction in the number of live, starch-
bearing wood rings from below upward was about the same as
in the other cases. The wood rays near the cambium were de-
void of starch. A slight amount of fat was present in the bark,
while that of the wood increased from a small amount at the
base of the tree upward to a maximum in the smallest twigs
where it exceeded the starch. In this case a starch maximum
occurred also at the base of the trunk, while in the branch bear-
ing part of the stem the starch was evidently being dissolved
from the cambium inward and in increasing extent upward.
Fats were abundant throughout the trunks and also present in
the wood of the larger roots but absent from the bark of roots.
But very little starch was present in the wood-rays at the base
of the trunk and the season’s growth of wood was devoid of
starch, while the previous year’s growth was almost free of it in
mid-June. From this region upward starch-free peripheral
wood increased up to the first branches, where it included the

Grossenbacher—Radial Growth in Trees. 15

outer four rings. In the branch-bearing portion of the stem the
outer rings again showed some starch. All the wood was rich
in fats which usually exceeded the starch present. In the
phloem only the youngest cells had appreciable amounts of fat.
That is, in mid-July more fat and less starch is present in spruce
than in June.

Very little starch occurred in the one-year roots but it in-
creased in amount toward the thicker roots so that in four-year
roots as much starch was present as there had been in the trees
cut before. The bark also contained much starch but very little
fat. No fat was present in the wood of the smallest roots but it
occurred in the larger ones and increased upward. The new
elongation growth of the roots and the bark on the thin ones, as
well as the young wood and phloem, were devoid of starch al-
though considerable was present in the large roots. Fat oc-
curred in the root wood and in occasional places in the bark.

. By the last of August an additional reduction had occurred in
the fat content of the bark and the starch in the bark had also
decreased from the ground upward while nearly the entire wood
cylinder had become practically starch-free. The bark of the
larger roots contained considerable starch but it was irregular-
ly distributed. In the youngest phloem it was absent. The
wood-rays in the larger roots and stumps had fairly large
amounts of fat present. In general it may be said that the
starch decreased in the aérial parts and increased underground
since last examined in July. The transition occurring at the
crown or stump where starch was less and fat more abundant
than earlier in the summer.

On September 25 the bark of the stem contained considerable
starch but it was present in decreasing amount from the first
branches upward to practically none in the season’s growth of
shoots. Nearly the entire wood cylinder was devoid of starch
excepting a small amount at its base or crown and in the inner
living rings. Both bark and wood were rich in fats especially
in the rays. The maximum fat content occurred about 3 m.
above ground where starch was practically absent. All except
the thin roots were comparatively rich in starch. In the wood
starch increased toward the stump. The larger roots also con-
tained considerable fat while the small ones had none.

On October 28 the bark of the stem near the ground contained

16 Wisconsin Academy of Sciences, Arts, and Letters.

t

very large quantities of starch, which gradually diminished up-
ward to the branches where it increased again but none was pres-
ent in the season’s shoots. In the wood of the stump the starch
was also abundant especially in the rays. It decreased upward
to the branches and in the season’s shoots only a little was pres-
ent near the pith. The fat content of the bark increased from
the ground upward but beyond the four-year-old branches there
was but little fat present. In general less fat than starch was
present in the wood of the stem but it gradually increased from
the ground up to the branches.

A marked starch increase in the wood since September was evi-
dent while the fat content had not been correspondingly reduced,
in fact it was considerable in the branch-bearing part of the
trunk. The distribution and relative amounts of reserve food
was very similar to that found on the preceding February. It
is therefore thought evident that starch does not diminish early
in the dormant season and that it is retained as starch through-
out winter.

The bark of the roots had an increasing amount of starch
toward the stump until a maximum was reached in roots 2 to 3
em. in diameter after which it diminished. The wood contained
considerable starch in as many as thirty of the outer rings near
the stump and then the number of starch bearing rings decreased
peripherally as it did in the stem from the ground upward. In
an excentric root with a radius of 44 mm. on one side and of 7
mm. on the other twenty rings contained starch on the thicker
side and ten on the other. The thicker side had 70 rings and
the opposite side 20 showing that during 50 growing seasons no
radial growth had occurred on the thinner side. The roots con-
tained considerable fat which diminished toward the stump.

In this case as well as in the tree cut in February the young-
est ‘phloem and the included portions of the phloem rays besides
the outer cortex contained very little starch while that portion of
the bark between them contained much starch. Fabricius thinks
that the characteristic ‘browning of the inner phloem so com-
monly noted in late winter and spring, which has been attributed
to the action of atmospheric electricity by Tebeuf,?* ‘probably has
a relation to this distribution of reserve food in the bark.

"6 Tubeuf, K. von. Beobachtung tiber elektrische Erscheinungen im

Walde.
Naturw. Zeit. Land-u. Forstw. 3:493-507. 1905.

Grossenbacher—Radial Growth in Trees. 17

From these jobservations on reserve food distribution in large
trees it seems evident that most of the starch is converted to fat
during spring and early summer, and reéconverted ‘to starch
again beginning in late September, so that the smaller portion
of reserve food passes the ‘winter as fat. Fischer’s?® observa-
tions do not agree with those of Fabricius but, since the former
based practically all his conclusions on specimens from stems
ten years old or less his conclusions are not surprising.

According to Fabricius there is a general increase of starch
also in spring but it is of short duration. By April 22 it had
largely disappeared from both'sides of the cambial region and
more especially toward the top of the tree, i. e., apparently in
proportion to cambial activity. At the same ‘time the process
of converting the reserve starch in the older rings to fat (which
continues all summer) is'also going on. The redeposition of re-
serve food is begun in the bark in the form of starch. In the
wood this process does not'begin till about the last of September
and not until October is the fat in the wood cenverted into
starch. The fat in the bark'is used up during summer and, from
the peripheral shoots downward, followed by a redeposition of
starch as the second growth is finished in late summer. Elonga-
tion growth of roots is said to oceur chiefly in June and July'and
again to a slight extent in October. During those periods they
contained considerable fat which afterwards disappeared.

This series of examinations has shown 'that the fat content of
roots is practically proportional to the amount of elongation
growth in progress and that when this growth ceases very little
or no fat is present, i. e., a causal relation seems to exist between
fat content and elongation growth. It is thought that perhaps
the growing tip secrets an enzyme which is carried up the root
by the ‘‘transpiration current,’’ and which converts starch to
fats. After the cessation of growth the fats are again changed
to starch.

A more recent contribution to this discussion is by Preston
and Phillips,*° but it also is based chiefly on determinations made
on young trees. The study covered the period from October to

29Wischer, A. Beitrage zur Physiologie der Holzgewdchse. Jahrb.
Wiss. Bot. 22:73-160. 1891.

3° Preston, J. F., and Phillips, F. J. Seasonal variation in the food
reserve of trees. Forest Quarterly 9:232-43. 1911.

2—S. A.

18 Wisconsin Academy of Sciences, Arts, and Letters.

June and included both hard and soft wood trees. It was found
that all starch disappeared in winter from Populus deltoides,
Saliz alba and Juniperus virginiana, while Quercus rubra, Ulmus
americana, Acer saccharum and Juglans nigra retained consid-
erable starch in the wood through the winter. Tilia americana
underwent a starch reduction but retained some in the phloem,
medullary rays, and xylem, while Carya glabra lost its starch
in small stems but retained about a fourth of it in larger stems.

None of these trees except Carya showed a reduction of starch
in the roots during winter. Large amounts of sugar were found
present only in spring as the buds were unfolding. The trees
tested had a maximum fat content in late fall and a minimum in
spring. These tests seem to show that broad-leaved hard wood
trees cannot be called starch trees nor those with soft wood fat
trees, as had been done by Fischer.

Niklewski* concluded from his study that the starch conver-
sion in soft wood trees like Tilia, Betula, ete. is practically com-
plete on the approach of winter, while in hardwood trees like
Prunus and Syringa it is only partial. It was found that fats
are more abundant in winter and also that a rise in temperature
increased the amount.

According to Wotczal** starch transformation begins in spring
in the distal parts of shoots and roots and proceeds towards the
older portions of the tree, although it starts later in roots than
in the shoots. But normally these two waves of starch trans-
formation starting in the roots and shoots do not encounter one
another, and in this way a starch residue remains in the older
wood and in the region of the root-crown. The deposition of
starch then occurs in the reverse manner throughout the tree,
i. e. it begins in the oldest parts and around the root-crown and
proceeds wave-like toward the distal ends of the shoots and roots.

The work by Fabricius reviewed above shows that remarkable
and apparently wave-like progressive changes occur in the state
and distribution of reserve foods in trees and that maxima and
minima of the different types occur in certain parts at rather
definite periods of the seasonal history. The above cited experi-

81 Niklewski, B. Untersuchungen iiber die Umwandlung einiger
stickstoffreier Reservestoffe wahrend der Winterperiode der Baume.
Beihefte Bot. Centralbl. 19 Abt. 1:68-117. 1906.

82 Wotczal, E. Die Staérkeablagerung in den Holzgewdchsen. Bot,
Centralbl. 41:99-100. 1890.

Grossenbacher—Radial Growth in Trees. 19

ments by Lutz show in addition that food distribution in stems
is related to the source and amount of elaborated food descend-
ing from the leaves although such factors do not seem to pre-
vent the eventual regional distribution so strongly brought out
by the observations of Fabricius, except in cases where the sup-
ply is very limited and apparently all used up or deposited on
its way down the tree. At any rate, in such instances too little
reaches the lower part of the trunk and roots to permit the oc-
currence of radial growth in those regions. Some recent defolia-
tion experiments by Kiihns** show that the radial growth occurr-
ing after defoliation usually does not extend to the base of the
stem and, therefore, results in an incomplete double ring. In
this case as in those cited by Hartig, Rubner, etc., the conclusion
seems warranted that radial growth was omitted on the lower
part of the trunk and roots chiefly because the downward stream
of elaborated food is used up before reaching that part of the
trunk. When the growth of excentric roots and an irregular
distribution of radial growth at any given circumference of a
tree-trunk, as noted by Lutz, are considered in relation to the
occurrence of reserve food, the problem becomes more complex.
Such cases make it necessary either to assume that elaborated
food is thus irregularly distributed in a tree or else that other
factors are involved in the distribution of radial growth.
Fabricius found that food is stored in a larger number of rings
on the thicker side of an eccentric root, but that does not neces-
sarily mean that the oldest starch-bearing rings on that side are
any older than the oldest starch-bearing ones on the thinner side
since rings are often entirely omitted on the narrower side. It
is at least possible that radial growth begins in spring in that
portion of a tree in which the greatest amount of food is stored
and in view of the fairly well established fact that growth con-
tinues longest in fall in such regions of maximum food content
this possibility is somewhat emphasized. Perhaps it might be
of interest first to consider the causes of excentric growth as far
as they have been determined before taking up the factors which
have been advanced by several authors as the cause not only of
the distribution of reserve focds but of the general form of tree
trunks.

*3 Kiihns, R. Die Verdoppelung des Jahresringes durch kiinstliche
Entlaubung. Biblio. Bot. 70:1-53. 1910.

20 Wisconsin Academy of Sciences, Arts, and Letters.

THE CAUSES AND THE OCCURRENCE OF EXCENTRIC RADIAL GROWTH.

In a study of the distribution of excentric radial growth on
trees it is well to note that excentricity may conceivably come
about in one or more of four ways and that in a sense such an
uneven growth of a stem at any height corresponds to the wave-
like uneven distribution at different heights of a tree. The four
ordinary ways excentric stems may be built up are (1) by the
entire omission of.radial growth in a part of the circumference,
(2) by the unequal rate of growth on different sides of stems,
(8) by the entire omission of summer growth on one side and,
(4) by the omission of spring growth on a part of the cir-
cumference and its occurrence at other places. In looking over
papers on excentric stems, etc., it is sometimes difficult to deter-
mine to which of the four classes the case under consideration
belongs but usually that is apparent.

Gravity and other factors of the environment as well as the
anatomic or physiologic characteristics of a species seem to be
the causes of excentric radial growth but as yet the matter is
not fully understood. That a difference may be found in trees
of different groups in regard to excentric growth, when subjected
to the same environment, is shown by some observations by
Nordlinger.** He cites an instance in which saplings of conifers,
beech, and oak had been bent over by the heavy snows of 1868
and afterwards grew in slanting positions. Three years later
sections taken at any point of the stems showed that pine, spruce,
and larch had developed three excentrie rings with the larger
radius below while on the oaks and beeches the three last rings
were thicker above. In one spruce only one very narrow ring
had been laid down on the upper side while the other rings had
been wholly omitted on that side. In both oak and beech radial
growth had been extremely slight on the under side during the
three years. This shows that different trees subjected to the
same environment may respond differently. That is, the specifie
characteristics of a plant to a certain extent determine the man-
ner of response to the environment.

Miiller’s*® observations seem to indicate that if excentric

a4], a.
3 Miiller, N. J. C. Beitrige zur Entwicklungseschichte der Baum-
krone. Bot. Untersuchungen 1:512-24, 1877. Heidelberg.

Grossenbacher—Radial Growth in Trees. 21

growth is due to the environment the branches on the upper and
lower parts of the same tree must be dominated by different
factors. On measuring the cross sections of 100 large horizontal
branches of beech trees he found that of those arising on the
stems between eight and fifteen meters above ground 36% were
epinastic, 60% hyponastic and 4% of equal radius above and
below; of those arising between fifteen and twenty meters above
ground 36% were epi—and 39% hyponastic and 24% had equal
radii above and below; of those taken twenty to twenty-four me-
ters above ground 64% were epi—and 28% hyponastic, with only
7% having equal radii above and below. ©

This opened up a phase of the problem, which is often left out
of consideration. It shows that the branches of some trees are
chiefly hyponastic on the lower part of trunks while they may
be predominately epinastic in the upper regions. From his tab-
ulated data the unmentioned and highly interesting fact may also
be gleaned that, of the 100 branches measured, 47 had the great-
est diameter in the horizontal plane and only 28 had the greatest
diameter.in the direction of gravity, while the other 25 were
isodiametric. Although no special attention was directed to
these facts by Miller he apparently was fully aware of them for
he concluded that gravity is not a factor in the distribution of
excentric radial growth, but that its distribution depends upon
illumination and the relative proximity to the channels of most
direct or greatest water and food conductance. Wiesner*® who
has given this problem much attention, says that all inclined
stems of conifers are hyponastic or what he calls hypotrophie,
and that those of broad-leaved trees with little or no anisophylly
become first epinastic or epitrophic and eventually often become
greatly hyponastic, while species with marked anisophylly are
first hypotrophic and subsequently become epitrophic, and finally
hypotrophic again. He maintained that excentric or heterotro-
phic radial growth of a branch is due to its position both in rela-
tion to gravity and to the axis from which it arises. On the
other hand Gabnay*’ concludes that the difference in the specific:
gravity of the elaborated food or of the cell content and the de-
gree of regenerative power possessed by the different classes of

8¢ Wiesner, J. Ueber das ungleichseitige Dickenwachsthum des Holz-
kérpers in Folge der Lage. Ber. Deut. Bot. Ges. 10:605-10. 1892.

87 Gabnay, F. Die Excentrizitat der Baume. Just’s Bot. Jahregber.
20:100. 1894.

22 Wisconsin Academy of Sciences, Arts, and Letters.

trees are the factors determining whether excentric growth shall
be epi- or hypotrophie. The specifie gravity of the elaborated
food of conifers was found appreciably greater than that of
broad-leaved trees. The regenerative power of a tree is said to
be inversely proportional to the specific gravity of its elaborated
food and it is held that the greater the regenerative power of a
tree the more epitrophic it is, while the lower its regenerative
power the more hypotrophic.

From his observations on the influence of the environment on
radial growth Kny** concludes that the excentricity of horizon-
tal branches is not only a reaction to gravity but that it is also
infiuenced by the relative illumination, transverse bark tension,
etc., as well as by some unknown factors. In some plants the
greatest thickness of one wood ring is on the lower side of a
branch while subsequent rings may be thicker above. The
branches of most of the broad-leaved woody plants were found
to have the upper half of the wood cylinder of greater thickness
than the lower, but quite a number of exceptions were also noted,
e. g. Tilia, Cydonia, Fraxinus, Gleditschia, Corylus and Alnus.
The branches of conifers on the other hand are thickened in ex-
cess chiefly on the lower side. In general it was found that one
type of excentricity is characteristic of certain natural groups
of plants, but isolated exceptions were often noted indicating
that gravity plays a, minor part in the distribution of radial
growth. The upper side of branches is subject to greater varia-
tions of light, temperature and moisture than the lower and it
was thought that perhaps bark tension might be less on the up-
per side owing to the greater distension of the bark on that side
by the variations of the temperature; yet since the results may
be just opposite in neighboring trees of different groups having
the same environment no conclusions were thought admissible.
It was observed that, owing to the fact that all leaves and buds
attached to the under side of a lateral branch develop and grow
most strongly, the axis is usually thicker on the lower side dur-
ing the first year, while in subsequent years the branches on the
upper side of a horizontal branch grow more rapidly than those
on the lower and thus result in changing hyponastic to epinastic
branches. A case is cited where the stems of Ficus stipulata

*sKny, L. Ueber das Dickenwachsthum des Holzkoerpers in seiner
Abhaengigkeit von aeussern Hinfluesen. pp. 186. Berlin 1882.

Grossenbacher—Radial Growth in Trees. 23

clambering up vertical walls were found to have both the wood
and phloem portions of the bundles thicker and of larger cells on
the wall than on the free side of ascending branches which is as-
sumed to have become inherited dorsiventrality.

Kny’s study of the roots of both hyponastic and epinastic
species showed that no regularity occurs in the excentricity of
radial growth and it was thought that local pressure relations
may determine the excentricity in roots. The lateral roots were
cut from small seedlings of Tilia, Picea and Gleditschia and,
after they had begun to develop new roots, they were placed in
darkened Knop’s solution and allowed to grow. No excentricity
resulted except in some cases where the upper radius was greater
at the origin of the root from the axis. An examination of hori-
zontal roots which had been exposed for years, showed that their
excentricity is the same as that of the branches of the same tree.
In a more recent paper he*® came to practically the same con-
clusions and maintained that the same factors which induce ex-
centric growth in aérial structures are in the main responsible
for their occurrence in roots. The atmospheric environment was
thought somehow to be the causal agent.

A new and rather striking application of the bark-pressure
hypothesis of Sachs and de Vries was made by Detlefsen*® in ex-
plaining excentric radial growth. He pointed out the obvious
fact that on the concave side of a curved stem radial growth must
necessarily decrease while on the convex side it increases bark
pressure chiefly because of the effect such growth has upon lon-
gitudinal tension of the bark. Owing to the presence of the
hard-bast fibers in the bark the reduction of the pressure on the
cambium becomes effective some distance on both sides of the
curve. The bark was usually found to be considerably thicker
on. the side of a stem having the greater radius and it was fre-
quently wrinkled or at least more rugged. He held, therefore,
that the excessive thickening in the upper angles of large lateral
roots and in the lower angle of branches is due to the reduced
bark pressure at those places following radial growth, and that
the ridges extending from such roots up the trunks are secondary

»Kny, L. ther das Dickenwachstum des Holzkérpers der Wurzeln
in seiner Beziehung zur Lothlinie. Ber. Deut. Bot. Ges. 26:19-50. 1907.

4°Detlefsen, E. Versuche einer mechanischen Erklaérung des ex-
centrischen Dickenwachsthums verholzter Aschen und Wurzeln.
Arbeit. Bot. Inst. Wiirzburg. 2:670-88. 1882.

24 Wisconsin Academy of Sciences, Arts, and Letters.

effects of the same thing. In case of branches, it was assumed
that their weight increases the longitudinal bark tension above
and reduces it underneath. Trees having one-sided tops were
said to also be affected by the increase of bark tension on the side
with fewer branches and a decrease on the top-heavy side, thus
resulting in excentric growth of the stem with the greater radius
on the side having more branches. A case was described in
which a large horizontal branch had a sharp lateral bend on the
concave side, which had resulted in a marked increase in radial
growth with only a slight increase on the lower side. On such
an assumption as this of Detlefsen it is conceivable that, after
the excentricity in the upper angles of lateral roots has once be-
come marked and a tree has attained some age, it may become
more and more pronounced until a buttress-like structure results.
However, he failed to mention epinastic branches.

Kny*! has also noted that bending roots of herbaceous plants
and allowing them to grow in the bent position results in exces-
sive growth of both xylem and cortex on the concave side.

According to Mer* the two chief causes for excentric radial
growth are those affecting the manufacture of organic food and
those influencing cambial activity. The factors affecting the for-
mer are the slope of the land, proximity to.other trees, fertility
of the soil, exposure, etc., while those influencing cambial activ-
ity are thought to be mechanical strains due to wind, gravity,
traumatism, etc. Sloping ground is said to induce an increased

growth on the hill and a reduced growth on tne valley side.
Trunks were more commonly found excentric in thick than in
thin forest stands and the excentricity was confined chiefly to
the lower parts. When affected by the proximity of another
tree the radius toward the influencing tree was shorter. Curva-
ture was held to be the most frequent cause of excentric growth.
Wounds were found to induce an excessive radial growth on the
opposite side of the stem; and excentricity was found to be con-
ducive to the occurrence of frost clefts.

“Kny, L. Ueber den Hinfluss von Zug und Druck auf die Richtung
der Scheidewdnde in sich theilenden Pflanzenzellen. Jahrb. Wiss. Bot.
87:55-98. 1902.

“Mer, E. Recherches sur les causes d’ excentricité de la moélle dans
le sapins. Rev. Eaux et Foréts. Ser. 2:461-71; 523-30; 562-72. 1888.
——3:19-27; 67-71; 119-80; 151-63; 197-217. 1889.

Grossenbacher—Radial Growth in Trees. 20

Cieslar*® performed an experiment which suggests the above
cited observations by Nordlinger in that he bent over the tops of
four eight-year-old spruce trees and tied them in a horizontal po-
sition in early summer, one was bent toward each of the four
cardinal points. All ascending branches were also fastened
horizontally. The trees were cut during the second wénter fol-
lowing the beginning of the experiment and the radial growth
was found to have become greater on the upright basal portion
of the stems on the side of the bent-over tops. The excentricity
increased from near the ground up to a maximum beyond the
middle point of the turn where the stem was horizontal. Start-
ing in the outer ring some distance above the inception of ex-
centric growth and extending even into the outer part of the
third ring, the wood on the side having the longer radius had a
reddish color, which also became darker upward in proportion to
the increase in the radius. That is, the rings produced the year
before the trees were bent were also affected by the bending. It
is also shown that the spring-growth of the affected rings is not
discolored in the lower part of the stained region.

Such ‘‘red-wood’’ as described above is very commonly present
in the under half at the base of pine and spruce branches. The
physical properties of ‘‘red wood’’ have been studied in some de-
tail and its histological characteristics have also received some
attention. Although it seems not to occur in stem structure de-
void of excentric growth, excentricity is not always accompanied
by ‘‘red wood.’’ The fact brought out in the above cited paper
by Cieslar that the summer wood may be affected while the
spring wood of the same ring is normal is especially noteworthy
because it shows that the factors producing ‘‘red-wood”’ are not
effective throughout the year.

Hartig** made an investigation of the occurrence and distri-
bution of ‘‘red-wood’’ in spruce and found that it is always pres-
ent on trees which have excentric trunks and are located in iso-
lated places or in thin and interrupted forest stands. Since
‘‘red-wood’’ occurs in portions of trees which appear to be sub-
ject to the greatest strains, Hartig thinks it arises in response to
the mechanical requirements of stems. He found that inclined

42 Cieslar, A. Das Rothholz der Fichte. Centbl. Gesam. Forstwesen.
22:149-65. 1896.

“Hartig, R. Das Rothholz der Fichte. Forst. Naturw. Zeit. 5:96-109;
157-69. 1896.

26 Wisconsin Academy of Sciences, Arts, and Letters.

tree-trunks had a greater radius on the side toward which they
slant and also have ‘‘red-wood’’ present on the side with the
longest radius. In one instance a tree on the west edge of a for-
est and therefore having most of its branches on the west side
was found to have a longer radius as well as abundant ‘‘red-
wood’’ on the east side. In another case trees along the western
edge of a forest had the typical excessive growth and ‘‘red-
wood’’ on the east side of the trunks up to the age of about 80
to 90 years, after which the new rings showed a lesser excentric-
ity and a smaller amount of ‘‘red-wood.’’ The change seemed
to have resulted from the presence of a new planting on the west
side which had attained some size by that time. Hartig con-
eluded that the mechanical or swaying effects of wind not only
causes excentric radial growth but also induces the formation of
‘‘red-wood’’ on the side of trunks subjected to longitudinal com-
pression. An instance is also cited in which the leeward side of
a tree-trunk is excessively thickened from the base up but which
was devoid of ‘‘red-wood’’ near the ground although it was abun-
dant farther up. A case is described where the distal part of a
young spruce stem had been bent into a complete turn and had
grown in that position during 27 years. Sections cut at various
points of the curve showed the occurrence of the greatest radial
growth and of ‘‘red-wood’’ on the sides where gravity and lon-
gitudinal compression resulting from the top-weight and wind.
action would require it. The excentricity of large spruce
branches and the accompanying ‘‘red-wood’’ was found to ex-
tend only about four meters out from trunks.

According to Hartig ‘‘red-wood’’ has comparatively large
intercellular spaces and the cells seem not to be very firmly at-
tached since they frequently fell apart in sections. The tra-
cheids are said to have especially thick walls the innermost thick-
ening layers of which are arranged spirally.

In a more recent summary of his investigations of wood Har
tig*® claims to have proved the relative influence of gravity and
longitudinal compression in inducing the formation of ‘‘red-
wood.’’ Spruce trees planted in large tubs were suspended in
an inverted position in a greenhouse and the distal part of the
stems were bent upward and allowed to grow during one sea-

‘5 Hartig, R. Holzuntersuchungen. Altes und Neues. Berlin. 1901.
pp. 99.

Grossenbacher—Radial Growth in Trees. 27

son. The excessive growth at the curve and the accompanying
‘‘yed-wood’’ was found to have developed on the under or con-
vex side of the curve. This was assumed to indicate that grav-
ity has more influence in the production of ‘‘red-wood’’ than
longitudinal compression.

Rubner** has given us some interesting observations on ex-
centrix as well as of more irregularly distributed radial growth
of trees. He called attention to the fluted or furrowed trunks
and buttressed trunk-bases so characteristic of certain species.
He attributed the ridges to excessive and the valleys to subnor-
mal radial growth. In Carpinus the deep, wide grooves in the
stem were found to occur at places where several compound
medullary rays are grouped together, while lesser depressions
or channels occurred along each individual compound ray, but
these lesser grooves were practically compensated for by the
greater phloem production so that the outer surface of the bark
did not show them. In portions of trunks represented by the
ridges the rays were small and it was assumed by Rubner that
the distribution of the large and small rays influences the rel-
ative amounts of radial growth of the ridges and valleys in the
wood cylinder. While Nordlinger*? assumed that the valleys
are due to an excessive bark pressure along the large rays owing
to the development of stone cells or abnormally long phloem-ray'
cells in the bark at such places. He notes the absence of marked
valleys and grooves in oaks devoid of broad rays, and that on
very large, old trees the outer rings often have the valleys be-
tween the large rays while the ridges occur along the rays. The
armpit-like depressions below some branches, according to Rub-
ner, occur under branches whose leaves elaborate only enough
food for their own use thereby leaving the region just below the
branch bases insufficiently supplied, owing to the deflection the
branch-bases cause in the downward current of food in the trunk.
These depressions are said to be chiefly confined to epinastic
species. In the valleys Rubner found the wood to consist main-
ly of thick-walled fibers and the radial arrangement of the cells
was perfect, apparently because the valley-wood is devoid of ves-
sels. The large ‘‘false rays’’ present in the valleys of Carpinus

*oRubner, K. Das Hungern des Cambiums und das Aussetzen der
Jahrringe. Naturw. Zeit. Forst-u. Landw. 8:212-62 1910.

47 Nordlinger, H. Wirkung des Rindendruckes auf die Form der
Holzringe. Centralb!. Gesam. Forstwesen. 6:407-13. 1880.

28 Wisconsin Academy of Sciences, Arts, and Letters.

were found to develop in the second and subsequent annual rings
by the elimination of most of the wood cells between adjoining
rays. Eames*® has noted a similar compounding of the simple
rays of white oaks. Rubner found that the ray cells in the val-
ley wood are shorter than those in the ridge-wood. The wood
in valleys often showed no indication of rings because the cells
were frequently all of the summer-wood type with a reduced
radial diameter. In the deep valleys many rings were found to
converge into a homogeneous layer of small cells many of which
had brownish contents. In some cases as many as twenty-two
year’s growth had occurred on the ridges while no growth re-
sulted in the valleys. In some such instances the cambium in
the valleys had become thick-walled and apparently lost its
power of growth and in others it had died and turned brown.
In the smaller valleys of trunks phloem production was found
excessive while on the ridges it was only slight. Rubner also de-
scribed instances in which no radial growth resulted on the lower
portion of tree-trunks during a number of years. He found
that long branches with sparse foliage have very irregularly dis-
tributed radial growth, often being wholly omitted in some por-
tions and present in others, although at times with imperfectly
differentiated cells. Similar irregularities were also noted by
Ursprung*® in branches of teak wood from the tropics; cross sec-
tions showed that in some growing seasons the cambium had
been active in only a part of the circumference.

The work reviewed above shows that several types of excen-
tric radial growth occur both in horizontal and upright struc-
tures and that some of them are apparently due to differences in
bark pressure and to an excentric distribution of the transpira-
iton current and metabolized food, while in others the cause of
the excentricity is not shown. For instance these authors have
not determined why radial growth should be distributed in scat-
tered patches on branches or tree-trunks which have an inade-
quate supply of food or why fluted trunks and buttressed stumps
should occur, although Detlefson made some interesting sugges-
tions regarding the latter. Rubner has shown that radial
growth is very slight in the valleys or grooves occurring in the

42 Hames, A. J. On the origin of the broad ray in Quercus. Bot. Gaz.
49:161-66. 1910.

**?Ursprung, A. Zur Periodizitat des Dickenwachstums in den
Tropen. Bot. Zeit. 62: Abt. 1:189-210. 1904.

Grossenbacher—Radial Growth in Trees. 29

trunks of Carpinus, etc., and that the wood of these valleys con-
tains the large aggregate rays while that in the ridges has simple
ones. That the presence of the aggregate rays has induced the
valleys by their early cessation of growth as Sorauer® held does
not necessarily follow, though it may be true, as it is more re-
cently implied by Bailey®* and others. In a number of recent
papers written by Jeffrey’s students” it is maintained that the
different types of rays and their method of development are of
great phylogenetic significance in showing the paths of evolu-
tionary development. Yet in the above cited paper by Bailey
it is also noted that changed nutrition may markedly modify the
rays and their distribution.

Some of Kny’s®* results obtained in his experiments seem to
indicate that the pressure under which rays differentiate in the
cambial zone has much to do in determining their size. He
found on applying a pinch-cock to twigs of Salix and Aesculus
Hippocastanum in spring that not only was radial growth almost
entirely inhibited on the compressed sides but that the ray cells
were broader in tangential direction and that in some cases a
doubling of the typically simple rays had occurred in both trees.
In the above cited paper on the causes of excentric growth Mer
also calls attention to the increase of radial growth on trunks
opposite a wound. This observation of Mer’s is of interest here
chiefiy because the occurrence of traumatic rays** in wood pro-

®°Sorauer, P. Handbuch der Pflanzenkrankheiten. Zweite Auflage.
1:587. 1886.

51 Bailey, I. W. The relation of the leaf-trace to the formation of
compound rays in the lower Dicotyledons. Ann. Bot. 25:225-41. 1911.

52 Bailey, I. W. Reversionary character of traumatic oak woods.
Bot. Gaz. 50:374-80. 1910.

‘Eames, A. J. On the origin of the herbaceous type in the Angio-
sperms. Ann. Bot. 25:215-24. 1911.

Thompson, W. P. On the origin of the multiseriate ray of the Dico-
tyledens. Ann. Bot. 25:1005-14. 1911.

Holden, R. Reduction and reversion in the North American Sali-
eales. Ann. Bot. 26:165-73. 1912.

Bailey, I. W. The evolutionary history of the foliar ray in the wood
of the Dicotyledons, and its phylogenetic significance. Ann. Bot.
26:647-61. 1912.

5s Kny, L. Ueber den Hinfluss von Zug und Druck auf die Reichtung
der Scheidewdnde in sichtheilenden Pflanzenzellen. Jahrb. Wiss. Bot.
387:55-98. 1902.

54 Jeffrey, H. C. Traumatic ray-tracheids in Cunninghamia sinensis.
Ann. Bot. 22:593-602. 1908.

Bailey, I. W. Reversionary characters of traumatic oak woods. Bot.
Gaz. 50:374-80. 1910.

30 Wisconsin Academy of Sciences, Arts, and Letters.

duced on the side of a stem opposite a wound is assumed to have
phylogenetic significance.

According to Groom® the evolution of the rays in Quercus is
not ag simple as: presented by Eames, Bailey, Thompson and
others for he found cases where the primary rays seemed to
branch like those of beech described by Jost®* as well as others
where the aggregations occurred in the manner described in the
above cited papers. Groom is inclined to the view that ray de-
velopment and architecture is based on physiological rather than
on phylogenetic factors and that it is impossible at present to
decide whether the narrow or the broad-rayed type is the more
primitive.

It is also worth noting that, although Nordlinger®” found the
valleys originating along the groups of broad rays and that oaks
without the broad rays are devoid of valleys, in case of very
large old trees the ridges were often found to occur along the
broad rays, while valleys were present between them, i. e. just
the reverse of the conditions obtaining in younger specimens.

Perhaps it might prove worth while to find out whether the
occurrence of valleys and ridges in such trees is due to differ-
ences between the rate of growth in the wood and in the rays
rather than being due to an early cessation of ray growth as
Sorauer had assumed. In case the formation and radial elonga-
tion of ray cells were very slow as compared to the radial in-
crease in the wood cylinder in general, it is conceivable that the
solid broad rays may have a dominating influence and retard
radial growth on both sides of them because of the firm attach-
ment between the rays and the surrounding tissues. If the claim
made by Klebs*’ that the presence of large quantities of elab-
orated food retards radial growth should prove correct and since
these large rays are the storage reservoirs for elaborated foods
it would also be understandable how they might be comparative-
ly slow growing in youth and comparatively more rapid in old
age, when radial growth has become slow.

The conspicuous ridges on the lower part of trunks correspond

5>Groom, P. The evolution of the annual ring and medullary ray in
Quercus. Ann. Bot. 25:983-1003. 1911.

5 Jost, L. Ueber einige Eigenthtimlichkeiten des Cambiums der
Baume. Bot. Zeit. 59:1-24. 1901

‘7 Nordlinger, H. Wirkung des Rindendruckes auf die Form der
Holzringe. Centbl. Gesam. Forstwesen. 6:407-13. 1880.

BX). -G,

Grossenbacher—Radial Growth in Trees. 31

with the occurrence of the upper lateral roots. In trees like the
elms, ironwoods, and oakg the excessive thickening in the upper
angle primary roots make with trunks are often exaggerated into
buttress-like enlargements which are continued as ridge-like pro-
longations extending some distance up the trunks. According
to Detlefsen®® the excessive radial growth in the upper angle of
lateral roots and in the lower angle which large branches make
with the trunks is chiefly due to a continued decrease of the bark
pressure at these places which results from radial growth. This
hypothetical explanation, however, requires an experimental
basis. The fact that the bark at these places is often cleft or
ruptured rather shows that radial bark pressure, at least, occurs
there. The pressure exerted against the bark by the growing
wood is not only sufficient to bring about tension at the root and
branch ridges but tension of sufficient magnitude to rupture the
bark in many instances. The experiments by Vochting®® in
which the distal tips were cut from Helianthus and other plants
with the result that the stems became somewhat fleshy and in
some cases rib-like thickenings developed over the leaf traces and
ran some distance down the stem, can scarcely be said to apply
owing to the fact that in Voéchting’s experiments the excessive
thickening was chiefly due to increase in the pith and cortical
parenchyma instead of radial growth of the stele.

Tt has been suggested or inferred by some of the above as well
as by other writers that greater cambial activity occurs in the
upper angle of roots at their origin from the stump than takes
place in the lower angle, because the downward current of meta-
bolized food is checked and accumulates more or less in the up-
per angle. The lower angle of the root is said to be more indi-
rectly and, therefore, more sparsely supplied with food and for
that reason one sided radial growth results. An additional
’ factor, which contributes to this excentricity, is doubtless the
pressure of the tree’s weight on the cambium of the underside
and another may be the reduced longitudinal bark tension sug-
gested by Detlefsen. Even in case of a tree with a deeply pene-
trating tap root a very marked radial increase on the lower side
of large primary laterals would tend to elevate the entire tree,
and a tree without a tap root must be carried chiefly by the large

S810:
lie.

32 Wisconsin Academy of Sciences, Arts, and Letters.

primary laterals and therefore exerts great pressure on the
cambium as Detlefsen® maintained.

According to another group of investigators to be cited in the
discussion on the distribution of radial growth, excentric growth
is not due to an independent distribution of metabolized food and
the other factors commonly assumed to be effective. Both food
and growth are held to be distributed by the mechanical effects
of the environment in conjunction with the weight effects of the
structure in question or by the rate and path of the transpiration
eurrent.

THE GENERAL FORM OF TREE-TRUNKS AND THE DISTRIBUTION OF
RADIAL GROWTH.

The distribution of radial growth on trees determines the form
of the stem and therefore its value as timber. Owing to the
economic importance of the shape of tree-trunks to the lumber-
ing industry foresters studied the distribution of radial growth
and its relation to the environment very extensively and have
collected many valuable data. Since the stem of a tree grown
in a fairly dense and uniform forest stand is relatively longer
and less tapering toward its upper end, free of branches and
therefore of more lumbering value than one grown in the open,
the differences in the environment of the two types have re-
ceived much attention.

No6rdlinger®™ noted that the yearly increase in thickness on the
branchless and branched parts of stems grown in a forest dif-
fered from each other. The annual distribution of radial growth
on the branch-bearing portion in a forest stand was found to be
similar to that on the entire trunk of a free-standing tree, which
bears branches nearly to its base. The thickness of the wood
rings in the branch-bearing part of stems was found to decrease
from the base upward. On the branchless portion of trunks in
dense forest stands the thickness of the recent rings was noticed
to have decreased from the branches downward although in some
cases the thickness of the new yearly growth remained practically
constant at the base of trunks. He thought that the presence
of elaborated food was not the only requisite for the occurrence

hae Oar
st. eG

Grossenbacher—Radial Growth in Trees. 33

of radial growth in any particular region of a tree-trunk for the
reason that the radial growth maxima in dense stands move up-
ward more rapidly than would be demanded by the reduction in
metabolized food.

Sanio® noted that in case of a dwarfed fourteen-year-old sap-
ling of Fraxinus excelsior growing in a swamp the spring wood
was for the most part very thin and usually had but a single
row of vessels while in some parts of the stem the rings were de-
void of vessels. He thought it likely that spring growth had
been wholly omitted at such places and that the ring there con-
tained only summer-growth wood.

R. Hartig®* has probably published more on the general dis-
tribution of radial growth than any other investigator. From
a study of overtopped pines and spruces between 20 to 30 years
old, he found that the rings became thinner from the branched
top downward and that in some cases as many as seven rings had
been entirely omitted on the lower part of stems. When rings
had been omitted during a series of years the lower edges of
the new rings or wood-sheaths were found to have receded farther
from the base each year. In another paper he™ called attention
to the fact that in overtopped trees a reduction occurs in the
yearly amount of wood produced from the branches downward.

In general a stem is said to have three more or less distinct
growth regions in each of which a typical distribution occurs.®
In the main axis of the branched top the cross sectional area of
the growth rings is said to increase from above downward. The
rings on the branchless shaft also increased in thickness from
the branches downward in trees having a well developed top,
but as stated above, the reverse was found true of a dominated
tree with a small top.

A more detailed study of the distribution of radial growth was
carried out by Himmerle® in connection with his observations

6¢ Hammerle, J. Zur Organization von Acer Pseudoplatanus: Biblio.
Bot. 50:1-101. 1900.

*2Sanio, K. Verleichende Untersuchungen tiber die Zusammenset-
zung des Holzkérpers. Bot. Zeit. 21:391-99. 1863.

*s Hartig, R. Das Aussetzen der Jahresringe bei unterdriickten Stim-
men. Zeit. Forst.-u. Jagdwesen. 1:471-76. 1869.
. % Hartig, R. Zur Lehre vom Dickenwachsthum der Waldbaume.
Bot. Zeit. 28:505-13; 521-29. 1870.

‘8 Hartig, R. Ueber den Entwicklungsgang der Fichte im Geschlos-
senen Bestande nach héhe, Form und Inhalt. Forst. Naturwiss. Zeit.
1:169-85. 1892.

38—S. A.

34 Wisconsin Academy of Sciences, Arts, and Letters.

on the elongation growth of young maple trees. He found that
the greatest thickness of each ring normally occurred in the
hypocotyledonary or crown region of young trees. The second
ring of the branches was thicker toward the end than in the mid-
dle but subsequent rings decreased regularly toward the distal
end. The third ring of a rather dwarfed, overtopped specimen
had its greatest thickness in the three-year-old branches and di-
minished toward the base until at the height to which the tree
had grown by the end of its first year, the ring was almost invis-
ible; at the hypocotyl or crown region and at least as far as 19
em. downward on the roots no growth at all had occurred during
the third year. The bark in all cases was thickest at the hy-
pocotyl or crown region.

From the papers cited in this section as well as from others
noted elsewhere it is very evident that the distribution of radial
growth is at least quite strongly influenced if not entirely deter-
mined by the environment and it will be interesting to examine
some of the papers in which the factors that have been advanced
as being the regulators of this distribution are discussed.

The publication of Schwendener’s®? epoch-making paper on
the mechanical principles underlying the structure of Mono-
cotyledons gave a view of plant anatomy from a new angle and
still exerts a marked influence on both physiology and anatomy.
Many measurements and calculations obtained from typical
Monocotyledons are presented in this paper in support of the
hypothesis that plant structures take on forms and have the sup-
porting tissues distributed in them in such a fashion as to meet
the mechanical requirement necessary to make such structures
most efficient in carrying their own weight as well as in resisting
injurious bending by the wind, ete. In replying to some severe
criticism of this paper he®* admitted that many inaccuracies oc-
cur in the calculations but maintained that on the whole it is
correct. The general principle developed in the first paper is
here also reinforced in its application to Dicotyledons but in a
less thoroughgoing way. It was noted that radial growth in a
tree-trunk seems to be distributed in a manner so as to meet the

© Schwendener, S. Das mechanische Princip im anatomischen Bau der
Monocotylen mit vergleichenden Ausblicken auf die tibrigen Pflanzen-
klassen. pp. 179. 1874.

*8 Schwendener, 8. Zur Lehre von der Festigkeit der Gewdchse.
Sitzungsber. K. Preuss. Akad. Wiss. Berlin. 1884:1045~—70. 1884,

Grossenbacher—Radial Growth in Trees. 35

mechanical needs in supporting the top in its environment. The
general form of trunks was found to conform more or less com-
pletely with shafts constructed to be of equal endurance through-
out and capable of supporting a given load (top) and wind-pres-
sure. It is said that owing to this fact a tree trunk grown in the
open and therefore bearing branches nearly to the ground is
thicker at the base than one grown in a forest and crowded by
other trees.

Some years later Metzger®® published some results and obser-
vations from which the striking conclusion is drawn that light,
warmth, moisture and food enable a tree to grow but that the
wind determines how it shall grow. He points out the self-evi-
dent but none the less interesting fact that a tree-trunk must not
only carry its own weight and that of the branched top but also
resist the wind action as it shifts the center of gravity while
swaying to and fro. The tree stems are said to be the pillars of
the forest and in order that the forest exist they must be both
rigid and at the same time elastic enough to withstand strong
winds. This is illustrated by him by imagining a wooden shaft
firmly fixed in a horizontal position at one end and weighted at
the other, thus resulting in the greatest strain at the place of at-
tachment. If such a shaft is to be equally liable to break at any
point of its entire length its cross sectional area must decrease
from the point of support to the application of the weight or
force in accordance with the physical laws involved, and the most
economical use of the material of the shaft would require such
a construction. By making numerous measurements and ealcu-
lations it was found that the proportional thickness and form of
tree-trunks below the branch-bearing tops was practically that
required of the shaft described above, except that most of them
are enlarged at the base or root-crown beyond the hypothetical
requirements. It is noted that tap-rooted trees in deep soil are
devoid of the excessive basal enlargement, and it is therefore
thought that the enlargement is only a result of developing an
adequate root anchorage for the tree. That portion of the stem
in the branching top was also found to conform to such a shaft.

In ease of horizontal branches it is held that their own weight
overbalances wind action as a formative factor, while in upright

*® Metzger, A. Der Wind als massgebender Faktor fiir das Wach-
sthum der Baume. Muindener Forst. Hefts. 3:35-86. 1893.

36 Wisconsin Academy of Sciences, Arts, and Letters.

branches like in trunks wind effects predominate over the weight
of the structures themselves as formative stimuli. Branches in
positions intermediate between these two extremes are said to be
correspondingly influenced by the two factors. Since conifers
of various sizes were found to conform very closely to the hy-
pothetical requirements, Metzger thought it logical to assume
that wind and the weight of the supported structures themselves
are the factors instrumental in shaping tree-trunks or distribut-
ing radial growth on them. When the lower branches of a free-
standing tree were removed, it was found that the annual
growths on the lower portion of the trunk were reduced in cross-
sectional area in very nearly the proportion required by the
hypothetical considerations of the upward movement of the
point of greatest stress. When a free-standing tree is encom-
passed by young trees radial growth of its trunk decreases from
above downward as required by this hypothesis. When forest
trees are left free-standing by the removal of surrounding trees
radial growth is found to increase on their trunks from above
downward and to decrease below normal on the upper part of
the stems. In conformity also with the above hypothetical re-
quirements the tall or over-topping trees in a forest of mixed
sizes undergo most radial growth on the lower parts of the trunks
while the overtopped trees grow more on the upper part of
trunks.

Although these conclusions were based on data, which were
obtained from spruce, Metzger’ thinks them applicable to the
distribution of radial growth of trees in general. According to
him the wind, acting as a stimulus through its mechanical effects
upon trees, also regulates in a general way, the distribution of
the elaborated food as well as that of radial and elongation
growth in accordance with the relation of the form of the top,
etc. to wind-exposure. It is said that during the first and sec-
ond year after the thinning of a forest most of the available food
is used up in increasing radial growth on the lower part of the
trunks so as to increase the wind resisting power of the suddenly
exposed trees, but afterwards elongation growth proceeds rap-
idly. In some cases of this kind it is held that the top may be

7 Metzger, A. Studien tiber den Aufbau der Waldbiume und
Bestinde nach statischen Gesetzen. Miindener Forstl. Hefte. 5:61-74.
1894. Miindener Forstl. Hefte. 6:94-119. 1894.

Grossenbacher—Radial Growth in Trees. 37

deprived of marked radial as well as elongation growth for sev-
eral years, and the long-continued scarcity of food in the upper
part of the top is said often to result in the dying back of the
upper branches and thus gives rise to stag-horn effects. The
length of time required for adjustment to the new environment
is said to depend upon the extent of a tree’s leaf surface. The
sprouts, which often arise on long bare trunks, are thought to
be induced by the swaying action of the wind thus tending to
develop a lower head.

An enormous amount of data and calculations on the relation
of the environment to radial growth and its distribution was also
callected by Schwarz” and published as a monograph which in
addition contains many very important observations on the life
and seasonal history of Pinus silvestris. It is noted that yearly
radial growth as measured by the area of its cross section in-
creases in trees until the age of about 20 to 30 years is reached,
but under very favorable environmental conditions its growth
may increase to the age of 100 years. His general conclusions
regarding the wind in its relation to the distribution of radial
growth are practically the same as those put forth by Schwen-
dener and Metzger. Some instances are cited where the tops of
trees had been broken off when about 30 years old and which had
since grown about 60 years with lateral branches diverted to
function as the main axis. In the region of curvature of the
branch which assumed the functions of the main axis excentric-
ity became very marked, with the greater radius on the under
side. It is thought that the excessive pressure or weight on the
under side was the stimulus to increased radial growth on that
side. In one ease, in which the curvature induced had been such
as to exert the greater pressure on the upper side in one place,
it was found that this upper side had the greater radius. Many
measurements on vertical stems also showed a greater radius on
the leeward side in regard to the prevailing wind. By tying a
young pine tree in a bent position excessive growth resulted on
the compressed side, i. e. it seems that a fixed, bent position ex-
erts the same influences on radial growth as the discontinuous
pressure due to wind swaying. Other measurements on slightly
inclined trees also showed a greater radius on the side toward
which the trees inclined. It is held that relative amounts of

Ey. es

38 Wisconsin Academy of Sciences, Arts, and Letters.

elaborated foods present in different regions of trunks is not
primarily responsible for the distribution of radial growth, for
on such an assumption the greatest growth would always occur
on the stem just below the branches, while as a matter of fact it
usually occurs within two meters of the ground. In fact it is
claimed that both the distribution of metabolized food and radial
growth are regulated by the wind-pressure-and-weight stimuli.
The wind effects are thought to induce the transfer of most of
the food elaborated in the leaves of a recently isolated tree to the
lower part of the trunk where increased radial growth is caused
by the increase of the mechanical wind-stimulation. Attention
is called to the fact that in case of excentric annual rings the ex-
centricity is chiefly due to an excessive production of the so-
ealled summer wood, thus upholding the view that swaying and
weight stimuli are especially effective during the latter part of
the period of radial growth. The data seemed also to show that
after trees with excentric rings are perhaps about 73 years old
or have begun to decline in their rate of growth the new rings
decrease markedly in excentricity and in conformity with that
it is noted that late season growth is less in trees which have
reached the age of decline in growth rate.

Schweinfurth” reported that about the Red Sea tree trunks
all have a greater radius on the south side owing to the occur-
rence there of a continued and strong north wind during the
summer. The presence of reduced branches on the north side is
thought to have caused the reduced growth on that side.

A more detailed application to Schwendener’s mechanical
principles of plant structure to excentric radial growth in
branches was made by Ursprung.”* He maintained that the dis-
tribution of radial growth of both stem and branvhes is deter-
mined by the compression-strain stimulus resulting from the
weight of the structure and the action of the wind. Non-verti-
cal stems and branches were usually found to have an elliptical
-eross section with the longer diameter in the direction of gravity.
This is said to increase the carrying capacity of the wood be-
cause the force required to bend such a branch in a vertical plane
is proportional to the third power of the vertical diameter and to

72 Schweinfurth. Sitzungsber. Ges. Naturfor. Freunde. Berlin 1867.
p. 4.
™ Ursprung, A. Beitrag zur Erklairung des excentrischen Dicken-
wachstum. Ber. Deut. Bot. Ges. 19:313-26. 1901.

Grossenbacher—Radial Growth in Trees. 39

only the first power of its horizontal diameter. He also con-
cluded that vertical stems may become excentric owing to one-
sided action of wind but that the effect on some trees might be
different on account of variations in the shape and the conse-
quent distribution of the weight of the top. The crooks in a
tree trunk are assumed also to be gradually eliminated by the
distribution of the radial growth in response to strain stimuli.
The same laws are thought to apply to the radial growth in roots
but because of the variation i in the environing soil they are not
always so regularly effective.

Vochting’* cut the tips from some potted one-year-old savoy
plants and placed them with their pots in a horizontal position.
He attached weights to some near their decapitated tips and. al-
lowed them to vegetate during some months. The vertical diam-
eter of the stems was markedly increased in the regions of great-
est strain while the stems of the check plants retained their
cylindrical forms.

The far-reaching applicability of this wind-gravity hpyothe-
sis originating with Schwendener and elaborated by Metzger and
others, according to which tree-trunks and other stem structures
have a form required of a shaft of equal endurance throughout,
has recently been questioned by Jaccard.”> He holds that the
hypothesis is untenable because measurements and calculations
made by him on a number of spruce trees resulted in a noncon-
formity of the hypothetical and actual forms of their trunks.
It was found that the portions of the trunks beginning with 5
m. above ground and extending to about 9 m. above ground were
practically of the form and dimensions required of such a shaft
but above and below that region the trunks were thicker than
required by the laws of mechanics. In one instance described
in detail, however, the trunk of a spruce practically conformed
to the required hypothetical shaft.

Although much more frequent strong winds are said to oceur
in western Switzerland the trees there were not found to differ
appreciably from those of eastern Switzerland where strong
winds are few. Jaccard maintained that during the growing sea-
son the wind is too spasmodic to be a factor in the distribution

We;
™ Jaccard, P. Hine neue Auffassung tiber die Ursachen des Dicken-
wachstums. Naturw. Zeit. Forst-u. Landwirts. 11:241-79. 1913.

40 Wisconsin Academy of Sciences, Arts, and Letters.

of radial growth, and besides, he holds that the distribution of
growth on a tree-trunk having concentric rings could not
conceivably be dependent upon wind action. From the meas-
urements and calculations it is concluded, however, that tree-
trunks are shafts of equal water conductance throughout. From
insufficient data and non-convinecing arguments it is concluded
dien-mentioned above; though targer than necessary for the-wind-
that the diameter of tree trunks above and below.the 5 to 9 m. por-
tion mentioned above, though larger than necessary for the wind-
gravity hypothesis are of just the size required of a shaft of
equal water conductance throughout. The morphogenic power
of the water current is thought to be proportional to the rate of
metabolism and transpiration. The rate of cambial division is
held to depend upon and be controlled by turgidity, and the in-
fluence of the environment is thought to affect radial growth
chiefly through the transpiration stream. In the calculation up-
on which this hypothesis is founded it was assumed that the
water conduction is confined to the outermost ring or wood
sheath.

This hypothesis has some defects in common with the one it is
supposed to supplant in that the distribution of radial growth is
assumed to be controlled chiefly by one factor, other factors be-
ing ‘effective only in so far as the basic one is influenced. Jac-
card has many difficult problems to solve before his hypothesis
to account for the actual distribution of radial growth in trees
can be considered a theory. The relation of the first radial
growth and its distribution in trees to the transpiration stream
in cases where such growth precedes actual unfolding of the
leaves will need to be explained in the promised detailed study
he is to publish in a future paper. Nor is it permissible to as-
sume as a fact that the water current is confined to the outer-
most ring of wood, especially when it is recalled that in certain
portions of trunks radial growth may be wholly omitted during
a number of successive years, and that many cases of girdling are
also on record in which trees operated on vegetated and fruited
normally during several years.

Wieler’® concluded that practically all water is conducted in

76 Wieler, A. Ueber den Antheil des secundaren Holzes der dicotyle-
donen Gew4dchse an der Saftleitung und tiber die Bedeutung der Anas-
tomosen fiir die Wasser-versorgung der transpirirenden Flichen. Jahrb.
Wiss. Bot. 19: 82-137. 1888.

Grossenbacher—Radial Growth in Trees. 41

the last ring but in a more recent study Jahn” made it appear
that the entire alburnum may be more or less active in water
conduction although perhaps as much as half or more of the
water is thought to be carried up the last ring.

Some evidence of the fact that wind is both a formative and
a limiting factor in plant growth is afforded by several scat-
tered papers on the influence of wind on vegetation, a few of
which might be briefly noted in this connection.

While making an experimental study of the effects of wind
on vegetation Bernbeck’® obtained some interesting results.
He found that both shoots and leaves of plants subjected to
wind of 14 m. or less per second were injured in proportion to
the amount of swaying and bending induced and that even del-
icate leaves of shade plants are not injured by the wind if they
are firmly held to prevent swaying or bending during the ex-
posure. It was found that the production of organic food was
reduced in leaves exposed to wind as compared to that accum-
ulating in protected leaves.

Gilchrist’? reported that potted plants of Helianthus annuus
subjected to artificial wind swaying and rocking did not grow
as tall as the checks while the diameter of their stems exceeded
that of the check plants. Some more recent observations by
Cavara®® show a similar effect of wind exposure on the struc-
ture of Iresine, Coleus, Aster, Zinnia, and Sempervivum.

Esbjerg** found that protecting various herbaceous plants
from strong winds by means of screens resulted in an increased
yield. An increase of 16 to 31% above that of the checks was
secured in the yield of grain from rye; the yield of ruta-baga
roots was increased from 7 to 17% and of mangels from 3 to
18%, while clovers and grasses showed a gain of from 4 to 23%
as a result of wind protection.

mae

7 Jahn, E. Holz und Mark an den Grenzen der Jahrestriebe. Bot.
Centbl. 59:257-67; 321-29; 356-62. 1894.

78 Bernbeck, O. Der Wind als pflanzen-pathologischer Faktor. In-
augural Dissert. Bonn. 1907. pp. 116.

7 Gilchrist, M. Effect of swaying by the wind on the formation of
mechanical tissue. Report Mich. Acad. Sc. 10:45. 1908.

80 Cavara, F. Some investigations on the action of wind on plant
growth. Expt. Sta. Record. 25:224-25. 1912.

81 HWsbjerg, N. Experiments with windbreaks. Expt. Sta. Record
23:435. 1910.

42 Wisconsin Academy of Sciences, Arts, and Letters.

Similar facts are also reported by Waldron® from North
Dakota. While from Porto Rico®* we learn that the northeast
wind prevailing there causes citrus trees to grow slowly and
one-sided in unprotected places; the bark looks dead and the
new shoots are variously twisted. A case is cited where two
similarly planted citrus groves are located across the road from
each other but one is protected by a windbreak while the other
is fully exposed. The trees had all been set three years and
were bearing in the protected grove while in the exposed one
they looked as though they ‘‘had just been set.’’ Wind-exposed
trees were also found heavily infested by scale-insects while the
protected ones were practically free from the pest.

In a very recent paper* it is stated that the wind induces
dwarfing and the rosette habit, although the structural modifi-
cations are attributed to excessive transpiration.

A like conclusion was recently also drawn by Choux.®* He
found that the stems of Neptunia prostrata and of Ipomea rep-
tans grown during the tropical dry season were not only smaller
but that their vascular systems were much more strongly devel-
oped than in those produced during the wet season. Starch was
abundant in the dry season plants and practically absent from
those grown in the wet season.

The hypothesis advanced by Schwendener and subsequently
elaborated by Metzger and Schwarz and the more recent one by
Jaccard are so simple and imbued with such insidious directness
that they are fascinating although not wholly convincing. <Af-
ter making a brief survey of the observations and experiments
by Jost, Lutz, Fabricius, Rubner, etc., it seems as though the
occurrence and distribution of radial growth could not be de-
pendent on a single factor. It appears for instance that the
distribution of elaborated food must in part at least depend upon
its place of manufacture and on the channels of its transport,
especially when the amount available is somewhat below the

Waldron, C. B. Windbreaks and hedges. N. Dk. Agrl. Expt. Sta.
Bul. 88. 1910. pp. 11.

88 Tower, W. V. Insects injurious to citrus fruits and methods for
combating them. Porto Rico Agrl. Expt. Sta. Bul. 10:16-20; 35. 1911.

®* Kroll, G. H. Wind und Pflanzenwelt. Beihefte Bot. Centralbl.
30 Abt. 1:122-40. 1913.

85 Choux, P. De Vinfluence de l’humidité et de la sécheresse sur la
structure anatomique de deux plantes tropicales. Rev. Gen. Bot. 25:
153-72. 1913.

Grossenbacher—Radial Growth in Trees. 43

actual needs. On the other hand from the work of both Jost
and Lutz it is also evident that the presence of food, transpira-
tion current and suitable environment alone do not result in
radial growth when no developing buds or shoots are present;
1. e., cambial activity seems somehow to be dependent upon
elongation growth or some enzyme activated or produced by it.
The determinations by Fabricius, however, have made it ap-
parent that the distribution of reserve food in tree-trunks seems
to be in accordance with some unknown law, which brings about
maxima and minima of food storage in more or less definitely
alternating regions. The marked differences in the amounts of
reserve food in the regions of maxima and minima could not be
attributed to differences in the storage capacity of the regions
for such differences would have been noted, nor to the distribu-
tion of the branches because the wave-like succession of maxima
and minima also occurred and it was usually most marked on
the branchless portion of trunks. There is some indirect evi-~
dence to be had from the cited papers which tends to show that
the places of the inception and longest duration of radial growth _
in a general way are the places of maximum food storage, and
therefore gives support to Mer’s** contention to the effect that
cadial growth begins first where most food is stored and is most _
active and persists longest in such regions. The Schwendener-—
Metzger-Schwarz hypothesis suggests another way out of the
difficulty by its assumption that wind action is responsible for
the distribution of both metabolized food antl radial growth.
But we cannot admit the far-reaching claim of these investiga-
tors that wind and gravity are the only formative factors con-
cerned in the distribution of radial growth especially since light
and transpiration hare been shown to be powerful formative

agents.
t

OBSERVATIONS ON THE DISTRIBUTION OF LATE RADIAL GROWTH ON
FRUIT TREES.

While studying crown-rot of fruit trees during a series of
years, I found that the initial bark injuries which afterwards
result in the disease usually cecurred in places at the base of

86 Mer, ©. Sur les causes de variation de la densite’ des bois. Bul.
Soc. Bot. France. 39: 95-105. 1892.

44 Wisconsin Academy of Sciences, Arts, and Letters.

tree trunks where radial growth continues late in fall. The
observations made to determine the distribution of late radial
growth showed that it is very irregularly distributed, yet that
when it occurs it is confined to certain parts of trees. Crane-
field’? has called attention to the general variation of radial
growth in branches. After two seasons observations he con-
cluded ‘‘that a wide difference existed between trees of the same
variety, age and external appearance, and that the difference
was often greater between different branches of one tree than
between different trees.’? In 1899 he found that the bark
peeled readily on all branches of apple, pear, plum and cherry
as late as August 15, and after that date the bark still peeled
easily for some time on the larger branches. In 1900 the bark
of branches over 1 em. in diameter slipped easily enough to make .
whistles as late as September 15, while two weeks later it would
not peel from any of the branches.

Although observations like these of Cranefield show that
marked variations may occur in the distribution of the last ra-
dial growth, it is apparent that its actual variation can only be
determined by much more detailed examinations at numerous
points not only of any one tree but of any one branch. Some
of the above cited observations on the general distribution of
radial growth and more especially those on excentric growth also
suggest the inference that late growth is often very irregularly
distributed and that it is perhaps frequently confined to regions
of trunks and branches where excentric growth occurs. In a
general way that represents the distribution of the late growth
occurring in fruit trees.

Radial growth in apple and other fruit trees was most com-
monly found to continue latest in fall around the base of the
trunk and its upper roots as well as about the bases of branches
and around crotches; but in some cases other regions also under-
went late growth. The distribution of late growth about the
base of the trunk is apparently subject to many variations de-
pending upon the place of origin of the large upper roots as
well as on the size of the top. Usually the last growth occurs
on the ridges of the roots approximately in the center of the
rounded angle a root makes with the trunk, although in some

87 Cranefield, F. Duration of the growth period in fruit trees. Wisc.
Agrl. Expt. Sta. Ann, Rpt. 17:300-8. 1900.

Grossenbacher—Radial Growth in Trees. 45

cases it was found to oceur equally late in the upward extension
of such a root-ridge on the trunk. Again, in some instances in
which trees had only two large lateral roots making a rather
narrow angle with each other, very late growth was found to oc-
eur in the valley-like angle between them. From an earlier pa-
per®® on crown-rot and the papers cited there it is interesting
to notice that the distribution of that disease on fruit trees con-
forms fairly closely to the distribution of late radial-growth oc-
curring at the: root-crown region. It was found that in cases
where only a part of the bark was affected it was confined to the
upper angles of lateral roots, or to the very deep angles between
two large laterals.

Pruning fruit trees very heavily often results in a decided re-
duction in the thickness of the next annual ring toward the base
of the trunk. This was found by pruning some fruit trees in
one of the seedling apple orchards of the New York State Agri-
cultural Experiment Station in early spring of 1912. The ra-
dial growth on the lower part of such heavily pruned trees also
continued several weeks later than it did on nearby checks.*

The result seems to agree with those obtained by Jost, Lutz,
and Kiihns®® in that a reduction of the foliage beyond a certain
amount resulted in greatly reducing growth toward the base of
the stem.

As stated above observations made regarding the occurrence of
crown-rot on fruit trees seemed also to show a possible relation
of that disease to the distribution of late growth. Some New
York apple orchards may be used to illustrate this relation. In
one instance” two varieties almost equally susceptible to crown-
rot were grown side by side and received the same treatment ex-
cept that the Baldwin variety was pruned up high while the
other or Ben Davis variety was allowed to grow largely unpruned
and therefore low headed. The Ben Davis trees had been set
for fillers and were not deemed worth the care bestowed on the

*§Crown-rot, arsenical poisoning and winter-injury. N. Y. State
Agrl. Expt. Sta. Tech. Bul. 12:389-94. 1909.

*The writer wishes to thank G. H .Howe of that station for having
the pruning done, and R. Wellington, now of the Minnesota Experiment
Station, for making some of the collections of specimens from these
trees into killing fluids.

hal Bag

°° Crown-rot of fruit trees: field studies. N. Y. State Agrl. Expt. Sta.
Tech. Bul. 23:18-20, 46, and plate 7. 1912.

46 Wisconsin Academy of Sciences, Arts, and Letters.

other variety since they were to be removed after the Baldwins
had attained some size. Nearly all of the Baldwin trees had the
bark injured about a decimeter above ground during the winter
of 1910-11, and over 80% had practically entire girdles of loos-
ened or injured bark so that they had become worthless, while
none of the low headed Ben Davis trees were affected. In an-
other case*! bark injury resulted high up the trunks of bearing
trees after a severe pruning.

It was also found that radial growth is often very late in thick
callus rolls about old cankers and sometimes on the under side,
or on the concave side of crooks in horizontal branches. The
bases of water sprouts or adventitious ascending shoots that arise
on the larger branches of excessively pruned young apple trees
also undergo very late radial growth and apparently for that
reason are winter-injured in those regions; as in some cases dis-
cussed on pages 40 to 42 of the above cited paper on crown-rot.
Very similar observations regarding the distribution of winter-
injury in the bark of trees had been made by Nérdlinger.®? He
also assumed that such places are injured because of their late
growth.

The reasons for the occurrence of late radial growth at certain
places on trees are doubtless the same as those underlying the
general distribution of excentric growth, and have not been fully
determined as yet. It seems, however, that the re-distribution of
bark pressure incident to radial growth, the distribution of
elaborated food, the location of the channels for water conduc-
tion, and the gravity-wind pressure effects advocated as factors
which regulate the distribution of radial growth, may afford at
least a partial explanation of the localization of late growth after
they have been submitted to a more careful quantitative study.

WHAT CAUSES RADIAL GROWTH TO APPEAR AS ‘‘ANNUAL’’ RINGS.

The general distribution of radial growth in trees has also an
indirect relation to the development of ‘‘annual’’ rings in that
the proportion of spring and summer wood of a ring at any level
of a stem is doubtless dependent upon the comparative distribu-

1). ¢. p. 24-27.
» Nérdlinger, H. Die September-Fréste 1877 und der Astwurzel-

schaden (Astwurzelkrebs) an Baumen. Centbl. Gesam. Forstw. 4:489-
90. 1878.

Grossenbacher—Radial Growth in Trees. 47

tion and duration of growth, in the early and late season, over
the different parts of a tree. That is, if in any particular re-
gion of a trunk radial growth starts very early in spring and
continues rapidly to the end of the spring-growth period a con-
siderable layer of spring wood will occur in that region; while
if spring growth starts late, proceeds slowly and stops rather
early the thickness of spring wood would be slight. If the dis-
tribution of summer growth is such as to add but little to a re-
gion where spring growth had been heavy and much where
spring growth had been slight, the rings resulting in the two
regions would have a very different appearance. To continue
the illustration further, if for some reason radial growth failed
to occur in certain parts of a tree-trunk until after the produc-
tion of summer wood had begun such parts would show only
small-lumened, thick-walled cells in the ring; while had the sum-
mer growth been eliminated in regions where spring growth oc-
curred: the resulting ring would consist of spring wood only.
From the papers cited above on the distribution of radial growth
it is evident that all the cases illustrated here do actually occur
even in the extreme forms used in the last illustration. It is ap-
parent, therefore, that in some environments and especially on
certain parts of trees the distribution of radial growth may have
a marked influence not only on the type of the resulting ring
but even on the nature of the wood in such portions of stems.
This evident relation between the seasonal distribution of radial
growth on a tree to the type of wood ring to be produced has
reseived practically no attention, although in von Mohl’s® paper
on the anatomy of roots it is noted that rings with only the spring
type of wood seem to result owing to the entire omission of the
summer growth; while Sanio®™* suggested a similar idea regard-
ing the absence of spring growth in parts of some rings of a
dwarfed Fraxinus grown ina swamp. Lutz® also noted the ab-
sence of summer wood in a pine, from which the buds had been
removed in March, the little growth that occurred was spring
wood. When the wood of roots or stems grown in certain en-
vironments consist largely of so-called spring wood, elaborate
explanations are usually manufactured to show that the high

48 Wisconsin Academy of Sciences, Arts, and Letters.

water requirements of such habitats induce the formation of
large vessels throughout the wood for the conduction of the
water needed. This may be typically illustrated by a paper of
von Lazniewski*** on alpine plans in which attention is called to
the fact that the rings in mountain willows are much thinner and
have a greater proportion of vessels per ring than those in trees
of the same species grown in the valleys. Yet it was noted that
the outer parts of the wood rings were usually only partially
lignified, indicating that radial growth had been prematurely
checked. The excessive number of vessels per ring of the alpine
trees was interpreted as being due to the greater demands for
water on the mountains, while the probable fact that the sum-
mer-wood portion of the rings had perhaps been wholly elimi-
nated by the environment was not even mentioned. Practically
the same observations although on a larger scale were made by
Rosenthal®* in a later paper and the conclusion was drawn that
the larger number of vessels per unit area of cross section in
willows grown on the mountains is an adaptation to a higher
transpiration rate.

A number of hypotheses have been elaborated in an endeavor
to explain ‘‘annual’’ rings, and more or less data has been col-
lected by their supporters to substantiate them but with indiffer-
ent success as judged by Krabbe®’, who some years after publish-
ing hig last researches on the subject, maintained that ring
formation cannot be satisfactorily explained with our present
knowledge of the factors determining the size differentiating
cells attain in different parts of the growing season, and of
the ones regulating the thickness of cell walls in different parts
of the rings.

It was recently pointed out by Klebs®* that periodicity in
plant growth occurs in all regions of the world having a periodic
climate, and that the dormant periods coincide with the cold pe-
riods of temperate climates and with the dry periods of the
tropics. He noted too, that some trees have partial and irregu-

%* Lazniewski, von, W. Beitrage zur Biologie der Alpenpflanzen.
Flora, 82:224-67. 1896.

* Rosenthal, M. Ueber die Aushildung der Jahresringe an der Grenze
des Baumwuchses in den Alpen. Inaug. Dissertation. Berlin. pp. 24.
1904.

®7 Krabbe, G. Einige Anmerkungen zu den neusten Erklarungsver-
suchen der Jahringbildung. Ber. Deut. Bot. Ges. 5:222-32. 1887.

hada ae ce

Grossenbacher—Radial Growth in Trees. 49

lar periodicity even in regions of the tropics having what ap-
pears to be a practically non-periodic climate.

In central Uruguay®* where the temperature never goes much
below freezing and where late summer is a dry season, some trees
have distinct yearly wood-rings, while in others more than one
ring is produced in a year. Robinia Pseudacacia and Melia
azedarach have fairly evident annual zones, but they also have
imperfect secondary zones due to a concentric arrangement of
large vessels. In Acacia the yearly zonation is less distinct but
the last wood is usually made up of cells with a reduced radial
diameter.

The measurements by Hall? show that the trunks of trees in
Uruguay usually increase in circumference during nearly ten
months of the year, and that in some cases they even increased
during the months of May and June (winter). He found, how-
ever, that the circumference of most trees decreased more or less
during winter, the deciduous trees more noticeably than the ever-
greens. Ursprung?™ found that a number of the evergreen trees
and shrubs of a tropical locality without any appreciable peri-
odicity of climate showed a zonation in cross sections of the stems
without the presence of any evident histological difference in the
wood of the different parts of zones. Some of these species are
said to become deciduous in localities having a periodicity in the
water supply with the result that the zonation of their wood be-
comes more marked. Holtermann’” also studied the relation of
climate to radial growth in the tropics and came to the conclu-
sion that the formation of growth rings in the wood is intimately
connected with the occurrence of periods of markedly different
transpiration rates, and that the larger vessels are developed to
meet the demands of increased transpiration. He holds that
tropical trees growing in a saturated atmosphere most of the.
time have no indication of zonation in the wood even though they

°° Christison, D. On the difficulty of ascertaining the age of certain
species of trees in Uruguay, from the number of rings. Trans. Bot. Soc.
Edinburgh. 18:447-55. 1891.
_ 300 Hall, C. E. Notes on the measurements, made monthly at San
Jorge, Uruguay, from January 12, 1885, to January 12, 1890. Trans.
Bot. Soc. Edinburgh. 18:456-68. 1891.

208 Lt

‘2 Holtermann, C. Der Hinfluss des Klimas auf den Bau der Pflan-
zengewiacbe. Anatomisch Physiologische Untersuchungen in den
Tropen. pp. 249. 1907. Leipzig.

4—S. A.

50 Wisconsin Academy of Sciences, Arts, and Letters.

are deciduous like some species of Leguminosae, Guttifereae and
Ficus. On the other hand it is noted that a seven-year-old tree
of Theobroma Cacao had developed 22 radial-growth rings, and
since it cast its leaves three times a year it is evident that the
number of rings corresponded with the vegetative seasons of the
tree. The real cause of zonation is thought to be an inherent
characteristic of a plant though the environment induces its
manifestation. .

According to Dingler’ leaf-fall is more dependent on the age
of the leaves than on the environment, for by cutting back decid-
uous trees in Ceylon some time before the normal period of leaf-
fall the new crop of leaves which immediately came out was re-
tained throughout the dormant season which is dry and very hot.
Unfortunately the effect upon radial growth was not noted but
from evidence given above it seems very likely that the periodic-
ity of radial growth always follows foliar periodicity in decid-
uous trees whether natural or induced.

In another paper he’ reported that the folier periodicity of
European fruit and forest trees grown in the highlands of Cey-
lon is very irregular even in different branches of individual
trees. In late October the trees of Quercus pedunculata could
be divided into five classes in regard to the condition of their
foliage, ranging all the way from cases in which chiefly old
spotted leaves were present (though some scattered buds were
swelling) to instances where no old leaves were present and the
new shoots occurred in all stages of elongation, although most of
them were full grown. Quercus Cerris had a more uniform
periodicity. In late October all trees bore two generations of
leaves: the old ones hard and spotted, althought still green, and
the young ones not yet full grown. In late November the old
leaves had practically all fallen and the new elongation growth
had been completed. European pears, peaches, cherries, plums
and apples were found to have practically the same periodicity,
producing two crops of leaves and flowers, though but one crop
of fruit per year. The trees are often almost leafless some time

108 Ningler, H. Versuche tiber die Periodizitaét einiger Holzgewichse
in den Tropen. Sitzungsber. Math.-Physical. Kl. Kgl. Bayer. Akad.
Wiss. Miinchen. 1911:127-43. 1911. ‘

14 Dingler, H. Wher Periodizitaét sommergriiner Baume Mittele-
uropas im Gebirgesklima Ceylons. Sitzungsber. Math.-Physical. KI.
Kgl. Bayer. Akad. Wiss. Miinchen. 1911:217-47. 1911.

Grossenbacher—Radial Growth in Trees. 51

in February or March. All stages of bud and leaf are said
usually to occur in these trees.

An experiment similar to that performed by Dingter had pre-
viously been made by Wright? in Ceylon. He lopped trees of
Mangifera indica and Terminalia Catappa in May and new
leaves developed from July to September, with the result that
no new leaves were produced on these trees in February and
March when others of those species developed new crops of
leaves. Some of the plants develop new leaves once or twice and
others several times annually, and immature leaves may be found
during every month of the year. Only a comparatively small
percentage of the Ceylon trees are said to be deciduous. Some
rapidly growing species were found to become defoliated at the
end of the first year and others at the end of the second; while
the more slowly growing ones may vegetate as evergreens until
the close of the fifth or sixth year before losing their leaves.
Usually, after a tree has once lost its leaves it loses them annually
but some species are deciduous only in youth and become ever-
green later. Some of the so-called evergreen trees are said to
also lose all the leaves in occasional years before the new crop ap-
pears. In some species periods of sparse foliation occur two or
three times per year and in others the foliage is more copious
on alternate years. It is held that the absence of any very
marked periodicity in the environment permits some plants to
follow their inherent periodicity of growth, while the annual
variation in the transpiration rate and atmospheric moisture are
thought to be the cause of the deciduous habit of others.

These observations on foliar periodicity by Dingler, Wright
and others seem to show that Dingler may be correct in his con-
tention that leaf-fall is more dependent upon the normal dura-
tion of life of the leaves than upon the environment. However,
if that should prove to be a fact, it would necessarily follow that
certain plants are deciduous not because of the leaf-fall but on
account of the failure of a new crop of leaves to develop before
the old ones drop. Such a view centers attention upon the causes
inhibiting growth rather than upon the causes of leaf-fall in the
study of periodicity, a method of attack adopted by Klebs in the
paper cited above.

106 Wright, H. Foliar periodicity of endemic and indigenous trees in
Ceylon. Ann. Roy. Bot. Gard. Peradeniya 2:415-516. 1905.

52 Wisconsin Academy of Sciences, Arts, and Letters.

It seems then that although trees having annual or more prop-
erly radial-growth rings are distributed all over the arborescent
world, one or more factors of their envionment must be effective
periodically in order that marked zonation occur. The more or
less regular recurrence of cold or dry seasons are the factors
usually noted in connection with periodically recurrent vegeta-
tive seasons, but doubtless any other recurrent environmental
factor influencing growth may also affect zonation, e. g., periodic
variation in the supply of inorganic foods as was suggested by
Klebs.?°* It should be noted, however, that wood zonations re-
sulting from recurrent dry periods of the tropics even in decid-
uous trees are not as marked as those occurring in temperate
zones where the dormant period is chiefly due to seasonal varia-
tions in the temperature and where consequently a greater sea-
sonal change occurs in the bark pressure.

The causes of the formation of radial-growth rings have been
studied mainly in the north temperate zone and, therefore, ex-
planations are largely. based on the environmental factors that
seem to be operative in that region. Seasonal changes in bark
pressure, in the supply of metabolized food to the cambium, and
in the rate of transpiration have been either separately or in
partial combination advanced as explanations for the occurrence .
of the large-celled spring-wood alternating with small-celled
summer-wood.

The bark-pressure hypothesis:—Sachs'™ seems to have been
the first to suggest that the difference between spring and sum-
mer wood may be due to a difference in the hark tension or pres-
sure obtaining in spring and summer. The idea was then tested
experimentally by de Vries?*® with the result that Sachs’ hy-
pothesis seemed to have been sustained. The experiments by de
Vries consisted in making some longitudinal slits in the outer
bark of various trees in spring and of applying ligatures to the
stems of others. On the following winter it was found that only
about one-half as many cells had been produced under the liga-
tures as occurred on other parts of the past season’s ring; while
in the regions where the outer bark had been slit the number of

08]. @: ;

197 Sachs, von, F. G. J. Lehrbuch der Botanik. 1. Aufl. 1868, p. 409.

108 Vries, de, H. Ueber den Hinfluss des Rindendruckes auf den ana-
tomischen Bau des Holzes. Vorlaiufige Mitlheilung. Flora. 33:97-102.
1875.

Grossenbacher—Radial Growth in Trees. 53

cells had become two to three times that produced in the normal
portions of the ring. Similar experiments also showed that the
amount of radial and tangential growth of cells differentiating
from the cambium is inversely proportional to the pressure ex-
erted on them. It also seemed that pressure acts as a selecting
agent in determining the proportion of vessels to wood fibers;
i. e. the greater the pressure the fewer the vessels and the more
numerous the wood fibers to be produced. De Vries concluded
therefore that bark pressure influences the rate of cambial di-
vision as well as the relative size cells may attain during differ-
entiation. Since bark-growth follows the enlargement of the
wood cylinder it was thought evident that bark pressure is
greater toward the end of the radial-growth period than at its
beginning. For these reasons de Vries held that a seasonal
change in bark pressure is the chief cause of seasonal growth ap-
pearing as ‘‘annual’’ rings.

In some later experiments, while studying wound wood, he?
found on lifting loose strips of bark with a knife on the concave
side of young tree-trunks held in a bent position, and then tying
it in place again in such a way as to prevent evaporation, that
‘numerous large vessels developed in the new wood produced un-
der the strips. He reiterated his former conclusion that bark
pressure is an important factor in determining the size of wood
cells and that it is largely ‘responsible for the difference between
spring and summer wood.

That bark tension does occur on enlarging stem structures had
been shown by Kraus’ as well as by Nordlinger*® but neither
of them secured quantitative results of value.

The influence of pressure on cambial activity and cell differ-
entiation have since been investigated from various viewpoints
and have led to different conclusions. Héhnei' found sharp-
angled transverse displacements in the bast fibers of many
Dicots at points where neighboring cells make an abrupt uneven

10° Vries, de, H. Ueber Wundholz. Flora. 34:2-8; 17-25; 38-45;
49-55; 81-88; 97-108; 113-21; 129-39. 1876.

109 Kraus, G. Die Gewebespannung des Stammes und ihre Folgen.
Bot. Zeit. 25:105-19; 121-33; 137-42. 1867.

0 Nérdlinger, H. Spannt die Baumrinde im Sommer nicht? Kritische
Blat. Forst-u. Jagdwiss. 52:(1):253-55. 1870.

131 Héhnel, von, F. Ueber den Hinfluss des Rindendruckes auf die
Beschaffenheit der Bastfasern der Dicotylen. Jahrb. Wiss. Bot.
15:311-26. 1884. -

54 Wisconsin Academy of Sciences, Arts, and Letters.

joint. Such transverse displacements or sharp double-bends
were found in about two-thirds of the fifty to sixty species ex-
amined. They were especially prevalent in Urticaceae, Apocy-
naceae, Asclepidaceae, Linaceae, etc., while in other families the
double-bends occurred only in certain genera. None were found
in the Rosaceae including the pomaceous group, nor in the Tilia-
ceae and Cupuliferae.

It was held that the sharp bends are due to bark pressure, as
indicated by the fact that in the plants in which these bends
commonly occur the bast-fibers are but slightly. or not at all
lignified. Héhnel held that if the double bends were not due to
growth or bark pressure they would not always appear at points
in the fibers where joints or breaks occur in the celis of the sur-
rounding tissues. The failure of the bends to become evident
until after the tissues are fully differentiated was taken to indi-
cate that bark-pressure becomes greater during the latter part of
the differentiation period. It also seemed that in case of Urtica,
Cannabis and Linum the bark pressure was often greater in the
lower part of the stem than above, for the angular bends were
frequently present on the fibers of the lower part while none oc-
curred in the upper. The transverse displacements were found
to be made up of two successive sharp bends which were notice-
able in all layers of the wall. In many cases some of the layers
were actually ruptured.

Krabbe’? made extensive studies of bark pressure and tried
to obtain some quantitative measurements. He increased bark
pressure by encircling tree-trunks with a chain much like that
now used on bicycles, except that it was wider. One end of the
chain was fixed to an iron peg driven into the tree and the other
ran over a pulley and had a weight pan attached. A piece of tin
a little wider than the chain was placed about the trunk under
the chain to distribute the pressure more evenly and to reduce
friction. Weights were put into the pans in accordance with the
determinations of bark pressure obtained before, and it was
found that the bark pressure had to be doubled and even quad-
rupled before any influence on the size of the cells or the thick-
ness of the yearly growth became evident.

112 Krabbe, G. tber die Beziehung der Rindenspannung zur Bildung
der Jahrringe und zur Ablenkung der Markstrahlen. Sitzungsber.
Akad. Wiss. Berlin 1882: 1093-1143. 1882. ;

Grossenbacher—Radial Growth in Trees. 5b

The ‘‘normal’’ bark pressure was determined by stretching
rings of bark over a smooth cylinder by means of weights until
the bark had attained the length it had while still attached to
the tree. In his later work" the rings of bark were straightened
out and weighted at one end to determine the force required to
stretch the bark to its former length, for it was found that the
results obtained in this way were the same as those gotten with
the more elaborate apparatus. The bark pressure of conifers was ~
found to be usually under one-half an atmosphere and that
of broad-leaved trees about twice as great. In case of conifers
the pressure seemed to increase in fall on an average about
0.8 gm. per square millimeter of cross section, while the average
of similar measurements on a number of broad-leaved trees indi-
cated a decrease of pressure in fall equal to 12.5 gm. per square
millimeter of cross section. He maintained that the Breaking
strain of bark is never reached by growth pressure. Bark pres- ~
sure was found greatest in regions of most rapid radial growth,
for instance on the side of excentric stems with the longer radius.

By using préssures from five to eight atmospheres the sum-
mer-wood type of radial growth was induced in spring on trees
having comparatively little difference in the size of spring and
summer-wood cells, while on trees having very marked differ-
ences between spring and summer wood it was practically impos-
sible to induce the formation of the summer-size of cells in
spring by increasing the bark pressure. In reducing the bark
pressure by means of longitudinal slits in the outer bark in sum-
mer, typical spring wood vessels developed in trees which nor-
mally have only a slight difference between size of spring and
summer wood cells; but in trees like Quereus and Fraxinus in
which a marked difference occurs between spring and summer
wood, the spring wood vessels could not be thus induced.
Krabbe therefore concluded that bark pressure remains practi-
cally the same throughout the growing season and that changes
in bark pressure could not be the cause of ring formation be-
cause it requires such a great increase to influence the size of the
wood cells.

u8 Uber das Wachsthum des Verdickungsringes und der jungen Holz-
zellen in seiner Abhangigkeit von Druckwirkungen. Abhandl. Kgl.
Akad. Wiss. Berlin. 1884, Anhang.1:1-80. 1885.

56 Wisconsin Academy of Sciences, Arts, and Letters.

Gehmacher™‘ also performed some experiments in the increase
and decrease of bark pressure on three to six-year-old trees and
shrubs. The outer cortex was slit in February and nearby on
the same stem a ligature of tightly wound wire was applied and
the stem allowed to grow until the end of the season.

The number of cork cells varied inversely as the pressure and
their radial diameter was decreased by 11% under increased bark
pressure, while under reduced pressure an increase of 13% above
normal resulted. <A similar effect was noted on the cortical
parenchyma cells except that both the radial and tangential di-
ameters were decreased under increased pressure and the inter-
cellular spaces were obliterated, while under reduced pressure
the cells became globular and the intercellular spaces were in-
creased in size above the normal. The difference between the
thickness of the cortical parenchyma under increased and that
under decreased pressure was enormous. In the wood the num-
ber of fibers increased and that of vessels decreased under added
pressure, while the number of bast fibers was greatly reduced by
increased pressure. Gemacher’s conclusion was that it does not
require the enormous differences of bark tension to influence the
size of wood cells as had been maintained by Krabbe.

Hoffman" also investigated the influence of pressure on cell
division and differentiation in the cambium of trees and con-
cluded that the forces which contribute to the development of
cylindrical stems rather than some other form are (1) bark ten--
sion and the consequent bark pressure, (2) radial-growth pres-
sure, and (8) the passive resistance of the wood. Cambial di-
vision and growth are said to continue only as long as growth
pressure exceeds bark pressure and it is thought that if bark
pressure is equal on all sides the axis must either be or soon will
become cylindrical on occurrence of continued radial growth.
This is shown by the fact that angular young shoots become
cylindrical on growing older. Even when the tension of the
bark is the same all around a branch bark pressure may be dif-
ferent at different points, being considerable at prominences and

4144 Gehmacher, A. Untersuchungen tiber den Hinfluss des Rinden-
druckes auf das Wachstum und den Bau der Rinden. Stizungsber. K.
Akad. Wiss. Wien. 88 Abt. 1:878-96. 1884.

118 Hoffman, R. Untersuchungen tiber die Wirkung mechanischer
Krafte auf die Teilung, Anordnung und Ausbildung der Zellen beim
Aufbau des Stammes der Laub- und Nadelhélzer. Inaug. Dissertation.
Berlin. 1885. pp. 24.

Grossenbacher—Radial Growth in Trees. 57

perhaps zero or even negative in depressions. Among the nu-
merous angular young twigs examined the greater pressure at
the angles did not prevent the development of normal spring
wood, but larger numbers of both spring and summer wood cells
were produced in the depressions than on the ridges until the
twig became cylindrical.

_ It was found that when a tree-trunk or branch presses against
some non-yielding object or the bases of the component branches
of a forked stem press against each other, radial growth is re-
duced on the side of contact when the pressure has reached a
certain intensity and that the rays spread outward and eventu-
ally became parallel to the obstructing surface. The continu-
ance of radial growth tends to separate or pull apart the com-
ponents of a forked stem or widen the upper angle a branch
makes with its axis. Branches thus firmly pressed against each
other eventually fuse and the rays then come to radiate from
the common center and further radial growth tends to result
in a cylindrical, united structure. It was found that the callus
developing at the cut end of a twig in water also conformed to
the general law of the mechanics of radial growth in that its
cross sections become semicircular with a rough outline; but the
surface becomes smooth as tension is developed by further
growth. When a rectangular piece of bark was cut from a tree
the first division of the cambium in the formation of a callus is
said to be by a radial wall or one at right angles to the wall
formed under normal conditions. Further growth and division
was also found to occur in accordance with the resistance to
growth and resulted in a structure having its center at the place
where the first cambial divisions took place. The rays in the
bark on both sides of the piece cut out become diverted not only
by the contraction of the bark at the time the piece was cut but
also by the lack of surface growth in the bark surrounding the
wound. The omission of surface growth is said to be due to the
lack of accustomed tangential pull formerly exercised by the ex-
cised piece. Growth is resumed only after the callus bark has
reached a tension comparable to that of the piece removed. This
resulted in increased radial growth in the entire region in-
fluenced by the wounding, as shown by a count of the number of
cells produced here as compared to that produced in other places.
When the cambium was first freed from its normal bark pres.

58 Wisconsin Academy of Sciences, Arts, and Letters.

sure its cells took on isodiametric forms which were retained un-
til the bark pressure became appreciable again and then reverted
back to the elongate form normal to the species. It is held that
the upper and lower edges of a wound do not produce callus as
copiously as the lateral ones because of the lesser reduction of
bark pressure, and the death of the cut cells which extend some
distance above and below the wound.

From his experiments in which ligatures were applied to
stems Sorauer!® concluded that slow radial growth combined
with high bark pressure results in twisted grain and that a re-
duction of bark pressure below normal not only induces more
cells to form from the cambium, but cells having a greater di-
ameter and a reduced length.

Newcombe? found that when external conditions prevent
growth, the unfinished tissues remain unaltered and thin walled;
that mechanical resistance or pressure prolongs the differentiat-
tion period, the cells remaining smaller and thinner walled.

The occurrence of numerous cocoons of bag-worms on various
species of trees and the fact that the narrow silken bands by
which they are attached to the twigs are often too strong for ra-
dial growth pressure to break, afforded von Schrenk'* an occa-
sion for a study of the effects of excessive pressure on radial
growth. In most cases the silken bands encircling the twigs are
burst early in the summer of the year following the time of the
attachment of the bags. In some instances in which the liga-
tures were too strong to be ruptured by the thickening twigs the
transfer of elaborated food was eventually checked and an en-
largement developed on the distal side of the constricting band.
In other cases the ligature was sufficiently distended by growth
to permit of some food transfer and resulted in the formation of
welts on both sides of bands. In some instances the pairs of
welts fused above the ligatures and reestablished normal connec-
tion and pressure. In arbor vitae the wood fibers of the first

116 Sorauer, P. _Handbuch der Pflanzenkrankheiten. Dritle Auflage.
1:764-66. 1909.

1 Newcombe, F. C. The influence of mechanical resistance on the
development and life-period of cells. Bot. Gaz. 19:149-57; 191-99;
229-36. 1894.

118 Schrenk, von, H. Constriction of twigs by the bag-worm and in-
cident evidence of growth pressure. Ann. Rpt. Mo. Bot. Gard. 17:153-81.
1906.

Grossenbacher—Radial Growth in Trees. 59

year’s growth were often found arranged at right angles to the
axis, under unbroken bands.

In the latter part of the second summer following the attach-
ment of the bags the portion of the twigs distad to the constric-
tion had much starch in the bark rays and pith, while that on
the basad side was practically devoid of it.

In hard-wood trees both bark and wood were found to have
continued growing under unbroken bands though welts developed
on both sides. The first wood cells formed under the ligatures
were normal but those developing afterwards had a shorter ra-
dial diameter and thicker walls than those under normal pres-
sure. The number of vessels appeared to decreaes in proportion
to the pressure. The wood fibers developing under high pres-
sure were found to have their long axis at right angles to the
twig or parallel with the compressing band, and the rays were
bent or buckled laterally unded pressure. It is held that the in-
creased pressure induces the formation of smaller wood cells not
because cambial division occurs before the cells have attained the
normal size but because the pressure hinders their enlargement
during subsequent differentiation.

A large number of tests made to determine the breaking strain
of the bands from both conifers and broad-leaved trees showed it
to be about 40 atmospheres; and, therefore, indicates that
Krabbe’s experimental results showing a growth pressure of 15
atmospheres are too low, since von Schrenk’s observations show
that the majority of the bag-worm ligatures are ruptured by
the enlarging twigs.

An osmotic-pressure hypothesis —In a paper on the devel-
opment of pits in the wood cells of the Abietineae Russow’?®
suggested another explanation of ‘‘annual’’ rings. He claimed
that the bark pressure hypothesis of Sachs which de Vries en-
deavored to support by experiment, cannot account for the oc-
currence of growth rings in the wood because the last phloem
cells of a season do not have a reduced radial diameter and on
account of the fact that two rings may be induced by defoliat-
ing trees. The bark-pressure hypothesis is also held to be dis-
eredited by the occurrence of growth rings in the tropies where

119 Russow, E. tiber die Entwicklung des Hoftiipfels, der Membran
der Holzzellen und des Jahresringes bei den Abietineen, in erster Linie
von Pinus silvestris L., Sitzungsber. Naturfor. Ges. Dorpat 6: 147-57.
1884.

60 Wisconsin Academy of Sciences, Arts, and Letters.

the bark is not distended by low temperature during a dormant
season. In another paper he??° added that in accordance with
the bark-pressure hypothesis the wood cells in roots ought to be
small while as a matter of fact they are large. On the other
hand he held that the changes in the radial diameter of cells
from spring ‘to fall can easily be explained by assuming the
presence in them of highly osmotic substances, which induce a
high hydrostatic pressure and as a result give rise to large cells
in spring, while toward the end of the radial-growth period the
hydrostatic pressure in differentiating cells is reduced owing
to a reduction of the osmotic pressure in them. By using solu-
tions of glycerine as plasmolysing agents Wieler?*? found that
osmotic pressure in herbaceous plants was less than that in the
living wood and ray cells of trees where it ranged from 13 to 21
atmospheres. No difference was found, however, between the
osmotic pressure in differentiating wood vessels and of that in
the cambium cells. He thought that the walls of differentiating
spring-wood cells are more distensible than those of summer
wood owing to their lower cellulose content.

Seasonal variation in the available, elaborated food as the
cause of “‘annual’’ rings:—After years of intimate study of
forest trees Hartig’®? concluded that since radial growth begins
in spring under suboptimal environmental conditions and while
the new leaves are very small or the buds are just bursting, the
nutritive conditions ‘of the cambium must also be suboptimal
and for that reason the spring wood has thin cell walls. As the
season advances the leaves attain full size which in connection
with the accompanying seasonal changes are conducive to the man.
ufacture of the larger quantities of organic foods which, accord-
ing to Hartig, are responsible for the production of the thicker
walled summer-wood cells. It is held that the chief difference be-
tween spring and summer wood consists essentially in the thick-
ness of the cell walls and that the improvement in the nutrition of
the cambium from early spring until the later summer is re-

120 Russow, E. Uber den Inhalt der parenchymatischen Elemente der
Rinde vor und wahrend des Knospenaustriebes und Beginns der Cam-
biumthatigkeit in Stamm und Wurzel der einheimischen Lignosen.
Sitzungsber. Naturfor. Ges. Dorpat. 6: 388-89. 1884.

121 Wieler, A. Beitrage zur Kentniss der Jahresringbildung und des
Dickenwachstums. Jahrb. Wiss. Bot. 18: 70-132. 1887.

12 Hartig, R. Hin Ringlungsversuch. Allgem. Forst-u. Jagd-Zeit.
65: 365-73; 401-410. 1889.

Grossenbacher—Radial Growth in Trees. 61

sponsible for the occurrence of ‘‘annual rings.’’ Hartig stated
however, that the differences in the nutritive conditions cannot
account for the change in radial diameter of wood cells nor for
the presence of the larger proportion of vessels in spring wood,
and maintained that the transpiration current determines their
size. He suggested that the reason so little difference exists in
the radial diameter of spring and summer wood cells of Populus,
Salix, Acer, etc., is to be found in the fact that these trees con-
tinue producing new leaves throughout most of the radial
growth périod' and because they have no duramen. Since the
water current in trees with duramen is necessarily confined to
the outer layers of wood its effects on cells differentiating from
the cambium are thought to be more marked and therefore re-

sult in greater differences in the diameter of spring and summer _

wood cells, e. g. in oaks, etc. According to Hartig, then, ‘‘an-
nual’’ rings are primarily due to the poor nutritive conditions
of the cambium in spring being followed by a period of more
abundant supply of metabolized food in summer, and secondari-
ly to a decrease in the intensity of the transpiration current
toward the end of the radial-growth period.

Wieler’*® came to a diametrically opposed conclusion regard-
ing the differences in the nutritive conditions about the cambium
in spring and summer. He thought that since the character-
istics of ‘‘annual’’ rings lie in the type of wood produced in the
early and late growing season and not in the succession of rings,
the relation of different nutritive conditions to the formation
of spring and summer xylem could be more easily determined
experimentally with herbaceous than with woody plants. This
was deemed permissible owing to the fact that in an examination
of 54 species of herbs belonging to 21 families the characteris-
tic reduction in the size of the xylem cells toward the end of the
growing season as is typical of the ‘‘annual’’ rings of woody
plants, was found in over half of them.

Seedlings of Ricinus communis were set into the soil of one-
fourth to one-half liter pots in spring, well watered and given
optimum light and temperature conditions, but they grew slow-
ly and remained dwarfs. In early summer four of them were
transplanted to the soil in a field and three of them into good
soil in four liter pots. Those remaining in small pots were

bases) laa +

By

62 Wisconsin Academy of Sciences, Arts, and Letters.

only about 27 em. high in January and their stems about 21 mm.
in circumference, while those in four liter pots were about 90
em. high and 50 mm. in circumference. Those transplanted to
the field became large plants with woody stems. Five dwarfed
plants, which were subsequently transplanted to a forcing bed,
had since made a rank growth and were retransplanted to four
liter pots. They wilted but eventually recovered their turgid-
ity, although the older leaves died.

Cross sections showed the xylem cells of the field plants
to be larger and the vessels more numerous than in those re-
tained in the small pots. In the plants transplanted to the field
the xylem cells around the pith were small and were surrounded
by larger ones toward the periphery. In case of those trans-
planted to four liter pots the same inversion of the normal po-
sition of large and small celled xylem occurred, but in addition
the outermost rows again had a much reduced radial diameter.
In the field plants which had been retransplanted to pots the
outermost cells also had a reduced radial diameter and thick
walls while within them was a zone of large, thin-walled cells
which had apparently been formed just before the last trans-
planting and as a result their walls remained unthickened.

_ Similar results were also obtained with Helianthus annuus.
Wieler concluded from these experiments that the abundant
supply of metabolized food to the cambium is the most important
factor in the production of spring wood and that the shortage
of such a food supply induces the formation of summer wood,
and that therefore ‘‘annual’’ rings of trees are due to an abun-
dant supply of organic food to the cambium in spring and a
reduced supply in summer.

Lutz’** was of the opinion that when the food supply to a
rapidly dividing cambium is comparatively low while water is
abundant the cells become large and thin-walled as is charac-
teristic of spring wood, while if the food supply is good and the
water is low the cells become small and thick-walled as in sum-
mer wood.

In a later paper Wieler’®* reiterated his former conclusions
though he admits his inability to prove that the small radial

ie].
3% Wieler, A. Ueber die Abhangigkeit der Jahresringbildung von den
Ernaihrungsverhaltnissen. Allgem. Forst-u. Jagd-Zeit. 67: 82-89. 1891

Grossenbacher—Radial Growth in Trees. 63

diameter of summer-wood cells results from a reduced supply
of food to the cambial region; nevertheless, it is held to be a
more likely contention than that maintained by Hartig to the
effect that summer-wood results from an increase in the supply
of metabolized food.

In this paper “Wieler cited similar experiments by Sachs*”* in
support of his conclusions, although Sachs noted that the fre-
quent addition of abundant nutrient solution failed to induce
more growth in small pots. Sachs held the dwarfing in small
pots to be due to a crowding of the root system into mats in such
a way as to greatly impair their absorptive functions.

The relation of rest and food supply to the production of
wood rings:—Mer’’ held that the winter rest of the cambium
and its consequent great activity in spring in connection with
the abundance of plastic materials at that time are the causes
of the production of large-celled spring wood. The cell walls
of spring wood are thought to remain relatively thin because
the food transfer through such a thick differentiating zone of
cells is comparatively slow, and the thick walls of summer wood
cells are assumed to be due to slow rate of cambial division or
to-the thinness of the differentiating zone and consequent ready
access of organic food to its cells. The sudden and consider-
able decrease in the radial diameter of the peripheral few rows
of wood cells in a year’s growth is held to be due to an arrest
of their development as a result of enfeebled cambial activity
rather than to an increase of bark pressure as maintained by
Sachs, de Vries and others.

A summary and comparison of the hypotheses:—The work
of Kraus, de Vries, Noérdlinger, Detlefsen, von Hohnel, Ge-
macher, Hoffman, Kny, Newcombe, von Schrenk, and Sorauer,
have made it apparent that pressure on the cambium affects the
rate of cell division as well as the size differentiating wood cells
may attain, but owing to the fact that no method has as yet
been developed by means of which quantitative measurements of
bark pressure can be made it is impossible to determine just
what relation bark pressure has to the production of ‘‘annual’’
rings.

126 Sachs, von, F. G. J. Vorlesungen tiber Pflanzenphysiologie. Leip-
zig. 1882. p. 623.
A87 1205

64 Wisconsin Academy of Sciences, Arts, and Letters.

The different degrees of hydrostatic pressure assumed by Rus-
sow as the cause of the difference between spring and summer
wood has apparenty also been implied by Hartig, Mer and
others in speaking of growth force, etc., but even more than in
the former case do the few qualitative tests need to be replaced
by quantitative measurements before the validity of the idea
could be tested.

Hartig has collected a mass of observational and even some
indirect quantitative data that seem to support his hypothesis
that the relative abundance of elaborated food determines the
thickness of cell walls and that the relative intensity of the
transpiration stream determines the length of the radial diam-
eter of wood cells, but the experiments of Jost, Lutz and others
show that although food and water may be present in great
abundance very little or no radial growth occurs when termi-
nal growth is prevented.

Wieler’s hypothesis that the abundance of metabolized food
in the cambial region in spring induces the formation of spring
wood and its reduction, summer wood is also lacking in that it
does not account for the cessation of radial growth on the re-
moval of the elongation structures. Besides, the experiments
with which he assumes to have made his contention probably in-
volved too many unknown variables to afford even a satisfactory
test. of the hypothesis.

The results obtained by Morgulis'”* in his experiments in al-
ternately feeding and starving salamanders tend also to make
one skeptical regarding the value of the hypotheses of both
Hartig and Wieler as explanations of ring formation because
Morgulis found ‘‘That the rate of growth is independent of the
amount of nutrition’’ and that ‘‘The impulse to grow plays the
leading part’’ and ‘‘determines the degree of utilization of the
nutriment.’’ Finally, he found too that ‘‘From all that has
preceded, the conclusion can be drawn that periodic starvation
is more detrimental to the organism than acute starvation fol-
lowed by a liberal supply of food. In the former ease the in-
dividual remains below the level of the normally fed animals;
in the latter case, on the contrary, provided the inanition has

128 Morgulis, S. The influence of protracted and intermittent fasting
upon growth. Amer. Nat. 47: 477-87. 1913.

Grossenbacher—Radial Growth in Trees. 65

not been carried too far, the restorative process may go even be-
yond the limit attainable under normal conditions.”’

Since Hartig laid especial stress on the difference in the thick-
ness of cell walls rather than the size of cells as the essential
difference between spring and summer wood his secondary fac-
tor, the relative intensity of the transpiration current, would
come in for first consideration because it is claimed to regulate
the size of cells. It seems possible that the full report prom-
ised by Jaceard??® on the tree-trunk as a shaft of equal water
conductance may throw more light on Hartig’s idea.

The possible relation of enzymes to the formation of ‘‘annual”’
rings:—In cases of this kind in which the hypotheses are so
numerous and the advocates of each can marshal at least a por-
tion of the observed facts in support of their views the truth
usually lies somewhere between them, and each conflicting ex-
planation will eventually contribute certain fragments to a
theory that will account for the known facts. The time for such
a theory has not yet come. However, since none of the pro-
posed hypotheses gives promise of becoming such an explana-
tory theory it may be pardonable to submit yet another with
the hope that the viewpoint thus suggested might lead to a new
attack on the problem.

From our present knowledge it seems that to be of any value
as a basis for work or a stimulus for the further study of radial
growth rings such an hypothesis must, by using all known and
some probable but undetermined facts explain how it is that
wood cells have a smaller radial diameter in summer than in
spring and why vessels are often wholly lacking in the later
summer wood.

It has been shown that an ‘‘annual’’ ring consists essentially
of a sheath or ring of wood produced during one more or less
continuous radial-growth period and that it is made up of two
types of wood which may merge gradually into each other or
join at a rather abrupt line. That portion of the ring devel-
oped in ‘‘spring’’ or during the early part of a new elongation-
growth period has larger cells than that produced in ‘‘summer’’
or after the closing of the first elongation, following the princi-
pal dormant season. In the case of trees in temperate zones
and many of those in the tropics which produce new leaves near-

bead POR

5—S. A.

66 Wisconsin Academy of Sciences, Arts, and Letters.

ly throughout the vegetative season the growth rings are not
very marked though they are usually apparent. Generally the
most reliable criterion for distinguishing the rings is the reduc-
tion in the radial diameter of at least the last row or two of
wood cells; yet in the tropics histological distinctions are said
to be practically absent in some trees, and their rings may only
be distinguished by slight demarking lines.

The work reviewed in this paper has shown that the environ-
mental factors which control elongation growth also influence
radial growth and that ordinarily the prevention of elongation
by the removal of vegetative points hinders growth in thick-
ness even when the environmental conditions are optimal and
the food and water supply abundant. Klebs**® assumed, in fact,
that large quantities of organic foods accumulating in plants
inactivates the enzymes concerned in elongation and therefore
brings about a cessation of growth in length. According to
him a timely increase in the water and inorganic nutrients may
reactivate or prevent inactivation of the growth enzymes and
thereby shorten or eliminate the dormant period.

With such a precedent one may also assume the presence of
enzymes which incite and maintain radial growth since there
are a number of phenomena to be noticed in connection with
growth in thickness that support such an assumption, as may
be gathered from the following papers.

.In an investigation on the reserve food in seeds Reiss™™? found
that cellulose is laid down on the inner side of cell walls of
many seeds and that it is largely redissolved on germination.
Schulze? made a similar study of lupine seeds and found con-
vineing evidence that the inner layers of the cotyledonary cell
walls are used up during germination. It seemed that the dis-
solving part of the walls is a hemicellulose which gives rise to
galactcse and arabinose on hydrolysis. Griisst®* also noted the
occurrence of the hemicelluloses, galactan and araban, in plant

23.Griiss, J. Ueber Lésung und Bildung der aus Hemicellulose besteh-
enden Zellwande und ihre Beiziehung zur Gummosis. Biblio. Bot. 39.
1896. pp. 14.

1201. ¢,

181 Reiss, R. Ueber die Natur der Reservecellulose und tiber ihre Auf-
Jésungsweise bei der Keimung der Samen. Ber. Deut. Bot. Ges. 7: 322—
29. 1889.

182 Schulze, E. Ueber die Zellwandbestandtheile der Cotyledonen von
Lupinus lutens und Lupinus angustifolius und tiber ihr Verhalten wah-
rend des Keimungsvorgangs. Ber. Deut. Bot. Ges. 14: 66-71. 1896.

Grossenbacher—Radial Growth in Trees. 67

cells, and that they may be dissolved or converted into gum by
enzymes. Potter"** called attention to the presence of an inner
cellulose layer in the xylem cells of many normal trees, and to
its especial abundance in the wood fibers of Quercus, Fagus,
Aesculus, Salix, Ulmus, Alnus, and Betula. He found that
after keeping wood in water during some days cellulose linings
became apparent in many cells in which none had been noted
before the water treatment.

Du Sablon?** concluded that when starch disappears in late
fall much of it is converted into reserve cellulose which is de-
posited on the inner side of wood-cell walls. In some cases this
lining was found to be comparativey thick and occasionally it
even had folds extending into the lumen of cells. It is said to
be readily soluble in dilute hydrochloric acid.

Schellenberg'*® made a more thorough study of the deposi-
tion and partial solution of hemicellulose in the wood and bark
of trees. He found a hemicellulose lining on the walls of fibers
in both spring and summer wood of Aesculus Hippocastanum,
Betula and other trees but it was not dissolved in spring. Since
similar hemicellulose linings in the cells of the phloem and corti-
cal parenchyma were found corroded in spring he concluded
that the lining did not dissolve in the fibers because protoplasm
was absent there. In the wood fibers of Vitis and Robina
Pseudacacia he noted the occurrence of especially thick hemi-
celulose layers in well matured wood and of much thinner ones
in those of immature wood. The protoplasm remains alive in
the wood fibers of Vitis and he accordingly found the inner lay-
ers corroded and dissolved in spring. He also found the same
selution of the inner unlignified layers in the bast fibers and
cortica Iparenchyma and collenchyma of Fraxtius excelsior.
Usually from a third to half of the unlignified layer in the cor-
tical parenchyma is dissolved when the buds open. He was of
the opinion that the deposition of hemicellulose in the bark
parenchyma continues after the leaves fall.

From these papers it is evident that a hemicellulose dissolv-
ing enzyme is active during the early part of a vegetative sea-

184 Potter, M. C. On the occurrence of cellulose in the xylem of woody
stems. Ann. Bot. 18: 121-40. 1904.

US],

188 Schellenberg, H.C. Ueber Hemicellulosen als Reservestoffe bei un-
sern Waldbiumen. Ber. Deut. Bot. Ges. 23: 36-45. 1905.

68 Wisconsin Academy of Sciences, Arts, and Letters.

son and that such an enzyme is not present or is inactive in the
latter part of the growing period as indicated by the fact that
hemicellulose is deposited in both the wood and bark at that
time. Sanio'®? found that in Pinus silvestris lignification did
not occur in spring wood until after the deposition of the secon-
dary thickening had been completed, that it began at the angles
of the cells and then involved the radial walls and later the tan-
gential walls. In the summer wood, however, the primary walls
were found to have lignified before the deposition of the secon-
dary thickening began, and it occurred in cells which were only
a few removed from the cambium. The final composition of
the cell walls of spring and summer wood seem also to differ, for
according to Wieler,*** the walls of spring wood contain a lower
percentage of cellulose than those of summer wood.

If the deposition and lignification of cellulose are in any way
dependent upon enzymotic action, there must be at least two
enzymes concerned because the two processes appear to be inde-
pendent of each other as indicated by Sanio’s observations. It
is evident that either of the processes would necessarily impede
or check further enlargement of cells differentiating from the
cambium. It, therefore, appears permissible to assume that the
enzymes involved in the solution of hemicellulose and the tardi-
ness of the lignification process in spring are important factors
in permitting the development of larger wood cells in spring
than those produced in summer, when the cellulose dissolving
enzymes are inactive and lignification occurs so quickly after a
cell is formed that in some cases it takes place even before sec-
ondary thickening has begun. The experiments by Jost and by
Lutz also give support to the idea that radial growth is largely
controlled by enzymotic activities which are somehow dependent
upon the process of terminal elongation. Perhaps the enzymes
concerned are liberated or activated in enlarging and bursting
buds in different parts of trees and are carried downward in the
metabolized food, or possibly enzymes produced in the enlarging
buds simply initiate certain activities which are transmitted
without the further aid of the enzymes as was assumed by

87 Sanio, K. Anatomie der gemeinen Kiefer (Pinus silvestris L.).
Jahrb. Wiss. Bot. 9: 66-68. 1873.
288], ¢,

Grossenbacher—Radial Growth in Trees. 69

Fick" regarding the action of the enzymes which coagulate
blood and milk.

The fact that stems and branches of trees are more pliable and
easily bent while in the midst of active spring growth than they
are at any other time, indicates that perhaps some enzymotic
softening of the mature wood occurs during the period of most
active growth. The upward bending of a branch on a decapi-
tated conifer also argues for the presence of some softening
agent during the time of most vigorous growth because of the
fact that such branches often bend in response to gravity at
places where lignification had previously occurred. In other
words, it seems that one of the most important factors in the
production of large wood cells in spring and smaller ones in
summer may be the presence of enzymes which retard lignifica-
tion and prevent rapid thickening of the walls and thereby per-
mit growth or hydrostatic pressure to develop large cells in
spring; while the absence or inactive condition of those enzymes
induces rapid thickening and early lignification of the walls in
summer and thus checks the enlargement of summer-wood cells.

It may be that the idea of growth force expressed by Detlef-
sen, Mer and others as well as ‘‘the impulse to grow’’ em-
phasized by Morgulis imply the same sort of notion as that ad-
vanced in the above scheme regarding the possible relation of
enzymes to ring formation, but in any case the hypothesis is
only a guess based on rather suggestive indirect evidence. Mer’s
conclusion that the winter rest of the cambium induces its
greater activity in spring seems to have something in common
with the outcome of some feéding experiments by Morgulis, to
the effect that in subjecting salamanders to alternate periods of
fasting and liberal feeding a greater growth resulted than by
more frequent and abundant feedings. A theory to account for
wood rings must also make use of the evidence brought out re-
garding the effect of variations in bark tension both longitudinal
and transverse, as well as of the influence of the transpiration
stream as suggested by Hartig and more recently elaborated by
Jaceard in his discussion of the distribution of radial growth.
It should be remembered, however, that transpiration is per-
haps greater during the time summer-wood is formed than it ig
while spring wood develops; to say that larger cells are pro-
duced in spring to meet the higher water requirements of the
approaching summer explains nothing.

189 Fick, A. Ueber die Wirkungsart der Gerinnungsfermente. Archiv.
Gesam. Physiol. Mens. Thiere. 45: 293-96. 1889.

70 Wisconsin Academy of Sciences, Arts, and Letters.

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