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ECOLOGICAL INVESTIGATIONS
UPON THE GERMINATION AND EARLY
. GROWTH OE FOREST TREES

BY

RICHARD H. BOERKER

A THESIS

Presented to the Faculty of

THE Graduate College in the University of Nebr.\ska

IN Partial Fulfillment of Requirements

FOR the Degree of Doctor of Philosophy

Department of Botany

LINCOLN, NEBRASK.^

January i, 19 i6

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I.— ECOLOGICAL INVESTIGATIONS UPON THE

GERMINATION AND EARLY GROWTH

OF FOREST TREES

BY RICHARD H. BOERKER

CONTENTS

Page

Prefatory Note

Preliminary Considerations -

Historical

Classification and Resume of Habitat Factors n

The Germination Process i r

Method of Attacking Problem at Hand .['. 19

Methods and Apparatus Used 21

The Control of Habitat Factors 24

Notes on Damping-off -^

The Effect of Habitat Factors upon Germination 34

The Effect of Habitat Factors upon Stem and Root Development .... 64
Tlie Relation of Size and Weight of Seed to Germination Per Cent.

and Early Development -0

Summary of Conclusions 32

Bibliography 00

PREFATORY NOTE

Almost every national industry makes provision for investiga-
tive work. Millions of dollars are spent annually to develop both
human and mechanical efficiency. It is immaterial whether the
nivestigations are for the purpose of utilizing- certain products
hitherto considered w^aste, or to make workmen more efficient,
or to employ the latest processes and inventions to better survive
in the competitive struggle— the results of this class of work are

2 Richard H. Boerkcr

considered a great, indispensable business asset, warranting whole-
some moral and financial support.

The history of our country reveals the fact that material in-
dustrial progress is largely in direct proportion to scientific re-
search and invention. This is especially true in the agricultural
pursuits. The various governmental bureaus, our state universi-
ties and agricultural colleges, and our many agricultural experi-
ment stations are intimately connected with and responsible for
the progressive agricultural development of our country. These
institutions form a vast ganglionic intellectual organization ; they
are rapidly becoming the centers of a new agricultural system
and, working from these centers outward, they are gradually
touching every phase of agricultural activity.

Forestry has joined the ranks of the great industries in develop-
ing the investigative side of the business and the establishment of
forest experiment stations and a forest products laboratory by
the Forest Service of the United States Department of Agri-
culture has been the first step in this direction. It has become the
business of these stations and this laboratory to study the funda-
mental laws governing the life of the forest and their effect upon
the final product— wood. That vast complex of environmental
factors— the habitat— is beginning to be analyzed to discover m
what ways man can help nature to produce more and better
timber, in a shorter length of time and at less cost than nature
has produced in the ages past. While perhaps, on account of
economic conditions, industrial investigations have been given
preference to purely silvicultural research, yet investigations m
estabhshing and growing forests have received no small amount

of attention.

Outside of these governmental endeavors very little has been
done along the lines of silvicultural research. State forest ex-
periment stations are practically unknown. It is true that the
foresters as well as the ecologists connected with some of our
agricultural experiment stations are contributing to this field, but
a beginning has scarcely been made. There is a great need for
state forest experiment stations or at least for foresters upon the
staffs of some of the agricultural experiment stations to help

Germination of Forest Trees 3

solve local forestry problems. Finally, there is no reason why
forest experiment stations established and maintained by private
endowment on the plan of the Desert Botanical Laboratory of
the Carnegie Institution would not be able to do a great service
along these lines.

The importance and need of silvicultural investigations
scarcely needs comment, yet it might be well at the outset to
emphasize certain fundamental concepts. Forests are one of our
greatest natural resources. Unlike coal, iron, oil, etc., they can
be grozvn to insure a continuous supply. Forests are not huge
ivarehouses of standing logs from zvhich zve can take our annual
supply ad infinitum; they are not merely aggregations of indi-
vidual frees; they are complex communities of living organisms
capable of response to environmental factors not unlike human
beings. It follows then that in order to replace what we take
from the forest, in other words, in order to groiv a nevcrf ailing
supply of timber intelligently and economically, we must under-
stand th.ese complex living organisms and communities, must
study their behavior and requirements and ascertain the condi-
tions under zvhich they grozv best. This domain is forest ecology
or silvics.

It has been asked. Does forestry in its present stage of develop-
ment need this kind of work? Is not this work ahead of the
times ? Is it not of too little practical value to demand our atten-
tion at present? It will be my purpose to show at this point
of my paper that, while this class of work is not absolutely
essential to forestry at the present time, it is extremely desirable
that it be begun in a scientific manner at the earliest possible
moment, in order to put American forestry upon a firm scientific
basis. The present status of forestry in the United States em-
phasizes the necessity of beginning soon. A brief word as to
our present stage of development may be in order.

Forestry either of an intensive or an extensive nature is being
practised in many parts of the country to-day. Both private and
public cori)orations are engaged in one or more of the main phases
of it, viz. : silviculture, forest protection, forest administration,
or forest utilization. In the field of forest protection gigantic

4 Richard H. Boerker

strides have been made in the last ten years on both public and
private holdings, and obviously this is the first step towards forest
management. Such intensive silvicultural operations as plantmg
and thinning are being practised principally in the east, while ex-
tensive forestry involving the selection and shelterwood systems
of management is almost the rule in the west. As might be
expected, in the west forest planting is still in its experimental
stage. On the whole economic conditions in the east have favored
the" development of both pubhc and private forestry and hence
this activity has been on a more intensive scale there than in the
west. That forestry in some sections of the country is not de-
veloping as fast as some conservationists might wish is due to the
fact that it is being held back by certain conditions and elements
of environment which by their very nature belong to a new
country with enormous natural resources like ours and over
which human endeavor has no control. It must be realized that
forestry never developed in any country in the world as fast as
it has in the United States in the last twenty-five years, and that
at the present time it is proceeding as fast as is consistent with
sound principles and existing economic conditions.

While the practice of forestry is making rapid strides, silvi-
cultural investigations are still in the infancy of their develop-
ment. In other words the practice of forestry and the science
of forestry have not developed in a ratio which would make them
mutually helpful. The greater development of the applied phases
of forestry is due partly to economic conditions and partly also to
a lack of appreciation of the value of purely scientific research.
The tendency has always been to magnify the industrial branch
of a science at the expense of the main body from which it had
its origin. Purely scientific botany has been largely lost sight of
in the^face of such of its branches as bacteriology, plant breeding,
pathology, etc. Similarly the science of silvics has had to give
way to seemingly more important phases concerned with the
utilization of forests. In these days of commercial ideals when
the value of most things is gauged by what they will bring on the
market, I fear that undue emphasis has been placed upon the
econom'ic or applied phases of a science. Hence it is not strange

Germination of Forest Trees 5

that we should measure the vahie of purely scientific work in
dollars and cents rather than in terms of scientific advance and
intellectual satisfaction. The test nowadays applied to any sci-
ence by the large majority of people is, How much money does
it influence? What industries has it created? What has it added
to the wealth of the world?

If purely investigative work in forestry must give a raison
d'etre, it might be well to call to mind the following facts : that
many of its problems strike the foundations of national pros-
perity and their value cannot be measured in dollars and cents ;
that some of its problems must be gauged by the future returns
they bring rather than by the present ; and that it is the avowed
purpose of scientific work to solve those problems in which the
so-called practical worker has failed to produce results. History
bears witness to the fact that those fields which have seemed
furthest removed from utility have often yielded the most fruitful
results. What seems of only scientific value to-day very often
turns out to be of great practical utility later. It is comparatively
easy to estimate the value of a piece of work when it is possible
to base that estimate upon what has been actually gained ; but
how hopeless is very often the task when we must base our esti-
mate upon the loss which it prevented. In such silvical investiga-
tions as the influence of forests upon stream flow, upon the water
supply of communities, and upon the health and prosperity of our
people money values fade into insignificance.

Silvicultural investigations as well as forestry business are
long time propositions. The value of such work is very often
measured not so much by the immediate financial returns it brings
as by the principles it helps to establish, which in turn may affect
our management and hence the financial returns niany years
hence. It is the ti)ne element more than any other that em-
phasizes the need for beginning the solution of some of our silvi-
cultural problems soon. It is believed by man}- that it will be at
least twenty-five years before intensive operations such as plant-
ing, thinning, and other silvicultural measures will be economically
possible in some parts of the country. Granted that tliis is true.
Is this too much time to devote in preparation for this work? If

6 Richard H. Bocrker

we keep on getting results in the next twenty-five years in the
same proportion as we have done in the past ten, will many of our
important problems be solved? Most silvicultural investigative
problems take many years to solve. Some nursery and planting
problems can be solved in from three to five years (if nothing
interferes), but most of even these take longer. In many cases it
takes from two to four years merely to raise stock let alone
experiment with it. It usually takes six months or more to de-
termine whether the stock set out will hve, let alone establish
principles in planting. The element of time is the largest factor
in this work ; we will need much of it, for failures will be numer-
ous and this will mean the loss of many years. Only long time
and carefully planned investigations can lead to stable and eco-
nomic forest management.

With the development of forestry it cannot be doubted that a
great deal of exact silvical and silvicultural knowledge is neces-
sary, and we must admit that a great deal of data is needed to-day
which cannot be furnished. We have unsystematic and indefinite
knowledge about many phenomena which await expermiental
proof. In fact, forestry is loaded down with a vast weight of
undigested facts, and pure science has only begun to relieve
forestry of this burden. The quickest and surest way for purely
forestry research to gain recognition is to show hov^' to attam
practical results which years of blind groping along ri;^plied lines
have failed to accomplish.

Our task is a gigantic one, greater than any investigative prob-
lems that have confronted or will confront European nations.
We have more species of trees important in forestry than all
European nations combined. Our varied topographic and cli-
matic conditions make our problems infinitely more complex and
numerous. But that should not discourage us. Big problems
concerning the forest have been solved in the past and are being
attacked to-day. We have worked out our problems in logging
and have developed machinery and methods unique in the history
of forest industry; we have developed a system of forest fire
protection unlike anything ever attempted by forestry-practicing
nations ; it remains for American ingenuity and enterprise to solve
the silvicultural problems which confront the American forester.

Germination of Forest Trees 7

Briefly stated the purpose of the present investigation is to
inquire into the effect of the more important habitat and seed
factors upon the germination and early development of certain
American forest trees in control cultures in the greenhouse for the
purpose of obtaining data that may be used in the silvicultural
management of these species.

This investigation has been conducted under the direction of
Professor Raymond J. Pool and I am indebted to him for his
friendly advice and counsel. I am especially grateful to him for
having read the first draft of this paper and for offering valuable
criticisms and suggestions. I wish to further acknowledge my
indebtedness to Professors P. B. Barker and H. J. Young of the
department of agronomy of the College of Agriculture for the
mechanical analyses of the soils used in these experiments and to
various members of the departments of botany and geology for the
many courtesies extended to me. Thanks are due to the various
district foresters, forest supervisors, and rangers, also members
of the Washington office of the Forest Service for their kindness
in furnishing so much of the seed used in these investigations.
Without this material assistance a large part of this work would
have been impossible. Grateful acknowledgment is also due to
my wife for much valuable assistance in counting seeds, in com-
piling the final data and in reading proof. Also, I cannot fail
to acknowledge the guidance and inspiration of the late Dr.
Charles E. Bessey throughout the course of these studies.

PRELIMINARY CONSIDERATIONS

Historical

The literature of the work done upon this problem is meager
and widely scattered. As has been noted before, both botanists
and foresters have worked in this field, so that papers from
widely dift'erent sources had to be considered. General observa-
tions were found to be much more numerous than results based
upon exact investigations. Too often one finds opinions and
views upon these questions with but very little data to substantiate

.8 Richard H. Bocrkcr

them. Foresters and botanists, in general, have proceeded on
the assumption that Hght and soil moisture are necessary for
germination. They have also noted that germination is acceler-
ated in sand as against a heavier soil like loam or clay. Little
has been done to inquire further into these relations. On the
whole the effect of habitat factors upon the early development of
plants has received more attention than their effect upon germina-
tion. In the following historical summary, light in relation to
germination and early development of plants will be considered
first, since probably more work has been done upon that particular
phase of the problem than any other.

One of the oldest notions regarding light and its relation to
plant growth is the one concerning the effect of artificial or
natural shade upon atmospheric and soil moisture conditions.
The forest experiment stations of Europe have long since worked
out this relation in the forest, so that to-day these results are
more or less well known to all foresters and botanists. Several
Americans, working on the eft'ect of artificial shading upon the
growth of tobacco, have brought out results similar to those
secured in connection with forests. Ilasselbring (3) has shown
that the transpiration of plants grown in the open is nearly 30
per cent, greater than the transpiration of plants grown under
cheese-cloth shade. The transpiration per unit of leaf surface
was nearly twice as great in the sun plants as in the shade plants.
Stewart (4) records the results of observations made in the
course of tobacco experiments in Connecticut on the climate and
soil conditions as affected by tents in producing a certain kind of
tobacco. He concludes that under the shade of tents the soil
retains more moisture, there is a greater relative humidity, and
there is a reduction in wind velocity, all resulting in plants which
are larger and of more rapid growth as compared to those grown
without tents. To sum up the eff'ect of .shade it might be stated
tersely : it lowers the air and soil temperatures and breaks the
action of the wind; these factors increase the humidity of the
air and this increased humidity results in less evaporation from
the soil and less transpiration from the plant ; the final con-
sequence is a greater soil moisture content with its correspond-
ingly good eft'ect upon the growth of the plant.

Germination of Forest Trees g

The effect of light upon the height growth of forest trees has
been used as a basis for determining the relative tolerance of these
trees. As early as 1866 Kraft (2) planted a number of different
species in the shade of older trees and measured their heights and
diameters several years later. Upon this basis Kraft arranged
the species according to their tolerance. Nikolsky (2) in 1881
carried on similar experiments with pine and spruce and showed
that the greatest length of stem was found in the trees which
grew in the shade ; the length of the entire plant above ground
increased with increase in shade ; the length of the main root as
well as the number and total length of the lateral roots, however,
diminished with increase in shade, while the total length of all
roots of plants which grew in great light intensity was greater
than the total length of all the roots in the shaded rows. At the
Swiss experiment station in 1893 Badoux (2) carried on experi-
ments on eleven tree species with different degrees of shading to
determine their behavior in different light intensities and thus
determine their tolerance. Fir and spruce had almost the same
average height growth at different degrees of shading. With
pines, larch, beech, and ash the growth on the contrary decreased
in proportion to the shading. In the case of basswood, blue
beech, and elm the growth in height was but little affected. The
work of Wiesner (2) from 1905 to 1909, in various parts of the
world, and of Clements and Pearson in the United States (2)
between 1907 and 1909 was only for the purpose of determining
the minimum light requirements of species as a basis for scales of
tolerance. The last two investigators took numerous readings
in the Rocky Mountains and noted the condition of seedlings
under various light intensities.

Burns (9) experimenting with white pine under lath shade in
the nurseiy found that shading delayed the time of germination
but that the final germination per cent, was about the same in
both cases. He likewise raised white pine seedlings in full shade,
half shade, and no shade and (at an age which he does not state)
measured the length of the hypocotyls, tap roots and lateral root
branches. He found the greatest length of hypocotyl in the
plants that had been grown in the full shade, the greatest length

lo Richard H. Bocrkcr

of tap root in plants that had been grown in no shade and the
greatest length of lateral roots and total root system in the no-
shade plants. This bears out Nikolsky's experiments along the
same line. An interesting conclusion reached by Burns is that
shade reduces the temperature of the soil and delays the time of
germination.

The work of Atterberg (9) which is quoted by Burns is given
here for completeness. Atterberg studied the relation of light
and temperature to the germination of pine seedlings. He found
that at a constant temperature of 23' C. 80 per cent, of the seed
germinated in the absence of light and 87 per cent, in the presence
of light during practically identical germination periods. Burns
concludes from this : " Apparently a high and changing tempera-
ture, light, and a moist seedbed are essential to satisfactory
germination."

" The investigations of Haak (5) and Pittauer (6) have very
little jjearing upon, the problem at hand. The former at the
mycological laboratory at Eberswalde studied the influence of
season, moisture, temperature, light days and dull, artificial and
natural light, color of light, intensity and duration of light, and
the influence of chemicals upon the germination of Scotch pine
seeds. He found that in lower temperatures germination begins
considerably later and proceeds much more slowly that in higher
temperatures, but that the final germination per cents, are about
the same in either case. He found that certain rays of light were
beneficial and certain harmful to germination. Pittauer studied
the effect of dift'erent degrees of light and extreme temperatures
upon the germii\ation of tree seeds of certain European species,
viz.: beech, black locust, and various conifers. He found that
germination proceeds more rapidly in light than in shade and is
most satisfactorily accelerated in diffused light.

Undoubtedly considerable work has been done in the United
States by the various forest experiment stations of the Forest
Service but these resuhs have not been, as far as my knowledge
goes, published. In a very recent article in Science, Graves (7)
speaks of such work being carried on at the Wind River Forest
Experiment Station in Oregon. A recent discovery at this sta-

Gcnnination of Forest Trees 1 1

tion showed that the seed of Pinus monticola of Idaho Hes in the
duff and htter beneath the mature stands for years and then
germinates when the ground is exposed to direct Hghting. This
is mentioned here, merely as another instance of the many of
record in which it is assumed that hght is to a large degree re-
sponsible for the germination of certain tree seeds.

Practically the only work of any importance on record concern-
ing the effect of soil moisture and soil texture upon the early
development -of forest trees is that of Tolsky (8). He studied
the relative effect of sandy and black soils upon the structure of
the root system of Scotch pine. He found on black soils that
pine developed principally vertical roots while on sandy soils
superficial roots predominate. In rich soils roots are guided in
their development by moisture, while in poor soils like sand,
activity is directed mainly towards extracting nutrition from the
soil. In poor soils nutrition is spread over a large area and in
order to get it in sufficient quantities trees need numerous roots.
Whatever the cause might be, Tolsky found more lateral roots
and more superficial roots in the case of trees grown in sand, and
this may be taken as the most significant part of his work.

Before discussing the present investigations, I feel that it would
be profitable to briefly summarize the edaphic factors of the
habitat with special reference to the physical properties of the
soil which play a physiological role in the germination of the
seed.

Classification and Resume of Habitat Faetors

The complex of climatic, edaphic. and biotic factors which
influences the life, growth, and reproduction of a plant is known
as its habitat. The study and investigation of habitats as entities
avails us very little unless we analyze a habitat into its component
parts and investigate each of these parts by itself.

Clements (i) classifies habitat factors into physical and biotic.
The former have to do in general with inanimate objects and the
latter with human beings and animals. He further divides phys-
ical factors into climatic and edaphic. Climatic factors are atmos-
pheric in their nature and the edaphic factors are concerned with

12 Richard H. Boerker

the soil. He further subdivides climatic factors into humidity,
light, temperature, wind, pressure and precipitation. The edaphic
factors are subdivided in a similar way into water content, soil
composition, soil temperature, altitude, slope, exposure, and
surface.

In glancing over this classification it becomes at once obvious
that all of these factors cannot affect the plant directly. Many
of those enumerated are in themselves very complex in their
nature. For example, slope, aspect, altitude, and surface could
each be subdivided into component factors, but if this is done it
will be seen that they resolve themselves into those factors men-
tioned above which are not divisible. In other words there are
about three master factors which are able to afifect plant life
directly, and all others are combinations of these. There is no
better way to bring out this idea than to give Clements' (i)
classification based upon the influence which each of these factors
may exert on plant life. He classifies factors into those that have
a direct bearing upon plant life, those that have an indirect bear-
ing, and those that have a remote bearing. Direct factors are
only those which produce qualitative structural changes in the
plant itself. Furthermore, the classification of habitat forms and
plant formations is based upon them, which fact merely em-
phasizes that they are fundamental. Indirect factors are those
that affect a formative function of the plant through another
factor ; and remote factors are those which are physiographic or
biotic in nature and must operate through at least two other
factors in order to produce a structural change in the plant. This
classification is as follows :

Direct Factors

Indirect Factors

Retiiote Factors

Water content

Temperature

Altitude

Humidity

Wind

Slope

Light

Pressure

Exposure

Precipitation

Surface

Soil composition

Soil temperature

Germination of Forest Trees 13

The germination of seeds depends principally upon edaphic
factors, hence climatic factors will receive little attention here
except in so far as they condition the former. It is taken for
granted that the morphological and the physiological significance
of water, light and heat to plant life are too well known to require
discussion here, especially since that phase of botany is funda-
mental in all ecological work.

The water content of the soil is by all odds the most important
edaphic factor in determining germination, for while other factors
may condition this process to a certain extent, none but water,
within certain limits, can prevent it altogether. In a synoptical
manner I will briefly call to mind the significance of this master
factor in germination and then briefly inquire into the important
soil factors and properties that bear directly on the investigations
at hand.

The amount of water in the soil has no direct relation to the
amount of water which plants can use. At the outset distinction
must be made between the different kinds of water in the soil
and which of these are available to plant roots. Usually three
kinds of water are distinguished, namely : hygroscopic water,
capillary water and free water. Hygroscopic water is that water
which plants cannot get owing to the enormous film pressure
which holds it. It is also known as the amount of water in an
air-dry soil. Capillary water is that water, most of which is
available to plants and is held against gravity around the soil
particles by capillary forces. Free water is that which is not held
either as hygroscopic or as capillary water. It is water influenced
in its movements by gravity and is therefore called gravitational
or hydrostatic water. Clements (i) calls these echard, chresard,
and holard respectively. It will be seen then, that the only water
available to plants is a part of the capillary water which surrounds
every soil particle and fills every small pore space.

The principal factors which influence the amount of soil mois-
ture available (capillary water) to plants are:

1. The amount of water reaching the soil.

2. The catchment of water by the soil.

3. The water-holding capacity of the soil.

14 Richard H. Boerker

4. The amount of evaporation from the soil.
• 5. The amount of water withdrawn by other plants.

6. The replacement of loss by capillary movement.

7. The amount lost by seepage, percolation, etc.

Of these factors, only four are important in the present investiga-
tions. These are the water-holding capacity of the soil, the
evaporation from the soil, the replacement of loss by capillary
movement, and the amount lost by seepage and percolation. The
water-holding capacity of a soil is determined by soil depth, soil
texture, and the amount of organic matter present. In soil tex-
ture two factors are important, namely, the size of the soil
particles, which affects the surface area of the particles and the
amount of pore space in the soil, and the density of arrangement
of these particles. It is largely for these reasons that loam will
hold more capillary water and will contain more air space than
sand or gravel. Evaporation from the soil naturally affects
greatly the amount of water available to the plant. This is
affected by climatic factors such as temperature, relative humidity,
and wind; and by soil factors such as texture, color, depth and
the character of the surface. The replacement of the loss of soil
water by capillarity depends upon the rise of water from the
water table. This rise is conditioned by the degree of saturation
of the lower soil layers, the texture of the soil, the height to which
the water must be raised and the character of the intervening soil
layers. A fine-textured soil like loam or clay is much more favor-
able in this respect than a coarse-grained soil like sand or gravel,
principally on account of its great ability to obtain water from the
lower soil layers. The amount of water lost by seepage and per-
colation depends largely upon the texture of the soil. The coarser
the soil the greater is the amount of water that percolates through
it and the less is the amount held by capillary forces.

As far as it determines the amount of soil moisture available
to plants, soil texture is certainly the most important physical
property of the soil and it deserves a foremost consideration in all
problems that pertain to the germination of seeds.

Germination of Forest Trees 15

The Germination Process {10, 11, 12, 15)

This period in the hfe history of the green plant is unique in
that the organism is independent of an external food supply and
also of all luminous energ)^ Germination may be called a period
of growth without photosynthetic activity, and it terminates at the
time the accumulated food in the endosperm is more or less ex-
hausted. During all this time it is without light; it does not
require it, but lives in total darkness beneath the surface of the
soil. While the seed has no use for light, it does require water,
oxygen, and a certain amount of heat in order to germinate suc-
cessfully. The dependent life of the plant begins at the termina-
tion of the process of germination, when the first ray of light
strikes the spreading cotyledons. Light sets the photosynthetic
mechanism in motion and this marks the beginning of the plant's
manufacture of food; henceforth it is dependent upon its en-
vironment.

The role of water in the germination process is to aid in the
transformation of the accumulated nutrient material into food
that can be used by the germinating embryo. In other words,
this factor is instrumental in taking this sunken capital and trans-
forming it into specie for circulation. But water cannot do this
directly; it must act through the agency of certain catylists or
enzymes. These enzymes transform insoluble and indiffusible
foods into soluble and dififusible ones which in turn move from
the endosperm to nourish the embryo.

Water is important to the seed for two reasons; its absence
determines the seed's power to live in a dormant condition, which
is one of its most important properties. If a seed is not dry it
cannot be preserved ; we cannot secure good seed in a wet autumn.
The second reason why water is important is because of its
chemical and mechanical action in germination. Hales at the
beginning of the eighteenth century showed that the absorption of
water by seeds is generally accompanied by a considerable mani-
festation of energy, which takes the form of swelling. Chemically
water acts as a solvent for the enzymes which render the ac-
cumulated foods soluble.

1 6 Richard H. Boerker

Practically all the accumulated foods in the endosperm must
be transformed by the action of enzymes, which in turn must
first be dissolved by water. Starch, which is insoluble in water,
is converted by means of the enzyme diastase into a soluble sugar.
Throughout germination the quantity of starch in the seed de-
creases ; the starch grains at first corrode and finally dissolve
completely. Many albuminoids (simple proteins) are likewise
insoluble in water and certain soluble albumens cannot diffuse
through membranes. A pepsin-like enzyme which develops dur-
ing germination acts upon the albuminoids, transforming them
into soluble and diffusible forms. Others are changed to crystal-
loids which after solution diff'use very readily. Fats and oils are
likewise insoluble. Certain enzymes during germination decom-
pose oil into its constituents, fatty acids and glycerin, the latter
easily soluble in water. It is well known that fatty acids when
set free assist the breaking up of oil in water into very fine drops
with the formation of an emulsion.

Heat is important in the germination of the seed in that it may
accelerate, retard, or even entirely stagnate the processes begun
by the action of water. It might well be said that the rapidity of
germination depends to a large extent upon heat, since it has the
power to modify the action of enzymes. Temperature likewise
affects the diffusion of liquids. A considerable part of the heat
used in germination is generated by respiration. This process
sometimes raises the temperature of the seed as much as 40-50°
F. above the surrounding temperature. Certain seeds owe their
ability to germinate at very low temperatures (below freezing)
to the heat generated during respiration. Certain arctic and
alpine plants are able to blossom in the snow for this same reason.

Seeds in water, seeds buried too deep, or seeds surrounded by
air deprived of oxygen do not germinate even if other conditions
are favorable. In other words, water and heat are of little avail
without oxygen. Even before water and heat can act through
the agency of the enzymes, in many cases another factor must
come into play to release the enzymes. The latest investigations
show that the formation of diastase is intimately connected with
respiration. In a similar manner respiration supplies the energy

Germination of Forest Trees 17

which oxidizes the fats and oils of the endosperm. It has been
noted that the quantity of oxygen absorbed is much greater in the
case of fatty seeds, hke those of the pines and birches, than in
the case of the starchy ones.

It has been known for a long time that seeds lose weight during
the process of germination although no solid matter is lost as near
as can be determined. If we take a certain quantity of seeds
and weigh them both before and after germination, being sure
to get the dry weight both times, we find that although the seeds
have increased in size, they have lost weight. This is due to the
loss of certain elements like carbon and hydrogen. In the process
of respiration the carbohydrates in the endosperm are broken
down, carbon and hydrogen are lost while the quantity of nitrogen
remains practically constant. In the process of respiration, the
products of combustion are carbon dioxide and water.

Respiration in the seed is quite different from that in the case
of leaves and other green parts of the plant. Seeds are generally
not provided with intercellular air spaces, but oxygen penetrates
to their interior chiefly by diffusion from cell to cell. Thus it will
be seen that the supply of oxygen to the deep-seated cells of the
seed is most liable to become insufficient. This of course retards
germination. If the supply of oxygen is reduced materially, due
to lack of soil aeration, germination may be prevented. The best
aerated soils are those that have comparatively large interstitial
spaces, like sands and gravels, and the poorest ventilated soils are
the heavy loams and clan's which are small grained and compact
and have minute interstitial spaces. The seeds of dift'erent tree
species naturally vary as to their soil requirements in this respect.
This explains why tree species of sandy habitats germinate so
poorly on clay soils.

From what has been said, it will be seen that water, heat, and
oxygen are the essentials for germination, and that the lack of anv
of these factors is sufficient to retard, if not entirely to inhibit the
process.

It is a well-known fact that seeds have a power of remainino-
dormant for a period without aft"ecting their vitality. The power
to retain this vitality is due largely to the nature of the seed-coat

1 3 Richard H. Bocrker

which insulates the embryo from heat, water and air and protects
it from mechanical injury. Cottonwoods, willows, elms so t
maples, and white oaks have a very short period of rest Usually
the period is not over six months, but basswood and hornbeam |
lay over from t^fteen to eighteen months. It has likewise been
noted that some tree seeds must lay over for a certam period
before germination can take place. The common experience of
attempting to germinate seeds in mid-winter which have been
gathered during the previous fall is proof of this phenomenon.
This leads me to a brief discussion of the process of after-npenmg

as it is called. ^ . . ■

Many seeds we know require a long time for germination in
spite of the fact that they are surrounded by the proper condi-
tions. During this period it has been found that certam chemica
and physical changes take place which are necessary before the
seed can germinate. The length of delay is apparently de-
termined by the persistence of the structure of the seed-coat
and to the conditions under which the seed is exposed, ihe
term " after-ripening" has come into use to designate the changes
in the seed during this period. Eckerson (17) concludes tha
most cases of delayed germination are due to the exclusion o
water or oxygen by the seed coats. But some seeds do not
terminate after all coats have been removed and the seed put into
germinating conditions, indicating that the delay is due to embryo
conditions. It is now certain that some changes ^nthm th.
embiTO are necessary for germination. In the case ot Crataegus
used by Eckerson it was found that food is stored m the embryo
in the form of fatty oils; neither starch nor sugar is present^
A series of metabolic processes takes place m the embryo during
the period of after-ripening. At first there is ^^^^'^ ^f^
accompanied by increased waterholding capacity. There f ollov ^
an increased activity and production of enzymes and as a result
the fats decrease and sugars appear. The appearance of sugars
which are soluble and diffusible marks the beginning of the germi-
nation of the seed.

All recent investigations both in America and abroad show how
extremely complex is the role of oxygen in germination. A set

Germination of Forest Trees 19

of conclusions based upon one species of plant apparently may
or may not hold for others. Shull's investigations (14, 15, 18)
are based mostly on Xanthium seeds. In his experiments he finds
no evidences of the diffusion of oxygen through an absolutely dry
seed coat. This is significant in that it shows an important role
of water in preceding oxygen in penetrating seed coats. Ex-
perimenting with Crataegus mollis Davis and Rose (16) find that
seeds treated dry or those placed under water do not go through
the process of after-ripening. Here again is evidence that both
water and oxygen are necessary. These investigators, working
on the effects of temperature upon the period of after-ripening,
conclude that favorable moisture conditions and temperature con-
ditions shorten the period. Atwood (19) confirms almost all of
the conclusions drawn by Eckerson although working on Avena
fatita. Crocker and Davis (20, 21) worked with water plants
and their results totally dift"erent than those described for land
plants need not be given.

Unfortunately these conclusions are not based upon forest tree
seeds. Such investigations have not been undertaken. This
phenomenon will probably explain many of the cases of delayed
germination which are well known to foresters. It is reasonable
to assume that the conclusions based on Crataegus would also
hold for such fatty seeds like the birches, spruces, hard maples,
etc. It is also reasonable to suppose that most tree seeds pass
through this period of after-ripening during the winter months;
if this is true it explains why it is often impossible to germinate
certain tree seeds immediately after they have been gathered.

Method of Attacking the Problem

There are two general methods of determining the causes in-
fluencing the behavior of seeds or plants growing under natural
conditions. These are the observational and experimental
methods. In the observational method we observe the kind of
vegetation produced in response to a certain complex of physical
factors and seek to find constant relations of one to the other in
order to draw conclusions. In the experimental method we may

20 Richard H. Boerker

either synthetize an artificial environment and proceed to study
the plant under definitely measured differences of light and water,
or we may measure the physical factors influencing the same plant
under various natural conditions. The observational method is
ill suited for most work on habitat relations because the habitat
involves an extremely variable array of uncontrolled physical
factors, and it is practically impossible to determine without actual
measurements which factor has the controlling influence and what
the relative importance of the others are. The most desirable
method for problems which will allow its application is the one
in which we synthetize an artificial environment. In this case we
keep certain factors constant and measure the variable one; in
this way, it is quite obvious, the environment is comparatively easy
to analyze. This method, of course, presupposes a greenhouse
and on this account is only of limited application.

There is no question that all these methods have their value in
their proper places ; the choice of one must vary with the problem
and the circumstances. The method of measuring the factors
influencing the same plant under various natural complexes is the
one probably of widest application in the field. The purely ob-
servational method, for work on the determination of habitat
factors, while of some value when other methods are impossible
of application has unsurmountable objections. Observers in vari-
ous parts have no common basis or standard ; their mental equip-
ment and fund of ecological knowledge vary greatly and they may
even have very different points of view. Some of these ob-
jections might be summed up in the term "personal equation."
Another danger in this method is that of applying local observa-
tions to large areas, in other words, in generalizing on the basis
of too meager observations. The conclusions drawn in the ob-
servational method are largely in the nature of opinions modified
as indicated above by the personal equation, while the experi-
mental method produces conclusions based upon actual figures
which are indisputable and carry the weight of scientifically
proven facts.

Another objection to the observational method in determining
the eft'ect of habitat factors is that this method studies the eflfect

Germination of Forest Trees 21

and not the cause of the factors. It is a most significant fact that
the same habitat factors do not always produce the same effects
upon vegetation even under apparently the same set of conditions.
The effect of two habitat factors or groups of factors may be the
same so far as the structure and behavior of the plant is con-
cerned, yet upon inquiry into the causes concerned we might find
in one case it was due to temperature and in the other to soil
moisture. In a similar manner it is known that other factors
besides light determine tolerance. In other words the study of
the effect of habitat factors upon plants does not always lead us
to safe assumptions as to what the underlying cause is. The only
safe method in this kind of work is to measure the cause, thus
employing a direct method instead of an indirect one.

Methods and Apparatus Used in These Investigations

The investigations herein described were carried on in the
middle room of the west greenhouse of the botany department
of the University of Nebraska. For the germination studies three
series of cultures were used, namely, the light, soil-moisture, and
soil-texture series. For the experiments and measurements in
connection with the early development of roots and stem a fourth
series was added, namely, the soil-depth series. In each series
three degrees were used. In the light series open light, medium
shade, and dense shade were used ; in the soil-depth series shallow,
medium deep, and very deep soil was used ; in the soil-moisture-
content series, dry soil, medium wet soil, and wet soil was em-
ployed ; and in the soil-texture series loam, sand, and gravel were
used. The values of each degree in each case will be given later.
As the experiments progressed it was found that the amount of
greenhouse space assigned to the work was not sufficient, so
that the open light culture, the wet soil culture, and the loam cul-
ture were combined into one since these were being run under
identical conditions. (For arrangement of cultures see page 33.)

The seeds for these experiments were obtained from any source
it was possible to get them. Large orders were sent to almost
all large commercial seed houses at one time or another. On the
whole the response from these orders was very discouraging. At

I

22 • Richard H. Boerker

the time the seed was wanted (early fall) many of the seed crops
had not been collected. Likewise it took time to determine
whether there would be any crops at all in the case of some
species. This resulted in delay in getting the work started. By
the middle of January eighteen species had been obtained from
commercial seedmen and of these only seven produced results that
were in any way satisfactory. On the other hand, through the
kindness of various members of the Forest Service throughout
the United States, twenty-six species were secured and practically
all of these produced good results. Due to these facts anyone
undertaking experiments of this kind in the future must look a
long ways ahead for a good seed supply. The following series of
tables gives the source of the seed obtained together with what
information was available as to date and place of collection. The
nomenclature used here and throughout this report is that used
by the Forest Service and is according to Forest Service Bulletin
No. 17 by G. B. Sudworth.

Species Supplied by the United States Forest Service

Species Place Collected Date

Piniis ponderosa California

Piniis ponderosa Pecos N. R, New Mexico 1913

Pimis ponderosa Weiser N. R, Idaho 1912

Pinus ponderosa '. . ■ Harney N. R, South Dakota 1912

Pinus ponderosa Bitterroot N. R, Montana 1912

Pseudotsuga taxifolia Pecos N. R, New Mexico 1913

Pseudotsuga taxifolia Caribou N. R, Idaho 1912

Pseudotsuga taxifolia Madison N. R, Montana 191 1

Pseudotsuga taxifolia Western Washington and Oregon 1911

Pinus Jeffreyi Kern N. R, California 1912

Abies concolor Durango N. R, Colorado , • • • 1913

Tsitga heterophylla Olympic N. F., Washington 191 1

Pinus lamhertiana Lassen N. F., California 1910

Libocedrus decurrcns Eldorado N. F., California 1914

Pinus palustris Florida N. R, Florida ?

Pinus coulteri Monterey N. F., California 1910

Abies magnifica Sequoia N. F., California ?

Sequoia washingtoniana . . Sequoia N. F., California 1912

Pinus divaricata Minnesota N. F., Minnesota 19^0

Pinus contorta Arapaho N. F., Colorado ?

Pinus resinosa Minnesota N. R, Minnesota 1910

Germination of Forest Trees 23

Lariv occidcntalis Colville N. R, Washington iQii

Abies lasiocarpa Priest River, Idaho I9 3

Abies grandis Priest River, Idaho I9i3

Picea sitkensis Coast of Washington iQii

Pinus monticola Priest River, Idaho I9i4

Species Supplied by Commercial Seedmen or Collected

Species Place Collected

I9I3

?

Pinus strobus Canada

Larix europea Europe • • • ■

Pinus ponderosa Black Hills, South Dakota I9i3

Pinus divaricata Northern Minnesota 19^4

Robinia pseudacacia Europe •

Catalpa speciosa Indiana 9 3

Quercus rubra Michigan 9 4

Acer saccharum Illinois

Liriodcndron tidipifera . . . Ohio ^9 4

Bctula papyrifcra Pennsylvania ^9 4

Abies balsamea Maine ^9I4

Pscudotsuga taxifolia ....Colorado ^9_^ 3

Pinus iaeda Southern states ^

Taxodium distichum Southern states

Liquidamber styraciflua . . . North Carolina I9i4

Acer saccharum Canada

Acer rubruni New Hampshire ^9H

Fravinus americana Indiana

Juniperis virginiana Missouri River, Nebraska IQH

Gleditschia triacanthos .... Lincoln, Nebraska 191 5

Pinus monticola Clacier Park, Montana I9i4

Catalpa speciosa Lincoln, Nebraska i9io

In the body of this report, in order to distinguish the cHmatic
varieties of a species, the name of the state in which the seed was
collected is given with the name of the species.

The first planting was done on October 28, 1914. From that
time on plantings were made as the seed arrived. The last seeds
were planted March 21, 1915- A^l experiments were conducted
between the first date mentioned and May i, 1915. a period of 184
days. In all cases ample time was allowed for the completion of
the process of germination. This time naturally varied with the
species. For most species three months was allowed but in the

24 Richard H. Bocrker

case of certain Pacific coast species four months was apparently
necessary. Three months ordinarily is plenty time enough;
usually if a seed in the forest fails to germinate in that time, it
usually does not germinate at all, especially in the west where
the dry period sets in after the spring is over.

Ten of the species mentioned in the foregoing tables failed to
germinate. These species were Larix enropea, Acer saccharum
(both), Liriodcndron tulipifera, Taxodium distichum, Liqiii-
dambcr styracifiua, Fraxinns americana, all of which were supplied
by commercial seedmen. If the data regarding the collection of
these seeds is bona-fide, their failure to germinate must be ex-
plained by the fact that they had not completed their resting
period. In the case of Jwiipcris virginiana, Gleditscliia tria-
canthos, and Pinus monticola, whose place and date of collection
is known absolutely there can be very little doubt as to why they
failed to germinate.

The soil used in all cultures (except the sand and gravel) was a
garden loam of excellent quality with a mixture of about 25 per
cent, of white sand. The mixture was prepared in the green-
house. This made a very good soil for experimentation purposes.
The sand used was common white, quartz sand with but a very
small per cent, of hornblende and magnetite. The gravel was the
kind used by the large construction companies around Lincoln
for concrete work. Mechanical analyses of representative
samples of these soils are given elsewhere.

All seeds were planted in rows at a depth which was 2^ times
the shortest diameter of the seed as near as this was determinable
by the unaided eye. The rows averaged 3 inches apart and about
24 inches in length. In general 200 seeds were used of each
species when the seeds were of medium size or smaller ; for some
of the western pines only 100 were used because of their large
size.

Tlic Control of Habitat Factors

As has been pointed out, the only safe way to study the effect
of the factors of the habitat upon the life of the plant is to
measure one variable factor while all the rest are kept constant.

Germination of Forest Trees 25

This principle is fundamental in mathematics; a single algebraic
equation with two unknown quantities cannot be solved. In each
of the series used in these investigations it was the intention to
have only one variable habitat factor. In this way the study of
cause and effect was much more clearly brought out.

The soil moisture determinations were made for four different
purposes :

1. As a check upon daily watering in similar cultures.

2. To show the eft'ect of shading on soil moisture content.

3. To show the minimum content in the soil moisture series.

4. To compare soil moisture content in loam, sand, and gravel.

The samples w^ere taken in certain cultures and at intervals
varying with the purpose. The samples for the moisture content
series of cultures were taken once a week, all others once a month.
Each sample consisted of from 50-100 grams of soil, and was
taken at depths varying with the development of the plants in the
culture. In order to provide against error each sample consisted
of from two to five portions taken from spots several decimeters
apart, care being exercised that no soil was dug near holes where
previous samples had recently been obtained. The samples were
always dug between the rows of seedlings. The samples were
immediately weighed and dried at a temperature of 95-105°
Centigrade to constant weight (24 hours). The per cent, of
water was computed upon the dry weight of the soil.

All the cultures except the dry soil and the medium wet soil
cultures were watered every evening. As was noted above, when
the amount of room for the germination tests became insufficient
the number of cultures was reduced from 12 to 10 by eliminating
two duplicate cultures. The only check moisture samples which
will be considered here are those that have to do with the three
cultures which were being operated under identical conditions.
Samples taken and recorded hereafter w-ill take care of the other
cultures and series. During the time (three months) that these
duplicate cultures were run, one set of soil samples was taken as
a check to determine whether they were being watered equally.
These figures follow :

I

26

Richard H. Boerker

Check Soil Moisture Samples 11/14/14
Depth 5.0 cm.

Name of Culture

Open light

Wet soil

Loam

Deep soil

Medium depth

Shallow soil

Moisture Per Cent.

27.9
24.1

27.7
28.2
28.5
314

1

These figures, obtained after more than two weeks of daily water-
ing, pretty well indicate the small amount of variation in moisture
content which results in a number of cultures under the same
conditions.

Light was controlled in the greenhouse by means of shade
tents. The east bench of the room was divided into three parts
and the portions at the ends of the bench were covered with
cheese cloth. The central compartment of the three was not used
on account of the shading influence of the tent to the south of it.
(See page 33.) The tent intended to develop medium shade was
made of^'a medium grade of cheese cloth, while the tent intended
for the dense shade was constructed of a double layer of heavy
cheese cloth. The light values developed in these tents and in the
full light of the greenhouse as determined by a Clements photom-
eter are given below :

Table of Light Values

Date

I1/21/14
II/27/14

Average.

Time

Open
Light

11:30 A.
12:00 A.

0.4250
0.4040

0.414s

MeJium
Shade

0.1775
0.1467

Dense
Shade

Number
Readings

0.0216

O.162I

0.0216

Clear
Clear

These values are based upon full sunlight just outside of the
greenhouse. These tables indicate that full greenhouse light is
approximately >^ of full daylight and that the medium and dense
shade tents have values approximately i/o and %o of full daylight
respectively.

Gennination of Forest Trees 27

It is quite natural to wonder how these values compare with
values that have been obtained in the woods. Probably the com-
parison of the light values obtained in the dense shade with some
of the lowest values obtained in the woods would be most interest-
ing. Clements (2) found light values from 0.12 to 0.05 under
mature lodgepole pine in Colorado. He observed that Douglas
fir occurred very rarely in densities below 0.05. Wiesner found
the same value in this case. Pearson in Arizona found that
western yellow pine seedlings grow fairly well in a hght intensity
from 0.309 to 0.414. White fir was found in good condition in
light intensities of from 0.027 to 0.068 and healthy young growth
of Engelmann spruce was found in intensities of from 0.033 to
0.062. In Oregon Pearson found such tolerant species as alpine
fir, Engelmann spruce, western hemlock, and Lowland fir grow-
ing in light intensities from 0.021 to 0.029. The western larch
however showed only poor development in a hght intensity of
0.353. This will be sufficient to indicate that the light in the
dense shade tent compares with some of the lowest Hght intensities
that have been measured in our western forests. In this con-
nection it is interesting to note that white pine, black locust, red
oak, and western yellow pine lived for two to four mouths in the
dense shade tent, as is evidenced by the fact that stem and root
measurements were taken on these species during the last days of
these investigations.

In connection with the light experiments a very important fact
soon became evident. In spite of the fact that all three cultures
were watered every evening at the same time and in the same
degree, it soon became evident from mere observation that the
top layer of soil by the following evening had dried out to very
dift'erent degrees in the three cultures. The open light cuUure
was noticeably the driest and the dense shade culture the moistest
so far as the top layer was concerned. This fact led to taking
systematic moisture samples to determine the exact difference
in moisture content. These samples were taken once a month,
three evenings in succession and these readings were averaged
into one reading. The table of soil moisture contents is given
below :

28

Richard H. Boerker

Table of Soil Moisture Content in Light Cultures in Per Cent.

Depth 2 cm.

Dat.
It/l0-I2

I2/I3-I5

1/4-6

2/I4-I6

3/I4-I6

4/15-17

Average

Open Light

Medium Shade

Dense Shade

12.3

15-3

19.8

12.0

19.0

19-3

6.1

iS-7

18.0

14.0

16.6

21.3

14.4

17.0

21.3

10.3

17.2

19.7

II-5

16.8

19.9

Soil depth was comparatively easily controlled, either by only
partially filling the flats with soil or by using deeper boxes. This
was done and the depths used were as follows :

Shallow soil 4.0 cm.

Medium deep soil 9.0 cm.

Very deep soil 30.0 cm.

The depth of the medium soil was the depth of all the other
cultures used in the light, soil moisture, and soil texture experi-
ments. No attempt was made to measure the soil moisture con-
tent periodically in the soil depth cultures, except as noted in
checking up the watering of the cultures.

Soil moisture was controlled in the soil moisture experiment
by watering the cultures at dift'erent intervals. The wet culture
was watered every evening, the medium wet culture was watered
every Wednesday noon and Saturday evening and the dry soil
culture was watered only every Saturday evening. The soil
samples that were taken were secured just before watering and
were taken at first every Wednesday noon and every Saturday
evening and later only every Saturday evening. Thus the soil
samples represent the minimum water content of the soil at the
end of one day, at the end of three and one half days, or at the
end of seven days. In the following table are given the soil
moisture per cents as taken at various depths according to the
stage of development of the majority of the seeds or plants in
the cultures concerned.

Germination of Forest Trees

29

Minimum Soil Moisture Content in Soil Moisture Cultures

Medium Wet
Culture,
Per Cent

Wet Culture,
Per Cent

II/4. .
II/7. .
Il/ll .

11/14-
II/21 .
11/28.
I2/S- ■
12/12 .
12/19.
12/26.
1/2..

1/9.-
1/16.
1/23-
1/30.
2/6. .
2/13-
2/20.
2/27.
3/6..
3/13-
3/20.

3/27-

4/3-.
4/10.

4/17-
4/24.

0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-9
0-9
0-5
0-5
o-S
0-5
0-9
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5
0-5

10. s
13-7

5-0
14.1
17. 1
13.8
12.4
16.6
12.1

9-4
II. 8
15-7
12.5

6.3

7-1
12.4
1

13.8

14.1

9-4

6.7

9-3

iS-6

12.4

14.0

8.0

12.4

23.0
30.0
25.0
24.1
21. 1
22.3
24-3
23-3
26.5

27-5
24.7
40.0
36.6
17.9
18.8
20.7
19.0
23-3
38.3
27.7
19.8
19.6
21.7
19.8
30.8
23-8
20.7

Average.

5-1

II. 8

23-9

Soil texture was controlled by the use of cultures of loam,
sand, and gravel. Soil texture affects principally the moisture
content and the air content of the soil, hence careful analyses to
determine both relations were made. The following moisture
determinations were made for sand, loam and gravel, which show
the amount of hygroscopic water, the volume of pore space, and
the amount of capillary water in each of these soils:

Table of Soil DETERiMiXATioxs

Texture of Soil

Hygroscopic Water,
Per Cent

Volume of Pore' Capillary Water,
Space in Per Cent Per Cent

Gravel, fine

0.14

36.54 5-4
39-34 5-0

41.14 2.8

average

Sand

loaiii

O.I I
0.92

39-04 4-4
33-51 16.6
53-32 1 38.0

^ Lest by accident in the soil oven.

30

Richard H. Boerker

The per cent, of hygroscopic water in the soils was the amount
of moisture the soils held at room temperature. The amount
of pore space in the soils was equivalent to the total amount of
water the soils would hold. This would, also be the amount of
air in the soil when air dry. In determining the amount of pore
space the soil used was air dry, hence the amount of hygroscopic
water in the soil had to be added to the amount of pore space.
The amount of capillary water was the amount of water the soils
held against gravity. The same soils were used in all three
experiments and the samples consisted of about 150 grams each
except in the volume determinations in which 100 cubic centi-
meters of soil were used in each case. This table shows very
strikingly the water and air relations in these soils. The great
amount of air in gravel when it is at its maximum capillary
water content is also shown approximately.

The mechanical analyses of representative samples of these
soils which were kindly furnished by the department of agronomy
of the University of Nebraska are given below:

Mechanical Analysis of Soils

Separate

Stones

Coarse gravel

Fine gravel

Coarse sand

Medium sand

Fine sand

Very fine sand ....

Silt

Clay

Volatile matter . . .

Total

Diameter, Mm.

Above 3

3-2

2-1

■I--S
• 5 --25
.25 -.1
.1 -.05
.05 -.005
.005 and less

Loam

7-936
II. 771

8.197
11.392

6.182
21.704
26.566

6.252

Sand

21.045
29.418
21.709
25.708
1.074

346
0.700

Gravel

38.639

40.382

14051

4-245

1.062

0.770

0.268

0.583

100.000

Besides these determinations soil samples were taken once a
month to determine how much moisture these soils held at the
end of a day in the cultures in the greenhouse. These results
bear out the findings in regard to capillary water held by the
soils shown in a preceding table. These moisture contents are
eiven below :

Germination of Forest Trees

31

Soil Moisture Content Twenty-four Hours After Watering
Depth ^.0 cm.

Date

Loam, Per Cent

11,14.

12/15.

i/iS.

2/15-
3/15-
4/15-

27.7

25.6

25.1

26.1
28.7
27.4

Average .

26.8

Sand, Per Cent

4-3

5-4
4.8
50
5-1
4-9

4-9

Gravel, Per Cent

2.4
2.1
2.5
2.0
2.4
1.9

2.2

The temperature and humidity of the air were determined by
a hydrothermograph which was checked every Monday morning
by means of a cog psychometer and humidity tables (the baro-
metric pressure used was 29 inches). The record sheets were
summarized and the results for the entire period are given below
by weekly averages :

November i

December

January

February

March

April

May

17-
24.
31-

7-
14.
21.
28.

7-
14.
21.
28.

4-

49

54

59

59

52

51

56

52

54

47

53

53

57

52

55

57

54

59

54

53

55

47

49

61

40
46
42
41
36
30
28
31
37
37
37
31

71.7
679
67.8
66.6
67.2
70.2
69.7
67.8
67.0
72.9
673
64.6

32
37
38
35

34
41
30

96

49

65-4

39

88

35

64.6

43

98

45

70.4

33

98

41

68.4

38

95

43

61.S

35

98

43

64.9

25

100

43

63.2

21

98

44

65-5

28

100

41

67.4

22

100

46

63.7

28

100

47

66.6

23

1 100

45

64.S

23

100

53

64.6

18

100

51

1 73-2

24

100

39

1 750

18

32 Richard H. Boerker

Soil temperature was not measured. With the air temperature
at an optimum point during the entire experiment it is reasonable
to assume that the soil temperatures were likewise always at an
optimum, at least they were never at such a low nor at such a
high point so as to affect materially the germination of the seeds
or the growth of the seedlings.

Notes on Dauiping-Off

No special investigations were conducted to determine what
species were most aft'ected and what conditions of light, moisture,
and soil were most favorable for the development of this group
of fungous diseases. This part of nursery practice is a problem
of no small importance in itself and the only data here given is
that which had to be taken in connection with this series of in-
vestigations. Therefore these are merely notes and suggestions,
which, while conclusive as far as they go, must be substantiated
in the future to be of any permanent value.

It was found that the pines were most affected. Pimts divari-
cata at the end of five weeks was aft'ected most. About 15 sepa-
rate cultures of 200 seeds each of this species were started and
most of these showed more or less serious eft'ects of the disease.
Several cultures of Pinus resinosa failed after six weeks. Piiius
palustris damps off in loam before it really gets its crown above
ground. In this case the loss was reduced in the sand and gravel
cultures. Both the New Mexico and South Dakota varieties of
Pinus ponderosa after five weeks damped oft" considerably, leaving
only from 10-25 P^r cent, of the original stand. The following
is a list of species in the order in which they were aft'ected in
loam under normal conditions of light and water. The first men-
tioned were aft'ected most :

Pinus divaricafa Pinus ponderosa (N. M.)

Pinus resinosa Rohinia psendacacia

Pinus palustris Pinus strobus

Pinus ponderosa (S.D.) Pinus taeda

It appears that the seeds of trees of certain habitats when
germinated in soils or under conditions dift'erent from those ob-
taining in their natural environment are affected worst. These
habitats are :

Germination of Forest Trees

33

1. Sandy soils

Finns divaricata
Finns resinosa
Finns palnstris
Finns taeda

2. Dry habitats

Finns ponderosa (S. D.)
Finns ponderosa (N. M.)

3. Poor soils

Robinia psendacacia.

1

34 Richard H. Boerkcr

Species that seemed to be affected most were those from the
Black Hills and New Mexico and those affected least were those
from the Pacific coast. Intermediate between these were those
species obtained from Montana and Idaho. The coast species
affected most was Sequoia washingtoniana. Psendotsuga taxi- ±
folia was much less affected than Pimts ponderosa taking into ac- T
count all the varieties of each.

The conditions and cultures which were favorable to damping-
off are of interest in that they emphasize many points already
known about this part of the subject. Loam is more dangerous
than sand or gravel due to its moisture retentiveness. The shade
cultures were more affected than the open light due to a greater
soil moisture content in the upper layers of soil. The moist cul-
tures were affected more than the dry ones and the shallow soil
cultures more than the deep soil ones due to a greater amount
of soil water per unit of volume of soil. Humous soils, soils with
decaying vegetable matter, and manure soils should be avoided
because they contain myriads of fungus spores. The data for
Pinus divariccta is given as representative of the three worst
affected species. The per cents, given below are those of the
number of plants killed (out of the total number that germinated)
within five weeks after planting the seed. Two hundred seeds
were planted in each culture :

Pinus divaricata Killed by Dampixg-off

Light Cultures Soil Moisture

Soil Depth

Open 35% Dry soil o%

Medium 26% Medium soil. . .24%

Dense 90% Wet soil 35%

Soil Texture

Deep 8% Loam 35^

Medium 35% Sand i*;

Shallow 6i%|Gravel o*;

THE EFFECT OF HABITAT FACTORS UPON GERMINATION

This problem was undertaken because it was felt to be of funda-
mental significance not only to silviculture but to ecology as well.
Not only was it desired to throw more light upon some of the
phases of this problem that had already been partly worked out
and to modify, if necessary, some conclusions that have been
drawn, 1)ut it was my intention to throw some light upon phases

Gcnnination of Forest Trees 35

of it that had never been attacked. Some of the questions that
are immediately called to mind by a mere statement of the prob-
lem are: Does light affect germination in any way? Does light
affect the germination of tolerant and intolerant species differ-
ently ? How does soiLmoisture content affect germination ? Do
drought-enduring species and moisture-loving species behave
alike in this respect? What is the effect of soil texture upon
germination ? Has the amount of air or oxygen in the soil any
significance in germination? Since soil texture affects mainly
the moisture content of the soil, does soil texture affect drought-
enduring species in the same way as moisture-loving species ?

The data collected upon the effect of habitat factors on germina-
tion will be presented in four parts. The effect of light, soil
moisture, and soil texture will be taken up in the order named
and following this there will be given a resume of the relative
eff'ect of all habitat factors. The three most important points
to be noted in germination, are the number of days it took until
germination began, the total number of days in the germination
period, and the final germination per cent. The rate of germina-
tion is shown by curves for certain representative species. The
length of the germination period was taken as the total number
of days during which any seeds germinated. Records were kept
long after germination ceased, so that the germination period was
ended at the time the last seed germinated. To give data as to
the period of greatest activity involves certain arbitrary standards
and this method, though tried in compiling the present data, was
abandoned. The effect of light, soil moisture, and soil texture
upon the periods of greatest activity is best shown by the curves
offered for certain representative species.

The original data was taken by two-day periods. Ever}' other
day the number of seeds that germinated were counted and re-
corded. In most cases these were immediately pulled up ; but
where growth measurements were to be taken later the seedlings
were allowed to grow.

The first three tables show the eft'ect of light upon the germina-
tion of eastern species, Rocky ^lountain species, and Pacific coast
species respectively. Three sets of figures are given under each

26 Richard H. Boerker

degree of light, namely, the number of days which elapsed before
germination began, the number of days in the germination period,
and lastly the final germination per cent.

The number of seeds used of each species in each culture made
was as follows : loo seeds each of Catalpa speciosa, Acer riibrum,
Gleditschia triacanthos, Pimis taeda, Pinus ponderosa (Idaho),
Abies grandis, Abies lasiocarpa, Pinus ponderosa (Mon.), Pinus
ponderosa (Harney), Pinus ponderosa (Calif.), P'^nus jeffreyi,
Pinus lambertiana, Pinus coulteri, Abies magnifica, and Pseu-
dotsuga taxifolia (Wash.) ; 25 seeds of Quercus rubra, 400 of
Betula papyrifera, and 200 seeds of all other species.

When a number of check cultures were combined as was noted
previously it became necessary to average the results obtained in
several cultures under the same set of conditions. Thus the check
cultures used in each series show the same data in every case.
Three cultures of each of the following species were averaged
together: Catalpa speciosa (Ind.), Pinus strobus, Quercus rubra,
Pinus divaricata, Robinia pseudacacia, Betula papyrifera, Pinus
ponderosa (S. D.), Pseudotsuga taxifolia (N. M.), Pinus pofi-
derosa (N. M.), and Pinus ponderosa (Cahf.). Two cultures of
each of the following species were averaged together: Pinus
palustris, Pinus resinosa, Pinus jeffreyi, Pinus lambertiana, and
Pinus coulteri. All other species in the check cultures were
planted but once.

In Table I 10 species out of a total of 14 germinated in the
dense shade before they did in the open light culture. Only
one species, Pinus palustris, germinated first in the open light,
one species, Gleditschia triacanthos, did not germinate in the open
light at all, and two species germinated simultaneously in all three
cultures. Pinus strobus germinated 8 days earlier in the dense
shade than in the open light, Pinus divaricata 2 to 4 days, Pinus
resinosa 10 days, Pinus taeda 2 days, Catalpa speciosa 2 days,
Quercus rubra 14 days, Robiniu pseudacacia 2 days, and Acer
rubrum 4 days.

In 9 cases the germination period is longer in the dense shade
than in the open light and, considering the shade cultures together,

Germination of Forest Trees

37

II species show a longer germination period in the shade than
in the light. The other three species did not germinate suffi-
ciently to make a conclusion possible.

TABLE I

The Effect of Light on Germination

Eastern Species

Species

Open Light

Began,
Days

Period,
Days

Final,
r er
Cent

Medium Shade

Began,! Period,
Days I Days

Final,
Per
Cent

Dense Shade

Began,
Days

„ . J Final,
Period, pgj

i^ays i Cent

Pinus strohus

Piniis divaricala

Finns divaricala (F. S.)

Pinus resinosa

Pinus paluslris

Pinus laeda

Abies balsamea

Calalpa speciosa

/ Calalpa speciosa (Neb.)

Quercus rubra

Robinia pseudacacia . . .

Belula papyrifera

Acer rubrum

Gleditschia Iriacanthos .

14
24
31
34
18
18
16
40
8

34

18

o

50
32
20
16
S3
6
30

28
16

30

10.7
54-5
39-5
30.5
10.5
19.0
II. o

i.o
91.0
28.0
28.8

1.0
17.0

0.0

16

54

II-5

10

26

63.S

14

26

37-0

16

24

50.0

32

82

12.0

34

6

33-0

18

38

10. 0

0

0

0.0

14

20

92.0

30

18

12.0

6

18

29.0

34

I

2.0

16

34

15.0

6

2

2.0

14

14
32
32
18
16
14
26

6
34
14

6

26
38
24
14
62

8
36

I
14
42

34

8.0

73-5

46.S

74-3

5-5

330

8.0

1.0

88.0

12.0

33-5
1.0

16.0
2.0

Three species had a higher germination per cent, in the open
light than in either of the shade cultures. Four showed the
highest per cent, in the medium light and six in the dense shade.
The greatest difference was shown in the case of Finns resinosa
whose germination per cent, was almost two and one half times
greater in the dense shade than in the open light culture.

The germination curves of Finus resinosa and of Finns divari-
cata are given on page 38. These are representative of the
eft'ect that light has upon germination. These curves show a
greater germination per cent, in the dense shade culture, a more
rapid rise of the germination curve in the dense shade and that
s:ermination begins sooner in the shade than it does in the light.

38

Richard H. Bocrkcr
The Effect of Light upon Germination

Fig. I. The germination curves of Pbnis rcsinosa.

Fig. 2. The germuiation curves of Pinus divaricata.

Germination of Forest Trees

39

TABLE II

The Effect of Light ox Germixatiox

Rocky Mountain Species

Species

Pinus ponderosa S.D.

Finns ponderosa iHarney

Pinus ponderosa | N.M.

Pseudotsuga taxifolia I N.M.

Pseudotsuga taxifolia ] Colo.

Abies concolor [ Colo.

Pinus conlorla i Colo.

Pinus ponderosa Mon.

Pseudotsuga taxifolia | Mon.

Pseudotsuga taxifolia j Idaho

Pinus ponderosa i Idaho

Abies grandis 1 Idaho

Abies lasiocarpa Idaho

Pinus tnonticola i Idaho

Open Light Medium Shade Dense Shade

:S3

•a in
c >>

Oh
32

10

14

14

14

40

12

26

12

42 j

24

50

14

80

i8

12

14

34

i8

30

36

52

36

36

30

30

24

.50

be s

58.0
52.0
56.0
63.0
91.0
38.0
22.0
10. 0
20.5
20.5
42.0
4.0

6.0
22.5

26
16

22
26
32
12
36
56
70
54
32
64
66
62
50
58

C3 >>
4)Q

56.S
58.0
82.0
69.0

73-5 '
54-0)

7-S
15.0
I5-S
49.0
52.0
16.0

7.0
20.0

14
14
10

14

14

34
26
12
16
36
60
72
8
44
64
82
60
28
60

58.5

63.0

79.0

65.0

82.0

56.0

3-5

9.0

350

50.0

430

10. o

6.0

36.5

In Table II 12 species out of a total of 14 germinated first in
the dense shade, the other two germinated simultaneously in the
dense shade and open light. The number of days difference
between the two cultures varied from 2 to 22 days. Pinus pon-
derosa (Harney) germinated 6 days earlier in the dense shade
than in the open light, Finns ponderosa (N. M.) 4 days, Pseu-
dotsuga taxifolia (N. M.) 2 days, Pseudotsuga taxifolia (Colo.)
4 days, Abies concolor 10 days, Finns ponderosa (Mon.) 8 days,
Fsendotsncja taxifolia (^lon.) 2 days, Pseudotsuga taxifolia
(Idaho) 8 days, Finns ponderosa (Idaho) 22 days, Abies grandis
14 days, Abies lasiocarpa 8 days, and Finns monticola 10 days.
The medium shade cultures in most cases represent a condition
intermediate between the open light and dense shade.

In 10 species out of 14 the germination \)e\-\oi\ was longer in
the shade than in the light. In 6 species the germination per cent,
was higher in the dense shade than in cither of the other two cul-
tures and in 12 cases out of 14 the highest per cent, was in either
of the two shade cultures as against the light culture. In other

40 Richard H. Boerker

The Effect of Light upon Germination

Fig. I. The germination curves of Pscudotsuga faxifolia (X. AL).

Fig. 2. The germination curves of Fiiius poiidcrosa (N. M.).

Gennination of Forest Trees

41

words, only 2 species had a higher germination per cent, in the
open Hght than in the shade.

On page 40 are given the germination curves of the two New
Mexico species, Pinus ponderosa and Pseudotsiiga taxifolia.
While these do not show a higher germination per cent, in the
dense shade than in the open light they show the characteristic
rapid rise of the shade curves and the fact that germination begins
earlier in the shade than in the light.

TABLE III

The Effect of Light on Germination

Pacific Coast Species

Species

Pinus ponderosa (Calif.)

Pinus jeffreyi

Pinus lamberliana

Pimis coulleri

Abies magnifica

Libocedrus decurrens

Sequoia washingloniana

Tsuga helerophylla

Picea silkensis

Larix occidentalis

Pseudotsuga taxifolia (Wash.) .

Open Light Medium Shade

42
31
70
52
44
29
16
66
22
o
22

61.0
22.0

2-5

iS-5

18.0

6.0

7.0

0.5

22.5

0

6.0

i^a

Dense Shade

62.0;
14. Oj

4.01
18.0
30.0

4.0

5-5

o

34-0

0.5
13-0

22
20
70
52
36
16

14
o

14
70

14

62

54
24
62
54
52
14

o
36

I
62

42.0

17.0

7.0

23.0

lO.O

9.0

8.5

0

38.0
0.5

22.0

Out of the II Pacific coast species listed in Table III, 7 germi-
nated in the dense shade before they did in the open light cuUure,
2 germinated simultaneously in the light and shade and 2 species
did not germinate sufficiently to warrant conclusions. Pinus pon-
derosa (Calif.) germinated 20 days earlier in the dense shade
than in the light, Pinus jeffreyi 11 days, Abies magnifica 8 days,
Libocedrus decurrens 13 days. Sequoia washingtoniana 2 days,
Picea sitkensis 8 days and Pseudotsuga taxifolia (Wash.) 8 days.

Six species showed longer germination periods in the shade,
three in the open light, and two species did not germinate sufficiently
to be considered. Only one species, Pinus jeffreyi, showed a
higher germination per cent, in the open light, two species showed

42

Richard H. Bocrker
The Effect of Light upon Germination

Fig. I. The germination curves of Pscudotsmja fa.vifolia (Wash.).

Fig. 2. The germination curves of Picca sitkciisis.

Germination of Forest Trees

43

a higher per cent, in the medium shade, and six a higher per
cent, in the dense shade than in the other cultures.

On page 42 are given the germination curves of Pseudotsiiga
iaxifolia (Wash.), and Picea sitkensis. Both sets of curves show
that germination begins sooner, the curve rises more rapidly and
the final germination per cent, is higher in the case of seeds
germinated in the shade as compared to light.

TABLE IV

The Effect of Soil Moisture ox Germixatiox

Eastern Species

Species

Dry Soil

.2 >>

C Ci!

10. 0

7-5
2

Pinus strobus i — ■

Piniis divaricala 40 24

Piniis divaricala (F. S.) 1 28 12

Pinus resinosa I 68 i

Pinus paluslris 1 • — ■ ! —

Pititis taeda — —

.4 bies balsamea { — —

Catalpa speciosa — —

Calalpa speciosa (Neb.) — -~

Quercus rubra , — —

Rohinia pseudacacia 10 30 15. oj 8

Betula papyri/era — — "

Arrr ruhrurti 24 i 3.0; 24

Medium Wet Soil

-

tJ w

«(J

br. c3

•u «

no *""

"'^

b u

Wet Soil

30

34

8.0

14

50

53-5

22

18

4.0

24

40

49.0

36

50

6.0

32
26

32

22
12
14
24

31

34

0: 18
- I 18

.0| 16

- 40

.0' 8
34
18

50 10.7

32 34-5

20 [39-5

16 30.5

53 10.5

6 ] 19.0

30 [ II. 0

I ' i.o

12 91.0

28
16

I 30

28.0

28.8

1.0

17.0

Tables IV. V, and VI consider the same species as the three
preceding tables from the standpoint of soil moisture instead of
light.

In 'Faille I\' in practically every case where a comparison is
possible germination started in the wet soil culture, and was de-
layed as the soil moisture content was reduced. Also the germi-
nation period is shortened with decrease in soil moisture. The
final germination per cent, in every case l)ut one was highest in
the wet soil. Pimis resinosa showed the highest per cent, in the
nuilium wet soil.

This tabic separates the species into classes based upon their
ability to germinate in dry soil, medium wet soil, or wet soil.

44 Richard H. Boerker

The Effect of Soil Moisture upon Germination

Fig. I. The germination curves of Piiius divaricuta.

Fig. 2. The germination curves of Robinia pscudacacia.

Germination of Forest Trees

45

According to that classification the most drought enduring are
Pimis divaricata, Pinus resinosa, Robinia pseudacacia, and Acer
rnbrum. It is rather unusual to find Acer riihrum in this cate-
gory but the seed has such a thin seed coat that water absorption
ts easier than in the case of a thick-coated seed. The intermediate
species are Pimis strobus, Pinus palustris, Abies balsamea, and
Catalpa spcciosa (Neb.). Among what might be called the
moisture loving species are found Pimts taeda, Catalpa spcciosa
(Ind.), Qiierciis rubra znd Betula papyrif era.

On page 44 are given the germination curves of Pinus divari-
cata and Robinia pseudacacia. These sets of curves show that
as soil moisture decreases the beginning of germination is delayed,
the germination curve rises less rapidly and the final germination
per cent, is decreased.

TABLE V

The Effect of Soil Moisture ox Germination

Rocky Mountain Species

Species

Pinus ponderosa S.D.

Pinus ponderosa I Harney

Pinus ponderosa 1 N.M

Pseudoisuga laxifolia .
Pseudolsuga laxifolia .

Abies concolor

Pinus conlorla

Pinus ponderosa

Pseudolsuga laxifolia .
Pseudolsuga laxifolia .

Pinus ponderosa

Abies grandis

Abies lasiocarpa

Pinus monlicola

N.M.

Colo.

Colo.

Colo.

Mon.

Mon.

Idaho

Idaho

Idaho

Idaho

Idaho

Dry Soil

26
34
22
12
14

48

Medium Wet Soil

Wet Soil

■r B

Oc

26

6

26.0
8.0

39-5
So
9-5

o.S
5-0

0-5

12
22
20
12
12
80
20
18
*8
20
14
66
84
18

.".a

(8 >>

hert

4>Q

36

18
16

44
42
26
66
58
26
32
72
10
I
38

48.0 10

17.0 14

61.0 14

54.0 12

60.5 12

12.0 24

3-5 14

6.0! 18

12. o 14

6.5 18

21.0 36

30 36

i.o 30

9.0I 24

32
14
40
26
42
50
80
12

34
30
52
36
30
50

1-3

58.0
52.0
56.0
63.0
91.0
38.0
22.0
lO.O
20.5
20.5
42.0
4.0
6.0
22.5

As in the preceding table, Table V shows that the beginning
of germination is delayed in most cases and that the germination
period is considerably shortened with the decrease of soil moisture
content. Only i species, Pinus ponderosa (N. M.) showed a
higher germination per cent, in the medium wet soil than in the
wet soil, all other species show a higher per cent, in the wet soil.

46 Ricliard H. Bocrker

The Effect of Soil Moisture upon Germination

Fir,. I. The germination curves of Pinus ponderosa (S. D.).

Fig. 2. The germination curves of Pscudotsuga taxifolia (Colo.)

Genn'niation of Forest Trees

47

It is evident from this table that the two most drought einkir-
ing species are Pinns ponderosa (S. D.) and Pinus ponderosa
(N. M.). While other species germinated in the dry soil their
germination per cents, were very small. Among the intermediate
species, as far as soil moisture goes, are Abies concolor, grandU,
and lasiocarpa, Pinus contorta, Pinus ponderosa (]\Ion.), and
Pseudotsuga taxifoUa (Mon.). It is interesting to see that with
one exception the only species that germinated in the dry culture
were either Pinus ponderosa or Pseudotsuga taxifolia. The
former from the' Black hills, New Mexico, and Southern Idaho
and the latter from New Mexico, Colorado, and Idaho. The line
is evidently drawn between Sotithern Idaho and ^Montana as to
whether these species will germinate in the dry culture or not,
since both species from Montana did not germinate in the dry
culture. Another interesting fact is that there are no moisture-
loving species in the Rocky Mountains so far as this classification
and these species are concerned, since there are no species that
germinated only in the wet soil.

On page 46 are given the curves of Pinus ponderosa and
Pseudotsuga taxifolia in their relation to soil moisture. These
curves show that germination is delayed, the curve rises less
rapidly, the period is shorter, and the final per cent, lower with
a decrease in soil moisture.

TABLE VI

The Effect of Soil Moisture ox Germination

Pacific Coast Species

Dry Soil

Medium Wet Soil

Wet Soi^

Species

.2 >»

fa b

(SO

68
80

90

■O in

2 >^

Z ct

12

6
8

-

i-5

£ is

Ph

6.0
12.0

30

5 ^

be rt

42
31

67
77

20

I

o.S

61.0

22.0

70

.^6

2.5

52

44
29
16
66
22

22

41
52
73
18
01
60

28

1.S..S

18.0

6.0

7.0

o.."?

22.5

1 -

Pseudotsuga taxifolia

' —

—

1 6.0

48

Richard H. Boerker

Classification of Species Based upon the Effect of Soil Moisture upon

Xerophilous Species

Rohinia pseudacacia
Acer rubrum

Pinus divaricata
Pinus divaricata

{¥. S.).
Pinus resinosa

Pinus ponderosa (S.

D.)
Pinus ponderosa (X.

M.)
Pinus ponderosa(ii.)
Pinus ponderosa

(Id.)
Pseudotsuga taxifolia

(N. M.)
Pseudotsuga taxifolia

(Id.)
Pseudotsuga taxifolia

/Colo.)
Pinus monticola

Libocedrus decurrens

Germination
Eastern Hardwoods

Xero-raesophilous Species

Catalpa speciosa
(N«b.)

Eastern Conifers

Pimis palustris
Abies balsamea
Pinus strobus

Rocky Mounkiin Species
Abies concolor

Abies grandis

Abies lasiocarpa
Pinus contorta

Pseudotsuga taxifolia

(Mon.)
Pinus ponderosa

(Mon.)

Pacific Coast Species
Pinus ponderosa

(Calif.)
Pi)ius jeffreyi
Pinus coulteri

Mesophi'ous Species

Catalpa speciosa

(Ind.)
Qucrcus rubra
Betula papyrifera

Pinus taeda

Tsuga heterophylla

Picea sitkensis
Pseudotsuga taxifolia

(Wash.)
Pinus lambertiana
Abies magnifica
Sequoia zvashington-

iana

In Table VI in everv^ case where conclusions were possible it
was noted that the beginning of germination was delayed and the
germination period was shortened with the decrease of soil mois-
ture. In every case the germination per cent, was highest in the
wet soil culture.

For some unaccountable reason Libocedrus decurrens ger-
minated in the very dry and wet cultures but not in the medium
wet one. However, the four drought resistant species stand out

Germination of Forest Trees

49

conspicuously : Pimis ponderosa, Finns jeffreyi, Finns coulteri,
and Libocedriis decurrens. This table shows that the Pacific
coast species are predominantly moisture-loving.

The foregoing table is a classification of all species used in the
soil moisture experiments upon the basis of whether they ger-
minated in all three soil moisture cultures, in two of them or in
only one of them. These three groups are called by the terms
xerophilous, xero-mesophilous, and mesophilous. Xerophilous
species are those that germinated in all three cultures ; xero-
mesophilous species are those that germinated in the medium wet
and the wet soil cultures ; and mesophilous species are those that
germinated only in the wet soil culture.

TABLE VII

The Effect of Soil Texture ox Germixation

Eastern Species

Loam

Sand

Gravel

^

^

^

Species

C in

' -6 a:

— " s

C m

■6 M

_-V.

c m

•O 01

. c

tS >^

C >.

C3U

5U

bna

c -^

■C rt

bB a

•r a

tS"

k^

U. u

^■^

j«a

a,

^a

^«

'^^

Finns slrobiis 22

Pinus divaricala j 12

Pinus divaricala (F. S.) 14

Piniis resinosa 24

Pinus paliislris j 31

Pinus taeda 1 34

Abies halsamea I 18

Catalpa speciosa 18

Catalpa speciosa (Neb.) 16

Quercus rubra 40

Robin ia pseudacacia 8

Betula papyrifera 34

Acer rubrum I 18

Gledilschia Iriacanthos I o

50
32
20
16
53
6
30

7 18
5 12
5 16
5 20
5 26
o 28
o 14
0 o

12

91.0

16

28

28.0

38

16

28.8

8

I

I.O

34

30

17-0

24

0

1 o.ol

34

34
38
18
54
54
12

34

0

12

46

18

I

18

I

II. o
72.0
32.5
85.0
12.5
41.0
18.5

0.0
92.0
24.0
395

30
130

1.0

34

10

16

16 i 8

22 i 62

38
40
18

40
16

o
16
30

8
34
18

0

I

40

o

I

54
18

7.0

45-5

28.0

16.5

9-5

1.0

7.0

0.0

9.0

16.0

II-5

0.5

8.0

0.0

Tables \'II, \'III, and IX show the eftect of soil texture upon
the same species.

Table \'TI shows that for the 12 species considered in the final
results only one germinated first in the loam culture. Three
germinated simultaneously in all cultures, three first in the sand,
and four germinated first in the gravel. Three species had the
longest germination period in loam, two in sand and 5 in gravel.

50 Richard H. Bocrker

The Effect of Soil Texture upon Germixation

I

Fig. I. The germination curves of Pinus divaricata.

ffy

;;;:;:j

;;;";;;;|;;;;;?

:ts^

■TTTT'ITT^^

^;;ii!i::

iiii

^^irrf^

;;t; £t^

in-Tr

:::::::±:f

: Si;::::;:::::;::::;:::::;::

;|

ini

:::::

X--4

::::

:,!;:::::::::::::::::::::::::::::;:::::

:::::::::|:

;;;;

1 1 |[ 1 1

;:|:

i;;;;;^:!

; iii:::;:;;::::;:;:;;:;;:;::;

::::

|l||||[||[[Ii

::::

+

::::

....

:|: 1

:;!

|:::::::::::M^:n:H^^::::::::l

1 .Uil

iMH

; i:i;:::;::::::::::..:::::i:

::::::::::|:|J: ::: :

::;:::,x-__^
: :|:::|^:.j ,;; J [J

^-

y

Fig. 2. The germination curves of Pinus resinosa.

Germination of Forest Trees

51

The two species that stand out as having the greatest germina-
tion per cent, in the loam are Onercus rubra and Acer ruhrum.
Nine species reached their highest germination per cents, in the
sand and in this group the following stand out most conspicu-
ously : Pinus divaricata, Pinus resinosa, Pinus palustris, Pinus
tacda, and Rohinia pseudacacia. Being species of sandy habitats
it is quite easy to see why they should germinate better in the
sand. In the gravel, which is a poor moisture retainer, it is in-
teresting to compare such a drought enduring species like Pinus
divaricata and such a moisture-loving species like Pinus taeda.

On page 50 are given the curves for Pinus divaricata and Pinus
resinosa.

TABLE VIII

The Effect of Soil Texture on Germination

Rocky Mountain Species

Species

Pinus ponderosa S.D.

Pinus ponderosa Harney

Pinus ponderosa N.M.

Pseudotsuga laxifolia N.M.

Pseudotsuga laxifolia Colo

Abies concolor Colo

Pinus conlorla Colo

Pinus ponderosa Mon.

Pseudotsuga laxifolia Mon.

Pseudolsuga laxifolia Idaho

Pinus ponderosa | Idaho

Abies grandis | Idaho

Abies lasiocar pa Idaho

Pinus monticola Idaho

Loam

« >• I .2 f^

■fclOCB u ctf

10
14
14
12
12
24
14
18

14
18
36
36
30
24

32
14
40
26
42

so
80
12

34
30
52
36
30
SO

Sand

•a m

m >.

2 >-

:SQ

PL,"

8

20

14

26

10

26

0
0

O 10
10

18

20

18

12

5' 20

o 44
0 46
o o
S 24

22
38
66
66
48
44
54
52
6
o
SO

57-5
4S-0
71-5
70.5
83-5
51.0
19-5
II. o
43-0
II. 0
S9-0
2.0
0
ii-S

Gravel

14
12

20
16
18
12
16
20
36
O
24

<S«

38

14

44
48
70

I
42
70
78
36

o
50

StJ

44.0
130
S7.S
63.5
80.0
34-0
40.5
4.0

44-5
43 o
71.0
30
o
13-5

Table VIII gives the results for the Rocky Mountain species.
Out of 13 species, 8 germinated first in sand or in gravel, only
one germinated first in loam, and four germinated simultaneously
in loam and in sand or gravel. Eight species show a longer
period of germination in sand or gravel than in loam, and 5
species show the same length of ])criod in either sand or gravel
and in. loam. Six species show a higlicr germinal ion per cent, in

52 Richard H. Boerker

The Effect of Soil Texture upox Germination

Fig. I. The germination curves of Finns pondcrosa (S. D.).

Fig. 2. The germination curves of Pimis contorta.

Germination of Forest Trees 53

The Effect of Soil Texture upon Germination

Fig. I. The germination curves of Fscudotsuga taxifolia (X. M.)-

Fig. 2. The germination curves of Ptiiiis pondcrosa (N. M.).

54

Richard H. Boerker

loam, four in sand, and four in gravel. It is significant to note
the large number of species in this table that germinate well in
the gravel.

On pages 52 and 53 are given the germination curves of Phius
ponderosa (S. D.), Piniis contorta, Pscudotsuga taxifolia (N.
M.) and Pinus ponderosa (N. M.). These curves show that the
germination usually begins earlier in the sand or gravel, that the
curve rises more rapidly for these soils and that the germination
per cent, is usually higher.

Table IX gives the results for the Pacific coast species. Out
of 9 species, two germinated first in the loam, the others ger-
minated first in either the sand or gravel. Three had longest
germination periods in the loam and six in either the sand or the
gravel. Only one species, Lihocedrus dccurrens, showed the
highest germination per cent, in the gravel, while six species
germinated highest in the sand.

On page 55 are given the germination curves of Pimis pon-
derosa and Pinus jeffreyi both from California. These curves
show substantially the same facts as those for the Rocky Moun-
tain species. These curves show that Pinus ponderosa does not
germinate so well on gravel as does Pinus jeffreyi a fact which is
significant when it is remembered that the latter will grow on
much poorer soil than the former.

TABLE IX

The Effect of Soil Texture on Germination

Pacific Coast Species

Species

0! >•

n"0

P^

Sand

US >H

Gravel

a >>

bo i4

o"Q

Pinus ponderosa (Calif.)

Pinus jeffreyi

Pinus lambertiana

A hies tnagnifica

Lihocedrus dccurrens. . . .
Sequoia washingtoniana .

Tsuga heterophylla

Picea sitkensis

Larix occidenlalis

Pseudotsuga taxifolia. . .

42

31
70

44
29
16
66

67
77
36
52
73
18
I
60

28

61.0
22.0

2.5

18.0

6.0

7.0

0.5
22.5

6.0

20
26
52
96
28
16
44
18

82
80
16
2
58
24
42
64

30
20

oj 80
0 50

38

44

30
86
18
48
68
I
8
64

5-0 36

48

17.0

19.0
9.0
5-0

22.0
0.5
1-5

24-5

6.0 J

Germination of Forest Trees 55

The Effect of Soil Texture upon Germination

Fi.i. I. The germination curves of Pinns pondcrosa (Calif.).

Fig. 2. The germination curves of Finns jcgrcyi.

56 Richard H. Boerker

Tables X, XI, and XII give the results of the effect of light,
soil moisture, and soil texture upon certain groups of species as
they were classified on page 48. While the foregoing tables
group the species and the final results on the basis of the geo-
graphical distribution of the species, these tables divide all species
into three groups based upon the amount of soil moisture neces-
sary for germination. The tabulation of the final data on this
basis is probably more significant than any other that could be
offered.

The data for the xerophilous species are given in Table X.
The average figures given at the bottom of the table show that
germination begins first in the dense shade, next in the medium
shade, and last in the light ; that the germination period is longest
in the dense shade ; that germination begins last in the dry soil ;
that the germination period is shortest in .the dr\- soil ; that
germination begins first in the gravel and that the shortest ger-
mination period is in the loam and gravel. Of the 14 species
given in this table, 13 germinated in the dense shade before they
did in the open, 9 showed longer germination periods in the dense
shade than in the open light, 12 germinated in wet soil before
they did in dry soil, 13 had shorter germination periods in the
dry soil than in the wet, and 9 germinated in gravel before they
did in loam.

Table XI gives the results for the xero-mesophilous species.
The average figures given in this table show that germination
begins first in dense shade, next in medium shade, and last in open
light ; that the germination periods are longest in the medium and
dense shade ; that germination begins last in the medium dry soil ;
that the germination period is shortest in the medium dry soil ;
that germination begins first in the sand or in the gravel ; and
that the germination period is shortest in the gravel. Out of 13
species listed in this table 9 germinated in dense shade before
they did in the open, 7 showed longer germination periods in the
dense shade than in the open light, 12 germinated in the wet soil
before they did in the medium dry soil, 12 showed shorter ger-
mination periods in the dry soil and 9 out of ii germinated first
in either sand or gravel.

Germination of Forest Trees 57

The data for the mesophilous species are given in Table XII.
The average figures at the bottom of the table show that germina-
tion began in dense shade, followed by medium shade and open
light; that the germination period is longest in the case of the
dense shade ; that germination began first in the loam and last in
the gravel ; and that the germination period was shortest in loam.
Out of the lO species listed in this table 7 germinated in the
dense shade before they did in the open light, 4 out of 8 species
showed longer germination period in the dense shade than in the
open hght ; and 7 showed shorter germination periods in the loam
and sand than in the gravel.

These three groups show exactly the same results so far as
light and soil moisture go. From the standpoint of soil texture
there are some interesting results. In the xerophilous species
germination usually begins in the gravel, in the xero-mesophilous
species it usually begins in the sand; and in the mesophilous
species it usually begins in the loam, as the average figures and
number of species in each case testify. In the xerophilous
species the germination period is shortest in the loam and gravel,
in the xero-mesophilous it is shortest in the gravel, and in the
mesophilous species the period is shortest in the loam. That
xerophilous species germinate sooner in the sand and gravel than
in the loam is due undoubtedly to the amount of oxygen in these
soils. This suggests that oxygen is more necessary for the ger-
mination of xerophilous species than is the case in mesophilous
ones. In the mesophilous species germination begins sooner in
the loam indicating that soil moisture is more necessary to them
than oxygen. In the case of the light and the soil moisture
experiments it has been shown that favorable moisture conditions
lengthen the time of germination. In these cases it was found
that the shortest periods were in the open light and in the dry
soil. This same theory is proven in the case of the soil texture
experiments. It is well known that loam is favorable for ger-
mination on account of its moisture-retaining properties and that
gravel is favorable on account of its great amount of aeration.
Sand is intermediate between these and combines enough of the
-oil moisture property of the loam with the aeration of the gravel

58

Richard H. Boerker

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Germination of Forest Trees

59

u

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'pOU3(£

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6o

Richard H. Boerker

so as to make it an ideal soil for germination. Hence we might
expect to find the longest germination periods in the sand. The
average figures show that this is the case in each group of species.
The shortest periods in every case are either in the loam or the
gravel because loam is unfavorable from one standpoint and
gravel from another.

In comparing the check cultures of the three groups of species
it will be seen that xerophilous species germinate first, xero-
mesophilous next, and mesophilous last. In other words the
drier the habitat the sooner germination starts, granting that the
conditions are favorable.

TABLE XII
The Effect of Light and Soil Texture on the Germination of Mesophi-
lous Species

Species

Light

Check
Culture

be m

ml— i

n^O

Medium
Shade

bo c«

col— I

^p

Dense
Shade

a >>

bo CIS

^Q

Soil Texture

Check
Culture

^<^

Sand

bo cu u a

Gravel

a >> .£ >>.

Finns taeda

Catalpa speciosa (Ind.).

Quercus rubra

Betiila papyrifera

Piniis lamberliana ....

Abies magnijica

Sequoia washingtoniana
Tsuga heterophylla . . . .

Picea sitkensis

Pseudolsuga taxifolia
(Wash.)

Averages-

35928.6

31-7

18

I
20
54

6

o
64

54
27.9

34

30.0

42

I
24
54
14

o
36

62

30.1

40

34 I I

70 36

44 52

54

46 |30

1 134 I
16 80 18

2 50 48

16 18 16 ,24 22

I I 44 ;42 50 8

'60 18 ]64 18 64

22 28 38 44 36 48

38.7!25.5i40.4 27.9 40.7 27.0

Tables similar to X, XI, and XII were constructed showing the
effect of these habitat factors upon the germination per cent, of
the species. This table is not given but the most significant facts
which it shows are given here and in a later table. It is interesting
to note that of the 37 species used in the experiments the highest

~ Catalpa speciosa and Tsuga heterophylla not included in averages of
light cultures. Catalpa speciosa not included in soil-texture averages.

Germination of Forest Trees

6i

germination per cent, did not always occur under the influence of
the same conditions. Considering all factors and all degrees of
these factors the highest germination per cents, occurred as
follows :

In open light 3

In medium shade 7

In dense shade ^ ^

In sand '^

In gravel 4

Total ^'

The distribution of these species in the three groups as de-
termined by soil moisture is not significant. The only interesting
fact to be found is that no mesophilous species germinated highest
in the gravel.

TABLE XIII

Summary of the Beginning of Germination by Number of Species

Light

Soil Moisture

Dry soil

Medium wet soil.
Wet soil

Total .

1-33
3.83
8.83

0.50
12.50

14.00

13.00

Number of Species

Germinated First in

Xerophilous

Xero-meso-
philous

Mesophilous

1 otal

0.33

2.33

11-34

2.33 0.50 3-17

2.33 1. 00 5.67

8.33 1 5-50 25.16

Total 14-00

13.00

7.00 1 34-00

1-33

4-33

21.34

27.00

Soil Texture

Loam

Saivl

Gravel

' 2.50

4.00

7-50

2.17
5-17
3-66

2.50
3.00
1.50

7-17
12.17
12.66

Total .

14.00

7.00

32.00

62

Richard H. Bocrker

I

Tables XIII, XIV, and XV take the same data as presented in
previous tables but the results are given by number of species
rather than by average numbers. The number of species in each
group which performed certain things under certain conditions
are given without respect to the name of the species. This is
perhaps a better way of drawing conclusions than to use average
figures. Each species is counted in its proper column ; if a
species, for example, germinated simultaneously in two cultures
it counted one half in each column.

TABLE XIV ''

Summary of the Length of the Germination Period by Number of

Species

Light

Number of Species

Shortest Germination
Period in

Xeiophilous

Xeromeso-
philous

Mesophilous

Total

Open light

7.00
4-50
2.50

5.00
3.00
5.00

2.50
3-50

1. 00

14.50

11.00

8.50

Medium shade

Dense shade

Total

14.00

T^ C\C\

7.00

34-00

Soil Moisture

Dry soil

12.00

12.00

Medium dry soil

Wet soil

1. 00
1. 00

13.00

Total

14.00 ! 13.00

27.00

Soil Texture

Loam ,. .

Sand

3.83

4-33
S.83

4.00
3.00
4.00

3-00
2.00
2.00

10.83

9-33

11.83

Gravel

Total

14.00

7.00

32.00

Out of 34 species 31 germinated first in either of the two
shade cultures and only 3 began their germination in the open
light. The tendency to germinate first in the shade is more
marked in the case of the xerophilous and the mesophilous species
and less marked in the xcro-mesophilous. Out of 27 species.

Germination of Forest Trees 63

over 21 germinated first in the wet soil. In both the xerophilous
and the xero-mesophilous species the tendency is to germinate
first in the wet soil. In the experiments on soil texture the
tendency is for the xerophilous species to germinate first in the
gravel and sand, for the xero-mesophilous to germinate first in
the sand, and for the mesophilous species to germinate first in
the sand and loam. This is a most interesting result, in view of
the moisture and air conditions in these soils. On the whole the
tendency is for most of the species to begin germinating in the
sand and gravel ; about 25 out of 32 species began germinating in
either of these two kinds of soils. In the soil texture data it is
interesting to compare the germination of xerophilous and meso-
philous species in the gravel. Such a comparison shows 7.50
xerophilous species germinated first in the gravel and only 1.50
mesophilous species.

From Table XIV it is apparent that out of 34 species 14.5
showed the shortest germination period in open light and that the
number of species of this kind decreases as the intensity of the
light decreases. In other words shade increases the length of the
germination period. In the soil moisture experiments the
shortest periods were in 25 species out of 27 found in the dry
or the medium wet soil. In the soil texture experiment the
species are very evenly distributed. Loam and gravel, the two
extreme soils from the standpoint of soil moisture and soil aera-
tion, show the greatest number of species and the sand culture
shows the least. This fact is in harmony with the idea that
favorable conditions, such as we found in the light and the soil
moisture experiments, lengthen the period of germination.

Table XV shows that out of 14 drought-enduring species 12
reached their greatest germination per cent, in the shade; out of
13 xero-mesophilous species 10 reached their highest per cent, in
the shade; and out of 8 mesophilous species 7 reached their
highest per cent, in the shade cultures. Out of a total of 35
species, 29 germinated highest in the shade cultures. Out of 2y
s])ecies tried in the soil moisture experiments 2}, germinated
highest in the wet soil and 4 highest in the medium soil. None
reached their highest per cent, in the dry soil cultures. Among

64

Richard H. Boerker

the xerophilous species the highest per cents, are in the loam and
sand, among the xero-mesophilous species the highest per cents,
are in the sand while in the mesophilous species the highest per
cents, are in the sand and loam. Out of 34 species, 18.5 ger-
minated highest in the sand, thus showing the value of this class
of soil for seed germination.

TABLE XV

Summary of the Final Germination Per Cent, by Number of Species.

Light

Number of Species

Greatest Germination
Per Cent in

Xerophilous

Xeromeso-
philous

Mesophilous

Total

Open light

2.00
3.00
9.00

3.00
7.00
3-00

1. 00 6. no

Medium shade

2.50
4-50

12.50
16.50

Dense shade

Total

14.00

13.00

8.00

3500

Soil Moisture

Dry soil

0.00

3.00

11.00

1. 00
12.00

0.00

Medium wet soil

Wet soil

4.00
23.00

Total

14.00

13.00

27.00

Soil Texture

Loam

6.00
5.00
3-00

1. 00
8.00

2.00

2.50
5-50
1. 00

950
18.50

6.00

Sand

Gravel

Total

14.00

11.00

9.00

34-00

THE EFFECT OF HABITAT FACTORS UPON STEM AND ROOT
DEVELOPMENT

Following the experiments upon germination, some of the
species were grown for several months for the purpose of ob-
taining root and stem measurements. Since damping-off re-
duced materially the number of seedlings as time went on, the
number of plants upon which final measurements could be taken
was naturally reduced. Hence the results are not based upon as
manv measurements as was originallv intended.

Genn'mation of Forest Trees

65

The species retained for this work were Finns ponderosa (S.
D.), Robinia pseiidacacia, Quercus rubra, and Finus strobus.
Stem and root measurements were taken upon the first three of
these species and stem measurements only upon the last one.
Each measurement represents the average of 10 representative
plants, except in case of Quercus rubra where from 3 to 14
plants were used depending upon the number available. The
measurements of the stems of Finus ponderosa and Robinia
pseudacacia were taken at two difl^erent ages, namely at the age
of two and three months, but the plants used at the age of three
months were not the same ones used at the end of two months.
Hence in the data the three months' old plants are not necessarily
larger than the two months' old plants, although they usually are.
Root measurements of Finus ponderosa and Rohiniei pseudacacia
were taken at the end of three months. Both stem and root
measurements for Quercus rubra were taken at the age of five
months.

The effect of light on stem and root development is shown in
the following table :

The Effect of Light on Stem and Root Development
Conifers

Siem Measurements

Root Measurements

Degrees

P. pondsrosa

P. strobus

P. ponderosa (3 Mos.)

2 Mos., 3 Mos.,
Cm. Cm.

2 Mos.,
Cm.

Tap.
Cm.

Laterals,
Cm.

Open light

2.76 2.59 4.31 5-93
2.90 3-13 5.50 S-8o
3-50 ... 1 6.3s

I. II

Medium shade

.62

Dense shade

Hardwoods

Stem Measurements

Root Measurements (Tap)

Degrees

R, pseudacacia

Q. rubra

Rob. pseud.

Q. rubra

2 Mos.,
Cm.

3 Mos..
Cm.

5 Mos.,
Cm.

3 Mos.,
Cm.

S Mos.,
Cm.

Open light

Medium shade

Dense shade

6.00
5.80
5.00

7.02
5-95

5-52

9.40
7-30
8.00

9.64
7.16
5.69

13.8
10.2
10.2

66

Richard H. Bocrkcr

From these tables it will be seen that Pinus ponderosa increases
its length of stem with a decrease in light intensity both at the
age of two and at three months. This is likewise true for Pinus
strobus. For Robinia pseiidacacia, however, both at the' age of
two and three months, there is a striking decrease in stem height
wdth a decrease in light intensity. Quercus rubra behaves the
same way, except that the length of stem is greater in the medium
shade than in the dense shade. This development is shown very
well by the accompanying photographs.

In the case of all species it is strikingly shown that the length
of the tap root and the total length of the laterals decrease with
decrease in light intensity.

In so far as the stem and its relation to light is concerned it
is quite evident that hardwoods behave differently from conifers.
As has been pointed out conifers tend to increase their height
growth with decrease in light intensity while hardwoods tend to
decrease this growth with decrease in light intensity. Evidently
conifers can adapt themselves to these unfavorable light condi-
tions better than hardwoods. In the hardwoods the reciprocal
relation of roots and stem in their dependence upon light is strik-
ingly shown.

The Effect of Soil Depth upon Stem and Root Development
Conifers

Soil

1
Depth

Stem Measurements

Root Measurements

Degrees

P/nus

pond.

P. strohus

P. ponderosa (3 Mos.)

2 Mos.,
Cm.

3 Mos.,
Cm.

2 Mos.,
Cm.

Tap,
Cm.

Laterals,
Cm.

Deep

2.85
2.76
2.60

2.69
2.59
2.68

4-35
i 4-31
1 4.25

9-51
5-93
3-97

•47
I. II

Shallow ....

4.61

Ha

rdwoods

Soil

Depth

Stem Measurements

Root Measui

ements(Tap)

Degrees

Ji. psei

dacacia

Q. rubra

Rob. pseud.

Q. rubra

2 Mos.j
Cm.

3 Mos.,
Cm.

5 Mos.,
Cm.

3 Mos.,
Cm.

5 ^los..
Cm.

Deep

6.45
6.00
5-70

1

1 7.20

7.02

1 6.04

6.50

9.40

; 5-90

15-55
i 9.64
1 3-30

20.4
1 13.8

Shallow

1 6.9

Germination of Forest Trees

^7

The foregoing tables show the effect of soil depth upon root
and stem development for the same species and ages of stock.

In the case of stem development in all species except Quercus
ruhra, the height of the stem increases with increase in soil depth.
The increase in length between the deep soil and the shallow soil
is not very great, i. e., in the pines it is never over 0.25 cm. and
in Robinia it is never over 1.16 cm. In Quercus rubra the
smallest height growth is in the shallow soil but the greatest
height growth is in the medium deep soil. It is interesting to
note that in all cases the greatest total length of stem and root
together is in the plants grown in deep soil.

As is to be expected the length of the tap root is materially-
decreased as the soil depth decreases. In Pinus pondcrosa the
tap root is 2^/^ times longer, in Robinia it is 5 times longer and
in Quercus it is 3 times longer in the case of the deep soil than in
the shallow soil. The length of lateral roots was taken only in
the case of Pinus pondcrosa and this species is representative of
what took place in all the other species. In this species the total
length of lateral roots increased with decrease in soil depth. In
the case of Robinia this is strikingly shown in the photographs.
This indicates that whether a tree has deep-seated roots or super-
ficial roots depends largely upon the depth of the soil in which
the tree grows. The terms " deep-rooted species " and " shaUow-
rooted species " have therefore only limited significance and the
real basis for these terms is in most cases the environment.

In the following table are given the data upon the effect of soil
moisture upon root and stem development:

The Effect of Soil Moisture upon Stem and Root Development
Conifers and Hardwoods

Degrees

Stem Measurements

Pinus pondcrosa

2 Mos., ! 3 Mos.,
Cm. I Cm.

Robinia pseud.

2 Mos., 3 Mos., 2 Mos.,
Cm. Cm. Cm.

Root Measurements^

Dry . . I 2.60 ....

Medium 1.80 2.02 ! 4.35 3.80 3.90 7.33 2.65

Wet 2.76 2.59 I 6.00 7-02 I 4.31 5.93 I l.u

3 Age, 3 months.
•* Age, 2 months.

I

e8 Richard H. Boerkcr

In connection with the soil moisture experiments a very in-
teresting fact was noted. Both Pinus ponderosa and Robinia
pseudacacia wilted on January i, just exactly two months after
the seeds were sown. The soil moisture at the time was de-
termined to be 6.6 per cent. It happens that at three different
times the moisture content was far below this figure. On Oc-
tober 28 the seeds were sown, on November 7 the moisture
content was 4 per cent., on the nth it was 4.6 per cent, and again
on December 5 it fell to 6.1 per cent. Robinia pseudacacia ger-
minated first on November 9 and the Finns ponderosa on No-
vember 26. It is evident from this occurrence that more mois-
ture is needed for the early development of the seedlings than is
necessary for germination. On the 9th of January this fact was
further emphasized. While taking root and stem measurements
and digging up the seedlings two germinating seeds of Robinia
were found. The moisture samples taken on this day show S-7
per cent, moisture in the dry culture. As a result of this condi-
tion no stem and root measurements appear in the dry column at
the age of three months.

In all species measured the length of the stem decreases with
diminishing moisture supply and the fact is noted that this de-
crease is greater in the case of Robinia than it is in the case of
Pinus ponderosa or Pinus strobns. This indicates the greater
drought resistance of the conifers as compared to the hardwoods.
In the case of the root development of Pinus ponderosa it is
shown that both the tap root and the total length of lateral roots
increase with diminishing moisture supply. For Robinia the
result was quite different, for it was found that the length of the
tap root decreases with diminishing moisture. While Pinus
ponderosa seems to be able to develop roots to reach the lower
moisture layers of soil, Robinia is unable to do this.

The following table gives the results on the effect of soil tex-
ture upon the development of the stem and roots of these species.
The greatest length of stem in Pinus ponderosa was found to
be in the case of the two-months-old seedlings in the loam and the
next greatest length in the gravel. In the case of the three-
months-old trees the greatest length was in the gravel and the

Germination of Forest Trees

69

next greatest in the loam. In the case of Pinits ponderosa clearly
the greatest length is either in the loam or in the gravel and the
shortest length of stem is in the sand. Loam and gravel are, as
we have seen, quite opposite when it comes to moisture retentive-
ness, hence the good development of plants grown in gravel must
be attributed to other properties of gravel, namely, the amount

The Effect of Soil Texture upon Stem and Root Development

Conifers

Degrees

Degrees

Ste

tn Measurements

Root Measurements

P. ponderosa

P. sirobus

P. ponderosa {3 Mos.)

2 Mos.,
Cm.

3 Mos.,
Cm.

2 Mos.,
Cm.

Tap, 1 Laterals,
Cm. Cm.

Loam

2.76 2-59

4.31
4.80
4.10

5-93
6.22
7.83

I. II

Sand

Gravel

2.15 2.06

2.6s 2.70

•94
4.01

Hardwoods

Stem Measurements

R. pseudacacia

2 Mos.,
Cm.

3 Mos.,
Cm.

Q. rubra

5 Mos. ,
Cm.

Root Measurements (Tap)

Rob, pseud. Q. rubra

3 Mos.,
Cm.

5 Mos.,
Cm.

Loam. .
Sand . .
Gravel .

6.00
4-75
3-80

7.02
4-75
4-25

9.40
5-90
5-70

9.64
10.85
10. II

13.80
1570
16.00

of air in the soil. Finns strobns shows the greatest height growth
in the sand. Robinia shows the greatest length of stem in the
loam and the least in the gravel. This is in peculiar contrast to
Finns ponderosa. For growth Robinia is evidently more par-
ticular about soil moisture than about the amount of air in the
soil. Onercns rubra shows the greatest height growth in the
loam and the least in the gravel.

The tap root of Finns ponderosa is of greatest length in the
gravel and least in the loam, and the total length of lateral roots
is greatest in the gravel. This naturally follows from the fact
that, as has been pointed out before, gravel allows water to
percolate rapidly and the top layers dry out very soon, hence the

I

-JO Richard H. Boerker

plant has to go deep for its moisture. In the cases of Robiiiia
and Quercus the greatest length of laterals and the greatest
length of the tap root was found in the sand or gravel, again
bearing out the fact that sands and gravels are poor soils for
retaining moisture.

THE RELATION OF SIZE AND WEIGHT OF SEED TO GERMINATION PER
CENT. AND EARLY DEVELOPMENT

During the process of counting between 100,000 and 125,000
seeds of various kinds for these experiments the fact that seeds
of the same species varied considerably in size came to the
author's notice very forcibly. In his experience in the woods as
well as in seed extracting it was often noted that many factors
may affect the size of seeds. In general, it may be said that the
size of the seeds of any one species depends upon one or more of
the following factors :

1. The size of the cone.

2. The position of the seed in the cone.

3. The development of the cone.

4. The age of the tree.

5. The physiological condition of the tree.

6. The site upon which the tree grew.

7. The climatic variety of the species.

It is an old experience that large cones produce large seeds
and small cones small seeds. The seeds at the extreme base and
the extreme apex of the cone are very often very much smaller
than in other parts of the cone. External conditions such as
temperature and moisture, may affect in no small degree the seed
while it is maturing, thus retarding its morphological develop-
ment. It has been observed that middle-aged trees produce the
largest cones and the largest seeds, while very young or very old
trees usually produce small cones and small seeds. The physio-
logical condition of the tree may affect the size of the seed.
Since seed crops are dependent upon the accumulated food in the
tree, it is reasonable to suppose that a paucity of such food ma-

Germination of Forest Trees 7^

terial will produce smaller seeds than in cases where there is a
great accumulation. It has been repeatedly shown that after a
seed year the amount of accumulated food in the medullary rays
and other food accumulation centers is reduced to a minimum.
The site upon which the tree grew, naturally, is intimately con-
nected with the amount of food material available for the embryo
of the seed. For the same reason the climatic variety of the
tree probably affects the size of the seed. At least, it is common
knowledge that the California variety of Pinus ponder osa has
seeds which may weigh from three to four times as much as
those of the South Dakota variety. While most of these points
remain to be proven experimentally, they have been indicated to
the writer by various experiences and are put forth as interesting
hypotheses awaiting experimental proof. Whatever the cause of
the varying size of seeds is, it is quite evident from the amount
of literature on the subject that this phenomenon has attracted
considerable attention in recent years both in silviculture and
agriculture.

That heavier and larger seeds furnish a better germination per
cent, than light ones has been recognized for a long time by
European silviculturists. The physiolog}' of germination indi-
cates that large seeds should succeed better, and repeated ex-
periments by Buhler, Friedrich, Haack, Eisenmenger, and others
establish this beyond much doubt. In fact forestry practice
throughout Europe and especially in Prussia shows that smaller
seeds produce fewer plants per hectare than larger ones in broad-
cast sowing. Favorable and unfavorable site and season condi-
tions produce far less variation in the final results in cases where
heavy seeds are sowed.

In 1904 Blumer (22) conducted at the seed laborator)- of the
United States Department of Agriculture a series of tests upon
certain American species of tree seeds. Pinus ponderosa irom
the Rocky Mountains and Pinics divaricata showed the highest
germination but Pinus ponderosa from Oregon germinated ex-
ceedingly slowly, a feature which also characterized Pseudotsuga
taxi folia from "the Pacific Coast. He noted great variation in
the number of seeds per pound for the same species, especially

72 Richard H. Boerkcr

for Pinns pondcrosa. In the case of this species the difference
was often as much as lOO per cent. ; usually the difference in other
species did not exceed 50 per cent. Schotte (23), of the Swedish
Forest Experiment Station, has shown that the size of the seed
and the size of the cones decrease with increasing age of the tree
in the case of Scotch pine. The work (24) done on seeds by
certain forest experiment stations in Europe in 1907 with spruce
showed that seeds from large cones germinate earlier than those
from small cones ; that the largest cones produce the largest and
heaviest seeds and hence the largest plants ; and that the eft'ect of
the size of seed upon the Hfe of the plant has been noticed only
in the first two years of its growth.

In Busse's (25) experiments pine seeds were graded by means
of a Kayser centrifuge into three grades according to weight.
The heaviest seed made up 68 per cent, of the stock seed, the
medium weight seed 27 per cent, and the light seed 5 per cent.
He recommended the first grade for field sowing but said that the
third grade should not be used. Sprout tests did not show anv
differences in germination results. Centgraf (26) examined 247
tests of pine seed as to the relation of the weight of 1,000 grains
to their germination. He failed to find a relation between weight
and germinative energy or germination per cent. In fact he
found that the heavier seed averaged a smaller germination per
cent, than the light ones. He concluded that the slower germina-
tion of l)ig seed is probably due to a thicker seed coat of the
heavier seed wdiich determines in part its weight and which takes
up water more slowly than thin coats of light seed. Some of
these results do not agree with the many experiments made by
foresters in Europe. These tests being made for commercial
purposes cannot therefore be taken as conclusive.

While the size and weight of seed has been recognized a? a
factor in germination it also has been recognized as a factor in
the early development of the seedling as has been indicated in a
few instances above. One finds statements in regard to this rela-
tion quite common in silvicultural works but very little material
to substantiate these opinions. The view held by many writers is
summarized very well by Scblich in his Manual of Porcstrv
(27) :

Genninatioii of Forest Trees 73

In the case of one and the same species large, heavy seed are better than
light ones. The former generally possess a greater power of germination
and the resulting seedlings show a greater power of resistance against
injurious external influences and a more vigorous development which in
many species is due to the greater quantity of reserve food materials
deposited in the seed. This superiority at the first start should not be
underestimated because it is recognizable long after the seedling stage
has been passed. In many cases the dominant trees grow out of the seed-
lings which had the better start.

The relation of size and weight of seed to germination per
cent, and later development has been worked ont to a much
greater degree of certainty in the case of agricultural and garden
seeds than in the case of forest-tree seeds. These facts have
already been quite firmly established in practice and already
adopted as a criterion of seed values. There is no reason why
weight of seed should not play as important a part in selecting
forest tree seeds as well as agricultural and garden seeds in the
future, as the source and germination per cent, of those seeds.

A considerable amount of work has been done by investigators
upon cereals, regarding the comparative value of heavy and light
seed used in planting. Most of the work has been done with
wheat, oats, and barley and the preponderance of evidence is in
favor of the large seed. The hypothesis upon which this work
has been based was the fact that, since the weight and size of the
seed determines largely the amount of food material immediately
available for the plantlet at the time of germination, it is reason-
able to assume that these factors might have some influence upon
the life of the plant and even upon the final crop.

Early experiments by Hellriegel, Wollny. Marek, and others
(2S) were favorable to the view that seeds of greater size and
weight generally give more vigorous plants than those smaller
and lighter. Hellriegel was of the opinion that differences at
maturity between the product of heavy and light seeds are in-
tensified when the conditions are unfavorable. Hicks and
Dabney (28) have made a test of the relative eft'ects of weight
upon vigor, using many kinds of seeds. In the case of radish,
vetch, sweet pea, cane, Kafir corn, rye. and oats the total weight
of the seedlings in each case favored the heavy seed. The differ-
ences in germination per cent, of light and heavy seed was not

74 RicJiard H. Boerker

conclusive. Only in the case of the corn was there a sufficient
difference to warrant a conclusion in favor of the heavy seed.
From the results of these experiments it seems logical to conclude
that in general more vigorous growth and consequently a better
stand in the field is secured by employing only the heavier seed.
The efl'ect of the size and weight of seed on production has been
with no other plant so extensively studied as in the case of the
wheat. The majority of results seem to favor the view that large
and heavy seed are preferable. Zavitz (28) showed that the
yield in bushels per acre was in favor of the large plump seed.

Trabut (32) found in the case of tobacco seeds that it was
possible to aft'ect a separation into heavy and light sorts through
the capacity of these two kinds respectively to sink and float in
water. It was found that the heavy seed produced plants which
were greener, more vigorous, and of larger size. The yield from
plants from the heavy seed was almost double that of the yield
from the light seed. Shamel (31) secured results similar to
these. Careful comparative tests of the light and heavy seeds of
tobacco have proved that the best developed and most vigorous
plants are always produced from the large, heavy seed while the
light seed produce small, irregular and undesirable plants. In
an experiment with Cuban tobacco seed Shamel found the
germination of heavy seeds almost perfect while less than five
per cent, of the light seeds sprouted. The plants from the heavy
seed grew more rapidly than those from the light seed and
reached the proper size for transplanting seven to nine days
earlier than the plants from the light seed.

In the case of cotton seed, comparative production tests of the
value of the heavy seed over the usual farm product have been
made by the U. S. Department of Agriculture (30). The yields
in pounds on equal areas in South Carolina show the gain from
the use of heavy seed in two different cases to be 10.9 per cent,
and 8.25 per cent, respectively.

Bolley (29) selected large and small grains from the same
heads of wheat and found that the large grains generally pro-
duced the largest yields. Waldron (29) found that short wheat
culms, shortheads, and those with a smaller number of grains

Germination of Forest Trees 75

bear on the whole grains of a greater weight. Walls (37), work-
ing upon the size of the grain and the germ of corn, concludes
that the heaviest grains do not necessarily have the best ger-
minating qualities and that plants from the heaviest grains attain
the greatest weight, other conditions being favorable. Concern-
ing the size of the germ he finds that the germinating properties
of the kernels containing different sizes of germs may be equal;
that the largest, hardiest, and most vigorous plants come from the
kernels with the large germs; and that the plants from the
kernels with the largest germs withstand the drought best. He
says in the selection of corn, in order to insure a good stand and
a large yield none but the large germed kernels should be used.

Harris (33, 34, 35) working on the differential mortality with
respect to seed weight of beans and peas secured similar results,
though in a different way. In the case of peas about 1,000 seeds
from each of ten early varieties were weighed and planted. In
seven cases out of ten the total weights of the seeds which ger-
minated was higher than the total weights of the seeds which did
not germinate. Cummings (38) worked with numerous kinds of
garden seeds. He quotes numerous investigators who worked on
corn, oats, wheat, sugar beets, cotton, and beans and practically
all the results show an increased yield through the use of large
seeds. He himself worked with squash, pumpkin, lettuce,
spinach, parsley, radishes, beans, garden peas, and sweet peas.
Here too the results were almost without exception in favor of
the large seeds. Not only were the resulting yields larger and
heavier but in most cases the yield was earlier. In the case of
the radishes the large seeds produced more uniform crops one
week earlier than the small seeds. Sweet peas showed earlier
blossoming, a larger total yield of blossoms and a larger number
of blossoms of good quality. On the whole the permanent ad-
vantages accruing from large seeds are a larger and greater
number of leaves, flowers and fruits.

Present Investigations

Having on hand several climatic varieties each of Pinus pon-
derosa and Pseudotsuga taxifolia. I was prepared to determine

76

Richard H. Bocrker

the effect of size upon germination per cent, for many varieties
of the same species. This study would also bring out some
interesting relations between these varieties, as for example, cor-
relating the size and weight of the seed with the site upon which
the trees grew.

The largest and the smallest seeds were separated from the
stock seed and counted, weighed, planted and carefully labelled.
Of each variety of Pimts ponder osa 500 seeds were used except
in the case of the California varieties. Due to the scant supply
of these only 200 seeds of each of these were used. In the case
of the Pseudotsuga taxifolia 200 seeds of each variety were used.
After germination began counts were taken every other day.
The tables below give the size of the seeds, weight of 500 seeds,
the number of seeds per pound, the final germination per cent., and

Size AND Weight of Seed in Relation to Germination Per Cent.
Pinus ponderosa

Source or Variety

Size

Size,
Mm.

Total

Weight SCO

Seeds,

Gm.

Seeds
Per Lb.

Final

Germ,

Per

Cent

Per Cent

in Favor of

Large

Seeds

South Dakota

3-5
5-9
4-6

10.065
20.720
10.845

22,530

ir.ooo
20,900

50.6
53-6
25.0

Harney, N. F., S. D. . .

Large
Small

3-0

Bitterroot, N. F., Mon.

Large
Small .

6-9
5-8

20.720
19.050

11,000
11,900

40.2
7-6

15-2

Weiser, N. F., Idaho.. .

Large
Small

8-1 1

4-7

30.400
17.100

7.450
13.250

8.0
60.0

0.4

Pecos, N. F., N. M. . . .

Large
Small

7-10
4-7

29-540
16.150

7.650
14,000

84.8
65.2

24.8

CaHfornia

Large
Small

7-9
7-1 1

11-14
7-10

10-14

23-470
35-500
67.000

9.650
6,350
3.385
8.725
2,900

73-4

63-5

73-5

8 0

8.2

P. jeffreyi

Large
Small

10. 0

Large

77.600

84.0

76.0

Pseudotsuga taxifolia

Caribou, N.F., Idaho.
Pecos, N.F.,N.M.. .

Washington

Colorado

Madison, N. F., Mon.

Small
Large
Small
Large
Small
Large
Small
Large
Small
Large

6.040
8.290
5-450
7.850
3.780
6.450
3-750
6.980
3-350
6.630

32.5
42.5
65.0
69.0
16.S
16.0
79-0
88.0

43-5
50.0

10. o

4.0

—0-5

9.0

6.5

Germination of Forest Trees 77

the per cent, in favor of the large seeds. In converting grams
to pounds it was assumed that 453.6 grams equals one pound.
The germination period for Pinus ponder osa was 120 days and
for Pseudotsuga taxifolia 100 days.

From these tables it will be seen that in every variety of Piniis
ponderosa the final germination per cent, is in favor of the large
seeds. In the case of Pseudotsuga taxifolia every variety except
one shows a final per cent, in favor of the large seeds.

It is well known that there are definite climatic differences
between the Rocky Mountains and the Pacific coast. The most
conspicuous proof of this is in the flora of these regions. In
general the Pacific coast is inhabited by relatively mesophilous
vegetation, especially near the coast, while the vegetation of the
Rocky Mountains is more xerophilous in nature. Again, the
Rockies themselves show marked differences in this very respect
in travelling from south to north and from east to west.

Probably the best way of studying the effect of great climatic
variations upon vegetation is to use polydemic species such as we
are considering here. Pinus ponderosa and Pseudotsuga taxi-
folia are conspicuous examples of this class of species. It is
well known that both these species reach a better development on
the Pacific coast than in the Rocky Mountains. It is likewise
well known that they reach a far better development in the
northern Rockies than in the southern. As a proof of this we
have but to go to volume tables of these species in the Woods-
man's Handbook by Graves and Ziegler. In the case of Pinus
ponderosa three tables are given, one for the Black Hills, one for
Arizona, and one for California and Montana. In studying
these tables it will be seen that the maximum heights and
maximum diameters and the average and maximum heights for
a given diameter increase steadily in going from the Black Hills
to California. In the case of the Douglas fir the same thing is
true in considering the volume table for Idaho and Wyoming
and that for Washington and Oregon. In the order of their
favorability for tree growth, as manifested by these species these
regions arrange themselves in (lie following order, the least favor-
able being given first :

78 Richard H. Bocrkcr

Black Hills

Arizona and New Mexico

Colorado and Wyoming

Idaho and Montana

Washington, Oregon, and California.

It is a striking fact in the case of Pinus ponder osa that the
size and weight of the seed and their manner of germination
follow exactly this same order. The smallest seeds come from
the Black Hills and New Mexico and the largest from California ;
the total weight of 500 seeds is least in the case of the Black
Hills variety and greatest in the California variety, hence the
number of seeds per pound is greatest in the former and smallest
in the latter variety. Furthermore, germination begins sooner,
the germination period is shorter and the germination curve rises
more rapidly in the case of the South Dakota and New Mexico
variety than in the case of the Pacific coast variety. Some of
these striking relations between seeds and site are also shown by
PseudotS'uga taxifolia. This species shows all these relations
except those of weight of seed and number of seeds per pound.
There seems to be no definite relation in this respect.

On page 79 the germin?tion curves of the climatic varieties of
Pinus ponderosa and Psadotsitga taxifolia are given and they
illustrate very forcibly whn.t has been said above concerning the
behavior of these curves.

In order to determine the efifect of the size of the seed upon
the size of the seedling shortly after germination, the seedlings
were dug up very carefully as they were counted and taken to the
laboratory and measured. The total length of the hypocotyl and
tap root was taken in each case, the seed being excluded from the
measurement. These measurements were kept separate for the
small and large seeds and the results are given below :

Pinus ponderosa (Idaho) — Age, 2 Days

200 seedlings from small seeds averaged 307 cm.

^00 seedlings from large seeds averaged 3-90 cm.

These 400 seedlings were classified according to their total length
as follows :

Gcniiiiiatioii of Forest Trees 79

The Effect of Climatic Varieties upon Germixatiox

FiG. I. The germination curves of Pseudotsuga taxifolia.

Fic. 2. Tiie germination curves of Pinus pondcrosa.

I

8o Richard H. Bocrker

Seedlings from Seeds
Size, Cm. Small Large

0-0.5 2 O

0.6-1.0 10 o

11-1.5 17 6

1.6-2.0 26 14

2.1-2.5 24 19

2.6-3.0 25 25

3i-3o 30 26

3-6-40 22> 31

4I-4.5 15 17

4-6-50 II 15

51-5.5 8 13

5.6-6.0 4 12

6.1-6.5 3 9

6.6-7.0 o 8

7.1-7.S o 2

7.6-8.0 I 2

8.1-8.5 I o

8.6-9.0 I I

Total 200 200

It will be seen that most of the plants from the small seeds fall
between the limits 0.6 and 5.0 while most of the plants from the
large seeds fall between the limits 1.6 and 6.0. In other words a
greater per cent, of small plants were found among the plants
that germinated from the small seeds. The average difference
in size of 200 plants of each kind was 0.84 cm. in favor of the
plants from the large seeds.

The measurements taken for another climatic variety of Finns
ponderosa were as follows :

Finns ponderosa (South Dakota) — Age, 4 Days

35 seedlings from small seeds averaged 4.6 cm.

51 seedlings from large seeds averaged 5.6 cm.

Here there is a difiference of i.o cm. in favor of the seedlings
from the large seeds.

Similar measurements were taken in the case of Pscndotsnga
taxi folia:

Pscndotsnga taxifoUa (Nezv Mexico) — Age, 4 Days

100 seedlings from small seeds averaged 358 cm.

100 seedlings from large seeds averaged 4-27 cm.

Germination of Forest Trees 8i

These 200 seedlings were classified according to their total length
as follows :

Size, Cm.
0-0-5

Seedlings from Seeds

Small Large

0 O

O.^I.O 0 °

I.I-I.5 I °

1 .6-2.0 9 3

-■-^•5 7 9

2.6-3.0 20 8

3-1-3.5 19 12

3.6-4.0 12 '^

4.1-4.5 10 '7

4.(^5.0 II ^2

5.1-5.5 5

5.6-6.0 5 7

6.1-6.5 I 6

6.^7.0 o ^

7.1-7.5 o .J

Total 100

100

It will be seen from this table that most of the seedlings from
the small seeds fall between the limits 2.6-5.0 while most of the
seedlings from the large seeds fall between the hmits 3-I-5-5-
Just as in the case of Pimis ponderosa above we see that the
greater per cent, of small seedlings are found among the seedlings
that germinated from small seeds. The average difference in
size of 100 plants of each kind is 0.69 cm. in favor of the plants
from large seeds.

The measurements taken for another climatic variety of Pseu-
dotsitga taxifoUa were as follows :

Fscudotsuga taxifoUa {Colorado)— Age, 4 Days

31 seedlings from small seeds averaged 34 cm.

76 seedlings from large seeds averaged 39 cm.

Here again there is a difference of 0.5 cm. in favor of the large
seeds. In comparing Pseudotsiiga taxifoUa with Pinus ponderosa
it is found that the size of the seed makes a greater difference in
the case of the latter species than in the case of the former. Also,

82

Richard H. Boerker

the difference in both cases is greater for the variety that comes
from the drier climate, that is, the South Dakota variety of Pinus
ponderosa shows a greater difference than the Idaho variety and
the New Mexico variety of Pseudotsiiga taxifolia shows a greater
difference than the Colorado. The data here presented upon this
phase of the problem, however, are not sufficient to warrant
conclusions.

GENERAL SUMMARY AND CONCLUSIONS

I. The Effect of Habitat Factors upon Gennination

1. Shade decreases evaporation and transpiration and thereby
increases the soil-moisture content of the superficial soil layers.
This increase in soil moisture content is best shown by the ac-
companying diagram. This conclusion agrees with the results
obtained by Stewart and Hasselbring who grew tobacco in shade
tents.

2. Shade accelerates germination, that is seeds germinate
sooner in the shade than in the light. This acceleration is due
to the increase in soil-moisture content spoken about above.

/o

opet7

Diagram showing soil moisture content in the three light cultures.

Germination of Forest Trees 83

Burns reached the conclusion that shade reduces the temperature
of the soil and delays germination. Evidently there is a delicate
balance between soil moisture and soil temperature, so that a
slight deficiency in either might delay the germination process.
In Burns's case the temperature of the soil was so low, that m
spite of the fact that there wvis sufficient soil moisture, germina-
tion was delayed. In the present investigations soil temperature
was kept at an optimum and measured differences in soil moisture
were sufficient to result in an acceleration of germination in the
shade cultures. One unfortunate fact about Burns's work was
that he failed to take into account soil moisture. In his experi-
ments it must be assumed that there was sufficient soil moisture
for germination. But there is nothing in his report which does
not indicate that there was too much soil moisture. • The recip-
rocal relation between soil temperature and soil moisture is well
known. Furthermore there is an intimate relation between soil
moisture and soil aeration and germination. Such factors as
these were evidently not taken into account to explain the delay
in germination in the experiments cited.

3. Shade increases the length of the germination period. This
bears out to a certain extent Pittauer's experiments which show^ed
that germination proceeds more rapidly in the light than m the

shade.

4. The germination curve of seeds sown in the shade rises more
rapidlx than the curve of seeds sown in the light. This con-
clusion does not agree with the results obtained by Pittaucr.

5. The final germination per cent, is usually higJier in the case
of seeds sown in the shade than those sown in the light. This
conclusion, based upon abundant evidence, is not in accord with
some work done by Atterberg which showed a greater germina-
tion per cent, in the presence of light than in the absence of it.

6. Light phiys absolutely no part in the germination of tree
seeds; in fact shade has been found to be exceedingly beneficial
to germination, other factors being e(|ual. In the work carried
on l)y Burns already referred to. there are at least two state-
ments that a certain amount of liglu is necessary for satisfactory
germination. Whether he means to imply by the term "light"

84 Richard H. Boerker

merely the luminous energy or the heat energy of the sun or both
is difficult to say. As a general thing it is impossible to have light
energy without a certain amount of heat energ}', but heat and
light affect plants so differently that the final eft'ect of these
factors is easily recognized. It is important to keep these two
concepts separate in order to avoid confusion. Graves also makes
the statement that light is necessary for the germination of
Western White pine. It is inconceivable how luminous energ)^
can play any part in germination, especially when the seeds are
below the ground ; it is likewise difficult to conceive what possible
effect light could have if it did reach the seed.

7. An inadequate supply of soil moisture delays germination.

8. An inadequate supply of soil moisture decreases the length
of the germination period.

9. A lack of soil moisture decreases the final germination per
cent.

10. The germination curves of seeds sown in wet soil rises
much more rapidly than that of seeds sown in dry soil.

11. Xerophilous species begin germination firsts xero-meso-
philous germinate later, and mesophilous germinate last.

12. The germination period of xerophilous species is sJwrter
than that for either the xero-mesophilous or the mesophilous
species.

13. In xerophilous species germination is accelerated in the
gravel and sand; in mesophilous species it is accelerated in loam
and sand. In general germination is accelerated in sand and
gravel due not to the amount of soil moisture in these soils (see
accompanying diagram) but to the amount of oxygen in the soil.

14. The germination peried is longest in the sand.

15. The germination per cent, is visually highest in the sand.

16. The rise of the germination curve of seeds sown in sand
is usually more rapid than of seeds sown in loam or gravel.

17. According to the table on page 29 of this report the volume
of air space in a given volume of soil is about 39 per cent, for
gravel, 33 per cent, for sand, and 53 per cent, for loam. In the
accompanying diagram is shown the amount of capillary water in
these soils at the time of watering and twenty-four hours later.

Germination of Forest Trees

85

This diagram shows very strikingly the water retaining capacity
of these three soils. Not only do sand and gravel hold less mois-
ture at the time of watering but they lose a much greater per
cent, of it in the course of twenty-four hours than does loam.

Diagram showing :

n soil moisture per cent, at time of watering;
■ soil moisture per cent, twenty-four hours later.

When we consider the amount of air space in these soils and the
amount of soil moisture each retains, the fact that loam usually
contains a great deal of moisture and very little air space and that
gravel contains very lillle moisture and a great volume of air
space is very strikingly shown.

86 Richard H. Bocrker

II. The Effect of Habitat Factors upon Stem and Root
Development

1. Piniis ponderosa and Pinus strobus show increased height
growth with diminishing light intensity. This conclusion bears
out the results secured by Nikolsky who worked with pine and
spruce and Burns who worked with Pinus strobus. On the other
hand Badoux showed that pines decrease their height growth
with increasing shade ; but these trees were grown to a height of
about six feet while Nikolsky and Burns experimented with much
smaller stock.

2. Robinia pscitdacacia and Oncrciis rubra show a decrease in
height growth with dijninisJiing light intensity.

3. Finns ponderosa shows a decrease in length of tap root and
in total length of laterals with diminishing light intensity. These
results again bear out the conclusions of Nikolsky and Burns.

4. Robinia pseudacacia and Oner ens rubra show a decrease in
length of tap root and total length of lateral roots with decreased
light intensity.

5. Pinus ponderosa, Robinia pseudacacia, Finns strobus, and
Ouercus rubra show increased height growth with an increase in
soil depth.

6. Pinus ponderosa, Robinia pseudacacia, Finns strobus, and
Ouercus rubra show an increase in length of tap root but a
decreased development of lateral roots with increased depth of
soil.

7. Pinus ponderosa, Robinia pseudacacia, and Pinus strobus
show a decrease in height growth with a decrease in the soil
moisture supply.

8. Finns ponderosa shows an increase in length of tap root
and an increase in total length of lateral roots with diminishing
soil moisture content.

9. Robinia shows a decrease in length of tap root with a de-
crease in soil moisture supply.

10. Pinus ponderosa shows the greatest height growth in the
loam and gravel, but Finns strobus shows the greatest height
growth in the sand.

11. Robinia pseudacacia and Ouercus rubra show the greatest

Germination of Forest Trees 87

height growth in the loam and the least in the gravel Compar-
ing this conclusion with No. 10 it is interesting to see that the
conifers do w^ell in either sand, loam or gravel, but that the hard-
woods do best in loam only.

12. Pimts ponder osa, and Querciis rubra show the greatest
length of tap root and greatest length of lateral roots in the gravel
and the shortest length in the loam; Rohinia pseudacacia shows
the greatest length of tap root in the sand and least in the loam.
In other words, root development is usually greatest in the gravel,
and least in the loam. This conclusion agrees in part with
Tolsky's results that pine in black soils develop vertical roots but
in sand develop a greater spread of lateral roots.

13. As far as height growth goes it is evident that pines, on
account of their greater drought resistance, may grow as well m
sand or gravel, or even attain a greater height in sand or gravel
than in loam ; while hardwoods which prefer moister soils grow
best in loam. That root development is greatest in gravel is due
undoubtedly to the fact that water quickly percolates through this
soil and hence the roots have to go deep for the moisture.
Reference to the diagram on page 85 will bring out these rela-
tions more clearly.

III. The Relation of Sise and Weight of Seed to Germination
and Early Development.

1. Large seeds of Finns ponderosa and Fscudotsnga taxifolia
produce a higher final germination per cent, than small seeds.
This conclusion contradicts the results of Busse and Centgraf
who found no relation between size of seeds and germination per
cent., but it proves the contentions of many old silviculturists
that large seeds produce a higher germination per cent.

2. At the age of from 2 to 4 days large seeds of Finns pon-
derosa and Fseudotsuga ta.vifoUa ])ro(luce larger seedlings than
small seeds. This conclusion i)roves at least in part Schlich's
statement on page 73 concerning the use of large seeds in plant-
ing and nursery work and bears out the contentions of practicing
foresters in h:urope that large seeds should be used in field
sowing. This conclusion likewise agrees with the mass of evi-
dence collected in connection with many cereal and garden vege-

88 Richard H. Bocrkcr

table seeds, namely that the use of large seeds results in a better
all round later development and a greater final crop.

3. The Rocky Mountain varieties of Pinus ponderosa produce
Smaller seeds, their germination begins earlier, their germination
period is shorter, and their germination curves rise much more
rapidly than in the case of the Pacific coast varieties of this
species.

4. Except for the size of the seed, the same relations hold for
the Rocky Mountain and Pacific coast varieties of Psendotsuga
taxifoUa. Blumer noted the slow germination of Pinus pon-
derosa and Psendotsuga taxifolia from the coast and he also
noted the great difference in size of the seed of Pinus ponderosa.
These observations are corroborated.

BIBLIOGRAPHY

1. Clements, F. E. Research Methods in Ecology, Lincoln, 1905.

2. Zon, R., and Graves, H. S. Light in Relation to Tree Growth. U. S.

Dept. of Agriculture, Forest Service, Bui. 92, 191 1.

3. Hasselbring, H. The Efifect of Shading on the Transpiration and

Assimilation of the Tobacco Plant in Cuba. Bot. Gaz., 57, 1914.

4. Stewart, J. B. Effects of Shading on Soil Conditions. U. S. Dept.

Agric, Bureau of Soils, Bui. 39, 1907.

5. Haak, J. Die Priifung des Kiefersamens. Zeitschrift fiir Forst- und

Jagd-wesen, April, May, 1912.

6. Pittauer, E. tJber den Einfluss verschiedner Belichtung und Extremen

Temperaturen auf den Verlauf der Keimung forstlichen Saatgutes.
Centralblatt fiir das gesammte Forstwesen, April, May, 1912.

7. Graves, H. S. The Place of Forestry among Natural Sciences. Sci-

ence, N. S., XLL: 117, 1915.

8. Tolsk}^ A. P. Work of the Forest Experiment Stations of Russia.

Review in Forestry Quarterly, III, 1905.

9. Burns, G. P. Studies in Tolerance of New England Forest Trees.

Vt. Agric. Exp. Sta. Bui. 178, 1914.

10. Flaberlandt, G. Physiological Plant Anatomy. English edition trans-

lated from fourth German edition, 1914.

11. Coulter, J. M., and Barnes, C. R., and Cowles, H. C. A Textbook of

Botany, 191 1.

12. Clements, F. E. Plant Physiology and Ecology-. New York, 1907.

13. Timiriazeff, T. A. The Life of the Plant, 1912.

14. Shull, C. A. The Oxygen Minimum and the Germination of Xan-

thium Seeds. Bot. Gaz., 52, 191 1.

15. Shull, C. A. Semipermeability of Seed Coats. Bot. Gaz., 56. 1913.

16. Davis, W. E., and Rose, R. C. The Effect of External Conditions

Germination of Forest Trees 89

upon the After-ripening of the Seeds of Crataegus mollis. Bot.
Gaz., 54, 1912.

17. Eckerson, S. A Physiological and Chemical Study of After-ripening.

Bot. Gaz., 55, 1913-

18. Shull, C. A. The Role of Oxygen in Germination. Bot. Gaz., 57,

1914.

19. Atwood, W. M. A Physiological Study of the Germination of Avena

fatiia. Bot. Gaz., 57, 1914.

20. Crocker, W., and Davis, W. E. Delayed Germination in Alisma plan-

tago. Bot. Gaz., 58, 1914.

21. Crocker, W. The Role of Seed Coats in Delayed Germination. Bot.

Gaz., 42, 1906.

22. Amerikanische Versuche mit Kiefersamen. Zeitschrift fiir Forst- und

Jagd-wesen, April, 1908.

23. Schotte, G. Work of the Swedish Forest Experiment Station. Re-

view in Forestry Quarterly, IV.: 51, 1906.

24. Die Zuchtwahl im Forstbetriehe und die Bestandespflege. Allg. Forst-

und Jagd-zeitung, December, 1907.

25. Busse, J. Ein Weg zur verbesserung unseres Kiefernsaatgutes. Zeit.

schrift fiir Forst- und Jagd-wesen, May, 1913.

26. Centgraf, A. Uber Beziehungen zwischen Tausendkorngewicht und

Keimenergy bei Kiefersamen. Allg. Forst- und Jagd-zeitung, June,

1913-

27. Schlich, W. A Manual of Forestry. Vol. II, London, 1891.

28. Duggar, B. M. Plant Physiology. New York, 191 1.

29. Waldron, L. R. A Suggestion Regarding Heavy and Light Seed

Grains. Am. Nat., 44, 1910.

30. Webber, H. J., and Boykin, E. B. The Advantages of Planting Heavy

Cotton Seed. U. S. Dept. Agric, Farm Bui. 285, 1907.

31. Shamel, A. D. The Improvement of Tobacco by Breeding and Selec-

tion. U. S. Dept. Agric. Yearbook, 1904.

32. Trabut, L. Bulletin 17, Service Botanique de I'Algerie. Directeur

du Service Botanique, Governement de I'Algerie.

33. Harris, J. A. On Differential Mortality with Respect to Seed Weight

Occurring in Field Cultures of Phasrolus vulgaris. Am. Nat., 46,
1912.

34. Harris, J. A. Supplementary Studies in the Differential Mortality

with Respect to Seed Weight in Germinating Garden Beans. Am.
Nat, 47, 1 91 3.

35. Harris, J. A. On Differential Mortality with Respect to Seed Weight

Occurring in Field Cultures of Pisuiii sativum. Am. Nat.. 48, 1914.

36. Nobbe. F. Ilandbuch der Samenkunde. 1876.

37. Walls, E. P. The Influence of the Size of the Grain and the Germ of

Corn upon the Plant. Bui. 106, Md. Agric. Exp. Sta., 1905.

38. Cummings, M. B. Large Seed a Factor in Plant Production. Bui. \'t.

.\gric. Exp. Sta., 177, 1914.

A Subject of Vital National Importance

Our National Forests

By RICHARD H. DOUAI BOERKER

PRICE $2.50

This is a short, popular account of the work of the United
States Forest Service in the National Forests of the West. It em-
braces such interesting and important phases of the federal forestry
ipolic} as: Why the National Forests were created; what natural
resources are found within their borders; how the Forests are or-
Iganized and administered ; why and how they are protected from
forest fires and other dangers: and how the vast resources are made
useful to the people.

I

Our National Forests

By RICHARD H. DOUAI BOERKER

Arboriculturist, Department of Parks, Cit\ of Nezv York.
With the United States Forest Service from 1910 to 1917.

PRICE $2.50

ADMIRABLY done. There is nothing else like it; it pre-
sents its subject lucidly, adequately and attractively. Thf
ordinary reader will find it thoroughly interesting as well as
informing. . . . It is accurate and up-to-date, and it combinei
the knowledge of an insider with the independent judgment of ar
outsider." — Herbert A. Smith, Editor, U. S. Forest Service.

This book especially emphasizes the relation of forests to th(
welfare of humanity. The National Forests are shown to be a
great heritage, of vital import to both the state and the nation
which we must pass on from generation to generation. Theii
proper care, protection, and development are therefore mosi
essential to the life, liberty, and progress of our country.

OUR NATIONAL FORESTS

The Forester will find in this book a concise, up-to-date ac-
count of the organization and administration of the National For-
ests Important data and information formerly scattered through
scores of pamphlets, circulars, and bulletins, most of which are not
readily accessible, are here brought together for the first time.

Students of forestry, schools, colleges, and other lusliiuuons
will find this a valuable book for instruction and for reference.
Tourists who are planning trips through the western country, where
it is almost impossible to travel without passing through one or
more of the Xational Forests, will find it a most mterestmg guide
and companion.

About 80 half-tone illustrations, reproduced from original pho-
tographs, are used to illuminate the text.

TABLE OF CONTENTS

Introduction: Forestry as a National Problem. The Extent and
Character of Our National Forests. Why the National For-
ests were Created. How the National Forest Policy has Bene-
fited the People.

I. The Creation and Organization of the National Forests.
Economic Conditions Which Led to Forest Conservation. The
First Steps in Federal Conservation. The First Reserves Es-
tablished in 1891. An Anomalous Condition— Forest Reserves
Without Forest Administration. The Administration Under
the General Land Office. The Consolidation of the Forestry
Work in the Department of Agriculture in 1905. The Pres-
ent Organization of the Forest Service.

IL The Administration of the National Forests. Personnel. How
the Forest Service Appropriation is Allotted. Equipment and
SuppHes. Improvements. Classification and Consolidation of
Forest Lands. How Young Forests are Planted to Replace
those Destroyed by Fire. Experiments and Investigations.

III. The Protection of the National Forests. Protection from Fire.
Protection Against Trespass, Insects, Erosion, etc.

IV. Sale and Rental of Forest Resources. Disposal of Timber.
Rental of Forest Range Lands. Special Uses. Administrative
Uses. Water Power, Telephone, Telegraph, and Power Trans-
mission Lines.

THE MACMILLAN COMPANY
64-66 FIFTH AVENUE NEW YORK

PLATE I

Fig. I. \'ie\v of the interior of the greenhouse, showing cultures and
In-drothermograph.

Fic. 2. \'ie\v of the interior of the greenhouse, showing cultures and
the cheesecloth tent used for the dense shade experiments.

PLATE 11
The Effect of Light upon Earey Development

Fig. I. The effect of light upon the development of Pinus pondcrosa
(S. D.). Ten plants each (i) grown in open light, (2) grown in medium
shade. 5^ natural size.

Eic. 2. The eft'ect of //(//// upon the development of Robinia pscudcicacia.
Three plants each (1) ,iirown in open li.uht, (2) in medium shade, (3) m
dense shade, i^ natural size.

PLATE III
The Effect of Soil Depth upon Early Development

Fic. I. The effect of soil depth updu the development of riiiiis pon-
dci-osa (S. D.). Ten plants grown (i) in deep, (2) in medium, and (3)
in shallow soil. K' natural size.

Fic. _'. 'idle effect of soil depth upon the development of Robinia pseu-
daeaeia. Three plants each (i) grown in shallow, (2) in medium. (3) in
deep soil. ' •. natural size.

PLATE IV
The Effect of Soil Moisture upon Early Development

Fk;. I. The effect of soil moisture upon the development of Finus pon-
dcrusa (S. D.). Ten plants grown in (i) medium dry soil, (2) wet soil.
V2 natural size.

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Imc. 2. The effect of soil moisture upon the developmenl of Kobiiiia
pscuciacacia. Three plants sr'^wn (i) in wet soil, (J) in medium dry soil.
J/j natural size.

PLATE V

The Effect of Soil Texture upon Early Development

Fig. I. The effect of soil texture upon the development of Pinus pon-
derosa (S. D.). Ten plants each (i) grown m loam, (2) grovi^n in sand,
(3) grown in gravel. ^ natural size.

Fig. 2. The effect of soil texture upon the development of Robinia pseu-
dacacia. Three plants each (i) grown in loam. (2) grown in sand, (3)
in gravel. M; natural size.

VITA

Richard Hans Boerker, born October 19, 1887, Brooklyn, N.
Y. ; prepared for college at Boys' High School, Brooklyn, N, Y. ;
received A.B. degree from Dartmouth College, Hanover, N. H.,
in 1909; graduate student in forestry at the University of Mich-
igan, 1909-1911, receiving the degree of M.S. in forestry in 191 1.

Engaged in private forestry work in New York and Michigan
at various times ; forester in the United States Forest Service in
Colorado in 1910, and in California from 1911-1914 engaged in
forest reconnaissance, silvical, and silvicultural work.

Graduate student in botany and silvics and Fellow in Botany
at the University of Nebraska 1914-1915, receiving Ph.D. degree
in 191 5. Since 1915 in charge of a private forestry enterprise
in New York State.

Author of numerous articles on forestry subjects, 1907-1915;
member of Sigma ,Xi ; American Association for the Advance-
ment of Science ; American Forestry Association ; Canadian
Forestry Association ; and the Ecological Society of America.

Richard H. Boerker

University of British Columbia Library

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FORM 310

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