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The Relation of Desert Plants to Soil
Moisture and to Evaporation

BURTON EDWARD LIVINGSTON

WASHINGTON, D. C.:

Published by the Carnegie Institution of Washington
August, 1906

CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 50

3 4

CORNMAN PRINTING CO.,
CARLISLE, PA.

CONTENTS.

Page

Introduction 5

Soil studies

General character of the soil 7

Water content of the soil in the dry season 8

Permeability of the soil to water and rate of downward movement 12

Retaining power of the soil for percolating water 14

Power of the soil to raise water from lower levels 17

Resistance offered by the soil to water absorption by roots 19

Supply of water to the soil

Atmosphere studies

General problem 24

Evaporation from a water surface — a new formof evaporimeter 24

Evaporation from the soil 34

Plant studies

Introductory

Water requirement for germination 40

Transpiration of desert plants 41

The general problem 41

Some measurements of transpiration; a new method for studying the

physiological regulation of this function 42

Generalizations from the experiments 63

Water requirement of certain desert plants 65

Osmotic pressure of cactus juices 70

Conclusion 72

Summary 75

Literature cited 77

THE RELATION OF DESERT PLANTS TO SOIL MOISTURE

AND TO EVAPORATION.

INTRODUCTION.

Every observer of desert vegetation has had his attention drawn to
the question of how certain plants of the arid regions are able to main-
tain a more or less active transpiration during long periods of absolute
lack of precipitation, when the soil in which they are rooted becomes
not only apparently air-dry but also attains exceedingly high tempera-
tures. It seemed that careful quantitative studies of the moisture con-
ditions in desert soil and desert atmosphere, and of the relation of these
conditions to the transpiration and life of desert plants, might throw
considerable light not only upon this problem of extreme xerophytism,
but also upon the limitations of plant life in general. Just as the alpine
summits of high mountains in all parts of the earth and the frozen
tundras of the arctic regions exhibit vegetable life under temperature
conditions which almost render it impossible, so the arid desert with its
centimeters of annual rainfall and its meters of annual evaporation
exhibits plant life under conditions of extreme dryness which similarly
approach a limit to the very existence of such life. It is thus plausible to
suppose that certain fundamental truths regarding the vital activities
of plants may be more advantageously studied in the case of organisms
existing under these extreme conditions than by confining attention to
what are considered the more normal circumstances of life and growth.

With the aid of a grant from the Carnegie Institution of Washington
the writer was able to spend the summer of 1904 at the Desert Botanical
Laboratory of that Institution at Tucson, Arizona, in carrying out a
series of quantitative studies on desert plants. The results of these
studies are embodied in the present paper.

Thanks are due to Prof. Frederic V. Coville and Dr. D. T. MacDougal,
who constituted the Advisory Committee of the Laboratory when this
work was done, as well as to Dr. W. A. Cannon, resident investigator,
for the excellent facilities provided at the Laboratory, without which
the work could not have been carried out. Mrs. Grace Johnson Liv-
ingston has rendered very valuable assistance in the preparation of this
paper, especially in the tabulating of the data and in the construction of
the curves.

6 THE RELATION OF DESERT PLANTS TO

The problems here dealt with concern the relations between certain
desert plants on the one hand and their physical environment, consist-
ing of soil and atmosphere, on the other. The importance of animal
life as an environmental factor in the desert is undoubtedly very great,
but no careful studies were made along this line. The results of the
investigations can be best presented under the three headings, "Soil
studies," "Atmosphere studies," and " Plant studies, " these to be fol-
lowed by a discussion of the interrelations existing between the facts
brought out by the three lines of inquiry.

It was more expedient and seemed altogether more desirable to make
a rather thorough study of the conditions obtaining on the shoulder of
Tumamoc Hill, in the immediate vicinity of the Desert Laboratory,
than to attempt broader and therefore less thorough studies embracing
other localities, such as the mesa below.the hill and the erosion channels
and washes of the Santa Cruz River, Rillito Creek, etc. , or of the more
distant and more varied Santa Catalina Mountains. A remarkable
uniformity in soils and vegetational characters is exhibited by all the
peaks and buttes of the Tucson Range, and Tumamoc Hill may be
taken as a type of these. Thus the results of the present investiga-
tions may be regarded as applicable to the whole range. All these
peaks are distinctly desert mountains, not attaining a sufficient alti-
tude to have moisture conditions which will allow any form of plant
growth less xerophytic than the Parkinsonia-Cereus society which covers
Tumamoc Hill. This society comprises, besides the giant cactus or
saguaro (Cereus giganteus) and palo verde (Parkinsonia microphylla) ,
a number of Opuntia species, both of the arborescent and prickly-pear
types, the barrel cactus (Echinocactus Wislizeni) , ocotillo (Fouquieria
splendens), cat's claw (Acacia greggii) , and occasional creosote bushes
(Covillea tridentata), together with several other shrubs and numerous
smaller plants. This vegetation has been briefly described by Coville
and MacDougal (1903) and also by Lloyd (1905).

The Santa Catalina Range, which rises on the opposite side of the
mesa, is more extensive than the Tucson Range and much higher. The
foot-hills and rugged slopes toward the mesa are very similar in soils
and vegetation to the Tucson Mountains, but as the ascent is made new
conditions are encountered, largely those of increased moisture and
lower temperature, and in the higher altitudes of the Catalinas are
streams of running water and forests of oak and needle-leaved trees.
The series of vegetational transitions from the willow and ash margined
Rillito Creek, across the great sandy washes, where the latter widens
in time of flood, on which dwarfed mesquite (Prosopis velutina) forms
practically the whole vegetational cover in the dry season; across the

SOIL MOISTURE AND TO EVAPORATION. 7

level mesa with its creosote bushes (Covillea tridentata) and several
arborescent species of Opuntia; up into the lower slopes of the moun-
tains, sparsely covered, like the Tumamoc Hill, with giant cacti, palo
verde, cat's claw, and both arborescent and prickly-pear forms of
Opuntia; still up into the intermediate region of scattered oaks, agaves,
and yuccas, with the beginnings of a real undergrowth of smaller
plants; and finally into the true forests of the high mountains— this
series of transitions would form as instructive a subject for ecological
inquiry as can be afforded anywhere. It was with a distinct feeling of
regret that the author returned from a reconnaissance trip through the
area of these transitions to take up the more definite problems on
Tumamoc Hill.

SOIL STUDIES.
GENERAL CHARACTER OF THE SOIL.

The shoulder of the hill on which the Desert Laboratory is situated
rises to an elevation of about 90 meters above the level of the broad
mesa below. The mesa surrounds it on all sides, excepting at the
south, where the shoulder connects with the flat-topped mountain itself,
which attains an elevation of about 200 meters above the plain. The
Laboratory building is thus located about midway between the base and
the top.

The mountain is composed mainly of volcanic rock broken into frag-
ments on the surface and darkened by weather to a deep brown or
black. On the slopes the pockets and crevices between these rock frag-
ments are filled near the surface with a heavy brown clay soil. On
the gently sloping and practically flat portion of the shoulder just above
the building this soil makes up most of the surface, the superficial rock
fragments being here not so numerous nor so large as on the slopes.
Even in those places which have the deepest soil, however, the pickaxe
and spade very soon reach either the bed-rock of the mountain or masses
of rock too large to be readily removed or excavated around. Thus deep
diggings are almost, if not entirely, impossible without penetrating the
rock itself.

On the mesa below the hill the surface soil is much more sandy and
gravelly and few large fragments of volcanic rock are found near the
surface. But this soil is underlaid at a depth of a meter more or less
by a curious hard-pan of soft and more or less fragmented limestone
called "caliche." This is not so hard but that it can be excavated with
a pickaxe and is quite permeable to water, although it certainly hinders
the downward flow of the latter to a considerable extent.

8 THE RELATION OF DESERT PLANTS TO

The caliche layer is perhaps an incrustation brought about by evap-
oration beneath the soil surface. In the dry season the soil becomes
air-dry to a considerable depth, and in this condition water must diffuse
as vapor through the interstices of the soil more rapidly than liquid water
can move from the moister layers below to the drier ones above. The
result is that the evaporating surface of the soil is often, and for long
periods, far below the soil surface, and, from this subterranean evap-
orating surface, water vapor diffuses upward through the dry soil-
layers to the air. As is well known, the soil of these regions contains a
large quantity of soluble salts. This soil solution, being lifted by evap-
oration, becomes concentrated, and finally the salts should crystallize
out at or near the evaporating surface. In this way the caliche hard-
pan may have been formed. Another hypothesis to explain the exist-
ence of this hard-pan supposes the caliche to have been formed at the
lower limit of penetration for precipitation water, the salts having
been gradually deposited as the soil was alternately wet and dry. To
definitely determine which of these hypotheses is more probable will
require further investigation.

Just as the caliche underlies practically the whole surface of the
desert mesa, so too the crevices and fissures on Tumamoc Hill are
largely closed by a similar formation at the depth of a meter or less.
Plant roots penetrate into the cracks of this hard-pan both on the hill
and on the mesa, and it is probably a very important factor in conserv-
ing the meager water supply.

WATER CONTENT OF THE SOIL IN THE DRY SEASON.

At the time of the beginning of the work, July 1, 1905, the desert
conditions on the hill were nearing their maximum for the year. The
surface soil about the Laboratory building was air-dry and seemed
thoroughly baked. Day and night air temperatures varied from 80° to
105° F. or above, and the relative humidity of the day time varied
between 8 per cent and 15 per cent of saturation. Rain had not fallen
since May 12, at which time 1.97 cm. fell, and strong breezes or even
gales were almost constant. The only plants which remained in good
condition were those which are particularly adapted in some manner to
dry habitats. The giant cacti had just finished flowering and were
ripening their pulpy fruits, much sought after by Mexicans and Papagos
and even by groups of American children who now and then came out
from Tucson. Prickly pears were also ripening their fruits, while bar-
rel cacti and several arborescent opuntias showed no marked growth
and of course bore no leaves. The creosote bush was green but not
growing, and was covered with ripe fruits; many plants of ocotillo had

SOIL MOISTURE AND TO EVAPORATION. 9

lost their leaves and stood as groups of gray, spiny wands; many others
were still green and appeared healthy, but practically all had scattered
their seeds. A single belated cluster of ocotillo flowers was found near
the top of the mountain on July 7. The trees of palo verde (Parkinsonia
microphylla) near the Laboratory had very largely lost their leaves,
thus also showing the effect of drought. Of the smaller plants, Encelia
farinosa still held its own as far as foliage was concerned, and a small
red mallow, Sphaeralcea pedata, together with a prostrate Euphorbia,
probaby E. capitellata Eng. , of somewhat the aspect of E. polygonifolia
of the East, were producing flowers and seemed perfectly vigorous.

The extreme dryness exhibited by soil, air, and vegetation, together
with the fact that the summer season of rains was rapidly approaching,
made it seem very important to take up immediately the question of the
actual amount of water contained by the soil at that time. Accord-
ingly a great number of diggings were made on the slopes of the hill
and on its top around the Laboratory, care being taken not to locate
any of these within possible reach either of the water tank, where
small amounts of water were usually escaping, or of the outlet of the
waste pipe below the building. Samples of soil were collected in this
manner from various depths and immediately placed in glass vials,
which were tightly stoppered and weighed. The samples were then
emptied into Stender dishes, of the form used for staining microscopic
preparations, and, in default of a suitable drying oven, left open in the
laboratory 5 to 15 days, being stirred occasionally to hasten evapora-
tion. When these had ceased to lose water they were returned to stop-
pered vials and their weight was again recorded. After the author's
return to the University of Chicago these samples were again weighed,
dried thoroughly in an oven at a temperature of from 105° to 110° C.,
and the amount of water thus lost was added to that which had been
lost in air-drying at the Desert Laboratory. The amount of water
present in the original samples was computed on the basis of volume
per cent. While for comparisons between different samples of the same
soil the water content may be determined in percentage of the dry
weight of the soil, this method fails to have even a practical value
when soils of different specific gravities are dealt with. This point,
while it has been mentioned by Whitney and Hosmer (1897, p. 7) and
others, has never been adequately emphasized from the standpoint of
plant physiology. From this standpoint the interesting questions are,
first, how much water is within reach of the plant, and, second, how
much of this water can be absorbed by the roots? In the answer to the
first question the specific gravity of the soil can play but a minor part,
the main factor being the volume of soil drawn upon by the roots and

10 THE RELATION OF DESERT PLANTS TO

the actual amount of water contained in this volume. The answer to the
second question depends upon the degree to which the soil holds its
water as related to the amount of absorptive power exerted by the plant.
Thus the availability for any species of any given volume percentage of
water in non-alkali soils is determined largely by the fineness of the soil
particles and by the physiological properties of the roots. It is only a
coincidence that extremely light soils, being mainly organic in their
nature, have a comparatively high power to withhold water from plants
growing therein.

In order to secure uniformity in compactness the wet volume of the
samples was used for this purpose. To obtain the wet volume, a suffi-
ciently large graduate was partially filled with water and the soil poured
in and thoroughly stirred to allow inclosed air to escape. The thin
paste thus formed was allowed to stand until settling was complete,
when the volume of the saturated soil was read directly on the graduate
scale. It was found that the amount of soil which would occupy, on
settling in water, a volume of 100 cc., weighed, in its oven-dry state,
85.0 grams. The same amount of soil, when merely poured into a
graduate without tamping, occupied a volume of 78.9 cc., and when
thoroughly tamped as it was poured in it occupied 68.4 cc. Thus the
percentage figures of moisture content obtained on the basis of volume
when allowed to settle in water are considerably lower than would have
been the case had they been computed on the dry volume when either
poured into the graduate or tamped. Since uniformity in tamping is
very difficult to obtain, the method of tamping could not well be used.
The dry surface layers of Tumamoc Hill usually crumble and com-
press beneath the foot, indicating that the soil of these layers, as it
dries out after being wet, occupies a greater volume than it would if it
were pulverized. Therefore it seemed that the natural volume would
be more nearly approximated by the method here used than by any
other, and at the same time a uniform treatment of the different
samples could be secured. From the data given above it is clear that
the moisture contents here given would have been if> or 17.6 per cent
larger had they been computed on dry weight, as is usually done in such
measurements.

By the method just described it was found that the soil samples air
dried in the laboratory contained from 2 to 3 per cent of moisture.
Samples of the upper 2 or 3 cm. of the natural soil, taken in the burn-
ing sunshine between July 1 and July 14, contained somewhat less
water, about 2 per cent. In most places on the hill it was impossible,
on account of rock fragments or caliche, to make small excavations to
a depth greater than 10 or 12 cm. Samples at this depth, lying against

SOIL MOISTURE AND TO EVAPORATION. 11

the rock, exhibited a total moisture content of 5 to 10 per cent of their
wet volume. A single sample taken at a depth of 15 cm. contained
13.04 per cent of water, and other samples ranged in moisture content
from 7 to 12 per cent. It is thus seen that there is considerable
variation in contained moisture at the same depth in different places,
largely due, no doubt, to the relation of the soil to the surrounding rock
fragments and underlying bed-rock or caliche. One digging was made
to a depth of 35 cm. and a sample taken from the soil at this depth,
lying against solid caliche, which apparently completely closed the
opening between the large fragments of volcanic rock which had been
followed in the digging, contained 15.16 per cent of water.

It was intended to make larger excavations and determine moisture
conditions at greater depth, but the beginning, on July 15, of the period
of heavy rains made this seem of no avail. The structure of the surface
layers of the hill, composed, as it is, of mingled rock fragments, offers
many chances for water from the surface to find its way to the lower
levels along rock surfaces, especially as all the superficial hollows and
rock pockets stand full of water for some time after each heavy shower.
The soil puddles and becomes itself very slowly permeable to water,
but the latter was shown, by diggings made shortly after the first rain,
to have attained the depth of the larger rock masses by following down
the surfaces of rocks which were exposed above.

From the moisture determinations which were made it is evident
that this soil does contain, during the driest season of the year, rather
large amounts of water, and this at no great depth. Spalding (1904)
found, about April 24, 1904, that a sample of soil of this same locality,
at a depth of 30 cm. , contained 8 per cent of its air-dry weight of water.
The same author says, ' 'Another sample from the hill [presumably at
the same depth] lost, by heating over an electric stove, 12 per cent of
its weight. ' ' From the relations of weight and volume given above it
is easy to reduce this result to the approximate percentage by volume
under water. As above stated, the per cent of water content calculated
on the dry weight of the soil is 17.6 per cent greater than that calculated
on wet volume. Thus Spalding's 12 per cent is 1.176 times the corre-
sponding water content figured by the method here used, and we have
the condition: 1.176v=12, wherein v is the percentage of contained
water on the basis of wet volume. From this it appears that v^=10.2
per cent.

It is probable that the method of drying over the electric stove
failed to remove all the water from the soil sample, and this may
partially account for the fact that the figure just derived is some-
what lower than would be expected from the determinations given

12 THE RELATION OF DESERT PLANTS TO

above. As has been noted, however, conditions other than depth
seem to play a part in determining the moisture content on this rocky
hill, great variations being manifested in a number of samples taken
from different places at the same depth.

The surprisingly large amount of water contained in this soil rela-
tively quite near its surface is probably very largely due, paradoxical
as it does indeed seem, to the excessively high rate of surface evapora-
tion. After a number of heavy showers, when the soil is quite moist
to an indefinite depth, the first few centimeters lose water much more
rapidly by evaporation into the air than it can be supplied by the much
slower process of diffusion upward through the soil films from lower-
lying layers. The result is that there is soon formed a very perfect
mulch of air-dry soil, similar to the "dust mulch" of the agriculturists.
In this condition the rate of water loss from the true evaporating sur-
face, which now lies at some depth within the soil, is governed, not by
the power of the free air to vaporize water, but by the rate of diffusion
of water vapor through the nearly air-dry layers which lie above. This
subject will be again considered in the chapter devoted to atmosphere
studies.

In the chapter devoted to plant studies the question of how much
water is needed in the soil in order that seeds of desert plants may
germinate and develop into seedlings will be considered, and experi-
mental evidence will be brought forward pointing clearly to the con-
viction already noted by Spalding (loc. cit. ) that sufficient moisture
is probably at all times present in the deeper layers of these soils for
the needs of transpiration and even growth of the desert plants which
root deeply enough to reach those layers. That there is considerable
variation in the water content of the deeper soil layers of Tumamoc
Hill is shown by the fact already stated, that at the end of the spring
dry season Fouquieria plants which had not lost their leaves were
numerous, while many others were leafless, the latter apparently indi-
cating a paucity of water in the soil within reach of their roots. It is
possible that the latter plants were so situated that their roots did
not reach moist soil on account of solid rock or large rock fragments.

PERMEABILITY OF THE SOIL TO WATER AND RATE OF DOWNWARD

MOVEMENT.

As has been stated, the season of summer rains began on July 15.
From this date until September 1 heavy showers were frequent, some-
times several on the same day, sometimes at intervals of several days,
and with each shower the surface of Tumamoc Hill was thoroughly
flooded with water. The precipitation flowed off from the general sur-

SOIL MOISTURE AND TO EVAPORATION. 13

face very rapidly, but always stood in the pockets and hollows for
several hours after the cessation of the rain. The surface soil became
saturated anew with each shower, but usually dried out quite thoroughly
before the next. The downward penetration of the water into the
lower soil layers continued, however, between the showers, and by
August 1 the soil had become quite moist to the depth of 20 or 30 cm.
This downward movement of water is hindered, as has been stated, by
the puddling of the soil, but is hastened by the presence everywhere of
oblique rock surfaces down which water movement is much more rapid
than it is through the soil itself. It is thus seen that during the rainy
season the deeper layers of soil receive considerable quantities of water
by direct downward movement from the surface. It is also probable
that greater or smaller amounts of water find their way through the
soil of the upper part of the mountain to the underlying rock and thence
flow down the slope beneath the soil surface and penetrate into all the
crevices, whether or not these are closed by caliche, the latter being
always quite readily permeable. Altogether, it is highly probable that,
for periods of many days during the latter part of the summer rainy
season, the entire soil of the mountain, with the exception of the first few
centimeters, is very moist and offers abundant opportunity for growth of
roots. The most superficial layers themselves are often very moist for
periods of several days at a time, when showers follow one another at
frequent intervals. Thus seeds might germinate at or near the surface,
when the soil contains sufficient moisture, and the seedlings might, by
the rapid downward development of roots, easily attain, before the end
of the rainy season, to depths where the water content is permanently
as great as 10 or 12 per cent.

Measurements of the rate of downward movement of water in the
soil when air-dry were made by several experiments. Six cylindrical
tumblers 5 cm. in diameter and 11 cm. high were filled to a depth of
of about 9 cm. with air-dry soil moderately tamped, water was poured
upon the surface of each so as to stand about 1 cm. above the soil, and
measurements of its rate of downward penetration were made from
minute to minute for a period of 15 minutes. The water above the soil
was kept at a nearly constant level by adding more as it disappeared.
The average rate per minute for the several intervals was determined
for the six soil columns. During the first minute the water penetrated
3.1 crn., during the second 0.8 cm., during the third 0.5cm., during the
fourth, 0.5 cm., and the rate of advance gradually diminished until at
the end of 15 minutes it had fallen to 0.2 cm. per minute, the decrease
being now exceedingly slow. During the entire period of 15 minutes
the water had penetrated, according to these averages, to a depth of
7.5 cm.

14 THE RELATION OF DESERT PLANTS TO

It seemed possible that the rate of movement here found was
too high for the natural soil on account of the fact that the latter is
apt to be more firmly packed than was the soil in these columns. There-
fore a similar tumbler of soil was prepared, tamped as firmly as possible,
and the rate of water penetration into it studied in the same way.
The upper surface of the column was not packed as firmly as the soil
below, and the initial rate of advance of the water was nearly as rapid
as that in the previous experiment. In two minutes it had advanced
2.5 cm., during the third minute its increment was 0.2, during the
fourth it was the same, during the fifth it was 0.15, and this rate
decreased so that at the end of 3 hours the soil was moist to a depth of
only 4.8 cm. below the surface. It thus appears that the rate observed
in the case of the first set of six tumblers is perhaps about twice as
great as in the natural soil. It was deemed worth while, however,
to study the decrease in the rate of penetration in the case of a longer
soil column only moderately tamped. This column was 4.5 cm. in
diameter and 93 cm. high, a column of water being kept about 2 cm.
high above the soil. The experiment was continued for 30 hours, obser-
vations being taken from time to time and the hourly rates of water
movement being calculated from the observed increments. These
rates, in centimeters per hour, are expressed in the form of a curve in
figure 1 (p. 15). In this curve abscissas denote time, the numbers on
the horizontal axis representing hours. The ordinates denote the rates
and are plotted at the middle of the time periods to which they corre-
spond. These rates are placed adjacent to the points marked by crosses,
which determine the position of the curve. It will be observed that,
after the first five hours, the rate of downward movement decreased
quite uniformly to the end of the experiment. During the last three
hours the rate was about 1.3 cm. per hour, the record ending when the
water had reached a depth of 42.4 cm. below the soil surface.

Data on the question as to the exact relation of these figures to the
natural conditions near the Desert Laboratory during the summer rains
were not obtained, but the fact was established that, as early as
August 1, the moisture of precipitation had penetrated to a depth of
from 20 to 30 cm. , thus connecting, by means of moist soil, the surface
layers with the lower-lying ones, which were moist at the beginning
of the rains.

RETAINING POWER OF THE SOIL FOR PERCOLATING WATER.

The power of soils to absorb and retain water and prevent its down-
ward flow into lower layers varies exceedingly according to their nature.
The coarser the soil particles and the smaller the amount of organic
debris contained, the smaller will be its retaining power. Thus, in

SOIL MOISTURE AND TO EVAPORATION.

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16 THE RELATION OF DESERT PLANTS TO

the case of sandy soils lying above the level of underground water, the
water of a heavy shower percolates rapidly and is drained away below,
leaving but a small amount in the upper layers. But in clay soils a very
much larger amount of water is held by capillarity and fails to drain
away. For this reason, in regions where long periods elapse between
rains, those upland soils which are more clayey in their nature are uni-
formly better adapted to plant growth during the periods of drought
than are the more sandy ones which retain less water. For a discussion
of this subject in reference to more humid regions the reader is referred
to publications by Warming (1902, p. 55), Schimper (1898, p. 94),
Livingston (1905), and Livingston and Jensen (1904).

As should be expected from its nature, the clay of Tumamoc Hill
has a high water capacity or retaining power. Determination of this
property was made by the usual method. A tin cylinder, 13 cm. high
and 8 cm. in diameter, with a perforated tin bottom covered exter-
nally with cloth, was used for this purpose. This vessel was partially
filled with soil, tamped in, and the whole was weighed. Then the cylin-
der was placed upright in water, so that the surface of the latter was
somewhat above that of the soil within, and water was poured in
above the soil until it stood several centimeters deep above the latter.
When the soil was thoroughly saturated the cylinder was removed and
allowed to drain until water ceased to flow out through its bottom, after
which a second weighing was made. Finally, the volume of the soil
when allowed to settle under water was determined, and the difference
between the two weights taken as the amount of water retained by the
soil. This was calculated in percentage of the wet volume of soil used,
of the dry volume tamped, and of the dry volume not tamped. An
average of five such determinations gave the amount of water retained
by this soil as 40.9 per cent of its wet volume, 59.8 per cent of its dry
volume tamped into the cylinder, or 51.8 per cent of its dry volume not
tamped. This water capacity, or retaining power, is very high, although
it does not reach that possessed by some of the heavy clays of Mich-
igan which the author has dealt with. One sample from that State had
a retaining power of 62.5 per cent of its dry volume untamped.

From the determination just given it is evident that the soil under
consideration retains, and prevents from draining away below, an
enormous amount of water, and to this fact is probably due the prev-
alence on the hill of a number of plant forms which derive most of their
water from near the surface. Cereus and Echinocactus are examples
of these. It is clearly shown by the work of Mrs. E. S. Spalding (1905)
on Cereus that this plant derives most of its storage water from the
surface layers when these have a high moisture content following a

SOIL MOISTURE AND TO EVAPORATION. 17

shower. These plants begin to absorb water and to swell almost im-
mediately after the surface soil about their bases is wet either by rain
or artificially. The more sandy and gravelly soils of the surface of the
mesa at the foot of the hill possess this property of holding water to a
much less degree, and water falling upon them readily finds its way to
the lower levels and finally to the drainage channels of the Santa Cruz
River and its branches. No doubt this sandy character of the mesa
soil furnishes the main reason why the vegetation on the mesa here is
so much more xerophylous in character than that on Tumamoc Hill.
Between the base of the latter and the Santa Cruz sand-wash prac-
tically the only plant to be seen in the dry season is the creosote bush,
and the specimens of this shrub here found are not by any means so
vigorous as those growing in the clay soil of the hill. The same con-
dition of things is to be observed in the relation of the mesa vegetation
on the other side of Tucson to the vegetation which occupies the foot-
hills of the Santa Catalina Range.

POWER OF THE SOIL TO RAISE WATER FROM LOWER LEVELS.

The power of a soil to raise water, by capillarity, from the lower-
lying layers follows very closely its retaining power. The rate at which
this water movement takes place and the height reached by the water
above the source of supply depends primarily upon the smallness of
the capillary spaces of the soil, and hence upon the fineness of the
component particles as well as upon their degree of compactness.

Capillary lifting power is most often measured by filling a vertical
glass tube with soil, placing its lower end in water, and measuring the
rate at which the water ascends the soil column, this being determined
by the change in the color of the soil as it becomes moist. A better,
though much slower, method for determining the maximum height to
which water will thus rise is to saturate a tube of the soil, place its
lower end in water, and determine the maximum depth from the upper
surface to which the soil becomes dry. The latter method was
attempted with the soil under consideration, but the author's time was
too limited to obtain any evidence therefrom.

By the other method results were obtained which warrant presenta-
tion here, although the soil columns used were undoubtedly much less
thoroughly packed than is the natural soil. The afternoon of August 2, a
vertical glass tube of 1.8 cm. internal diameter was filled with air-dry
soil, after having its lower end closed by tying a layer of cloth over it.
The tube was tapped rapidly on the floor while the soil was slowly poured
in, so that the latter was fairly compact when the tube became
filled. The lower end of the filled tube was placed in a vessel of water

THE RELATION OF DESERT PLANTS TO

and the height of the moist column of water was noted from time to
time. At 4"20'n p.m. the height of this column had attained to that of
the water outside the tube and from this time on the rise of the liquid
through the soil was due entirely to the capillary power of the latter.
The level of the external water was kept approximately at the same
height by addition of water as needed, evaporation from the free water
surface being avoided by covering this with oil. After September 3
several readings on this apparatus were very kindly made by Dr. W. A.
Cannon.

The results of these determinations are given in Table I. The first
column gives the times of observation, the second the observed height
of the column of moist soil above the water level outside the tube, and
the third gives the average rate per hour of the rise of the liquid
during the period just ending.

TABLE I. — Rise of Water in Air-dry Soil.

Date and hour.

Height of
moist soil
above wa-
ter level.

Rate of
ascent
per
hour.

Date and hour.

Height of
moist soil
above wa-
ter level.

Rate of
ascent
per
hour.

cm.

Aug. 2, 4h 20™ p.m.

Aug. 8, 8h5om a.m.

57-9

0.104

5 ii p.m.

4.8

5.16

Aug. 10, 8 30 a.m.

61.9

.083

6 oo p.m.

7-8

3-96

Aug. 12, 8 oo p.m.

65.6

.061

7 oo p.m.

1 1.2

3-40

Aug. 15, 6 oo p.m.

69.0

.048

7 40 p.m.

13.2

3.00

Aug. 18, 10 oo a.m.

71.0

.031

9,00 p.m.

16.4

2.40

Aug. 19, 2 30 p.m.

72.0

•°35

Aug. 3, 7 oo a.m.

29.2

1.28

Sept. 3, 5 oo p.m.

82.7

.029

12 50 p.m.

33-3

.683

Sept. 19, 7 oo p.m.

88.2

.014

9 oo p.m.

38.0

.587

Sept. 30, 9 oo a.m.

92.2

.015

Aug. 4, 8 30 a.m.

42.5

•391

Oct. 3, 10 oo a.m.

94.2

.027

6 30 p.m.

45-7

.320

Oct. 10, 12 oo m.

96.7

.015

Aug. 5, 9 oo a.m.

48.9

.220

Oct. 22, 1 1 oo a.m.

99.8

.on

6 30 p.m.

50.6

.179

Nov. 2, 2 oo p.m.

IO2.6

.010

Aug. 6, 12 30 p.m.

S3-2

.144

Nov. 26, 1 1 oo a.m.

IIO.2

.013

Aug. 7, 12 oo m.

55-7

.106

A curve of these results is given at A, figure 2 (p. 15). Time incre-
ments are plotted on the horizontal axis in days and rates per day in
centimeters on the vertical axis. The curve shows graphically the
decrease in rate of upward advance of the moist soil column as it rises
above the water level.

From these data it is to be observed that during the first three days
the water has risen in this soil a distance of 50 cm., and that it had
risen a meter in 81 days. At the end of the last-named period its rate
of advance was about one-tenth millimeter per hour.

SOIL MOISTURE AND TO EVAPORATION. 19

Another experiment, showing similar results, was performed with
the same soil after the author's return to Chicago. This extended over
a period of only ten days. The results are given at B, figure 2 (p. 15) .
It is seen to be the same form of curve as the previous one.

RESISTANCE OFFERED BY THE SOIL TO WATER ABSORPTION BY ROOTS.

There are in general three conditions under which plant roots fail
to absorb water from the soil. First, the soil may not contain an
adequate supply; second, the supply may be adequate but the solutes
dissolved in the water may not permeate the protoplasm of the root
hairs and may be of so great a concentration that plasmolysis occurs;
and, third, the soil may contain poisonous substances which injure the
roots and make absorption impossible, even though the physical concen-
tration of the soil solutions may not be great. Although the soils of
Tumamoc Hill contain a rather high percentage of soluble salts, it
is not probable that the second of the conditions just mentioned is ever
effective here to prevent water absorption. As the soil dries out, how-
ever, plants finally wither from lack of moisture, and this may be due
to either or both of the other two conditions.

The first condition, lack of adequate water supply, may be effective
to check absorption in two ways. First, the actual water content of the
soil may be too low, and, second, there may be sufficient water in the
soil to supply the plants in question for many days, and yet the plants
may suffer because the rate of movement of this water may not be
sufficiently high to supply the soil layers immediately surrounding the
roots as fast as these layers are exhausted by absorption. These two
conditions are closely related and difficult to separate. Also, as water is
removed from the soil, the concentration of the soil solution may
increase, so that it is somewhat difficult to distinguish, as the critical
point is approached, between actual paucity of water and the effects
of high osmotic pressure.

No attempt was made to analyze these factors by experiment, but
some interesting data were obtained in regard to the tenacity with
which this soil holds water against the osmotic pressure of a sugar
solution. Whether or not absorption in roots is primarily a phenomenon
of osmosis, we may be sure that the osmotic condition of the root hairs
is of fundamental importance in the process. If the root hairs are
plasmolyzed absorption can not proceed normally. Therefore it is of
the utmost importance to study the relations existing between an osmotic
cell and soils which contain various amounts of water, and it was along
this line of inquiry that experiments were instituted.

THE RELATION OF DESERT PLANTS TO

The suggestion is due to Whitney and Cameron (1903, p. 54) that
we may attack this subject by means of an artificial root hair, in the
form of the ordinary osmometer, consisting of a semipermeable mem-
brane precipitated in porous clay and filled with a solution of known

osmotic pressure. Following this suggestion
a number of porous clay cylinders or cups
were obtained for the preparation of osmotic
cells. These were designed especially for this
work. They are hollow cylinders of unglazed
porcelain, 12.5 cm. in length and having an
internal diameter of 2 cm. and a thickness of
wall of about 3 mm. One end is closed and
rounded, the same thickness of wall being
retained here as at the sides. The other end
is open and grooved within so as to give good
surface of contact for a rubber stopper, while
the thickness of the wall is doubled here for a
distance of 2 cm. back from the edge, the
thickened portion terminating in an external
shoulder. A working drawing for one of these
cells is shown in figure 3.

In preparing the osmometers, the precip-
itation membrane of copper ferrocyanide de-
vised by Pfeffer (1877) was employed. The
cylinders were boiled in distilled water to expel
air and allowed to cool under water. They
were then filled with n/10 potassium ferrocy-
anide solution and were placed upright in a
beaker containing copper sulphate solution of
equivalent strength, the surface of the exter-
nal solution being just below the upper edge
of the porcelain. In this condition the cells
were allowed to stand from two to five days,
at the end of which time a good semipermeable
membrane of copper ferrocyanide was usually
found to have been formed within the porous
wall. When the membrane was judged to be
complete the cells were removed, thoroughly
washed with water, and filled with a 1.5 molecular solution of cane sugar.
The opening was tightly closed by a rubber stopper with a single per-
foration, through which passed a glass tube of about 4 mm. bore. The
tube extended above the stopper a distance of about 50 cm. In the act

5 T

N 2.0cm

1 L

•••

/2.5cm

k 1

FIG. 3. — Mechanical drawing for
porous clay cylinder for use
in osmotic experiments and
in evapori meter.

SOIL MOISTURE AND TO EVAPORATION. 21

of closing care was taken to include no air and in pressing the stopper
into place the column of solution was forced up into the tube to a
height of several centimeters. After the cylinders were filled and
stoppered they were placed in water for several hours and only those
which failed to leak sugar were used in the experiments.

After testing with water the osmometers were placed in soils from
the vicinity of the Desert Laboratory, containing various amounts of
water, and observations upon the height of the column of solution were
made at intervals for a period of from 10 to 24 hours to determine
whether water movement took place from the cell into the soil or in the
opposite direction. Of course the osmometers act like water ther-
mometers and slight changes in the height of the columns will accom-
pany variations in temperature. A thermometer was placed in the soil
and in the critical cases care was taken to have the soil temperature at
the time of observation approximately the same as at the start. In
these experiments the soil was placed in tin cylinders of the form used
in determining its power to hold water, but without perforations in the
bottom. The soil was worked up with the required amount of water
and was tamped firmly into the cylinder around the osmometer, the
upper suface of the soil being on the same level as the top of the rubber
stopper.

Five different osmometers, each used several times, gave the fol-
lowing result: In soils containing 5, 10, and 15 per cent of water by
volume the column of sugar solution gradually sank, showing that water
was being extracted from the cell. In the 20 per cent soil a very slight
rise was noted in some tests and an equally slight fall in others; this
soil seems to have approximately the same attraction for water as has
a 1.5-molecular cane-sugar solution. In the 25 per cent soil the column
of sugar solution rose, showing that the cell was absorbing water from
the soil.

We may conclude, then, that the force with which the 20 per cent
soil resists absorption of water by one of these osmotic cells is about
equal to the osmotic pressure of a 1.5-molecular cane-sugar solution,
or, according to Morse and Frazer (1902), about 54 atmospheres. This
pressure is surprisingly high, much higher than the osmotic pressure
of most plant cells, and suggests that either the osmometers here used
do not form as good contact with the soil grains as do the root hairs,
or else that osmotic pressure does not indeed play the important part in
water absorption which has hitherto been assigned to it.

At the University of Chicago, during January and February, 1905,
a number of experiments similar to the above were performed upon a
very finely divided quartz sand. The sand used was the finest one of

22 THE RELATION OF DESERT PLANTS TO

the experiment of Jensen and the author (1904) upon the relation of
size of soil particles to plant growth. This sand has a water capacity
of about 46 per cent by dry volume.

In these experiments the osmometers were filled with a solution of
cane sugar having a concentration of 2 gram molecules per liter. Rub-
ber stoppers with two perforations were used and a thermometer was
inserted in each cell beside the glass tube, so that the temperature of
the solution could be recorded with the readings on the height of the
column, and corrections could be made for temperature variations.

It was found that the cell failed to absorb water from sands with
a water content of 1.5 per cent by volume; that neither absorption nor
water loss occurred in a sand of 5 per cent, and that absorption took
place from those of 7.5 and 10 per cent. It thus appears that the force
by which water is held in the 5 per cent sand is about equal to that of a
2-molecular cane-sugar solution, or at least 72 atmospheres.

While the experiments with this form of ' ' artificial root hair ' ' have
not been carried far enough to justify any theoretical interpretation of
the results obtained, enough has been done to show that this method
offers a very valuable means for quantitative studies of the mechanics
of root absorption. It is hoped that further work may be done along
this line. A comparison of the results obtained upon the same soil by
this means and by means of the artificial root hair of Briggs and McCall
(1904) should throw light upon both the tenacity with which moisture
is held by a soil and the rate of movement of soil water.

SUPPLY OF WATER TO THE SOIL.

Situated about 80 meters above the Santa Cruz sandwash, Tuma-
moc Hill must receive all of its natural water from precipitation. The
annual precipitation here is practically the same as that at Tucson, for
which station records are available. These records, for fifteen years,
as given by Coville and MacDougal (1903, pp. 26, 27), show a mean
annual precipitation of 30.10 cm. (11.74 inches), which is distributed
mainly in two rainy seasons— one in winter and early spring and one in
midsummer. This is shown clearly in Tables II and III, the first of
which presents mean monthly precipitations and the second the actual
record of precipitation at the Laboratory from May 11 to December 31,
1904. The data are for the 24 hours ending 8 a. m. on the date given.
For curves of the annual precipitation and average temperature at
Tucson, the reader is referred to Cannon (1905). Dr. Cannon has
kindly furnished the author with the data for Table III.

SOIL MOISTURE AND TO EVAPORATION. 23

During the summer rainy season of 1904 the surface soils of
Tumamoc Hill were often wet and almost continually moist. As has
been noted, it is probable that during the heavy showers considerable
quantities of water penetrate to the deeper soil masses along rock
surfaces and a relatively large amount is often held for several days in

TABLE II. — Mean Monthly Precipitations at Tucson, Arizona.

Month.

Centi-
meters.

Inches.

Month

Centi-
meters.

Inches.

January

2.O7

O.7Q

July...

6 ic

^ 40

February

2.7J

.QO

August

U.I ^

6 67

2 uO

March

1. 07

.77

September

"> O7

i 16

April

.60

.27

October.. .

-•v/

1 fid.

May

• l6

.14

November

-> 08

•U4

June

.67

.26

December ,,.,

2 c6

I OO

**yj

TABLE III.— Precipitation Record from May g to December 31, 1904..

Date.

Rainfall.

Date.

Rainfall.

Centimeters.

Inches.

Centimeters.

Inches.

May ir

1. 21

1-97
.10

.10

.90
.72
.038
.038

2.38

1. 21

•59
.87

.18
Trace
1.46

0.47

•77
.04
.04

•35
.28

.015
.015

•93
.48

•23
•34
•07

•57

August 17

0.077
1.51

Trace
2.49
.28
1.28

.13
.051
.103
.28
1.38
•44
.077
.56

0.03

•59
.20

May 12. . ..

August i Q

May ic..

August 27

June 18

August 25

Tulv ic..

September 2.
September 13
September 17
October 24....
October 28....
November 4.
December 5.
December 8.
December 9.
December 23
December 31

•97
.11
.50
,05
.02
.04
.11
54
•17
•03

.22

T 1 3

ulv 27..

4 / J
Tulv 2C

Tulv 26...

Tilly T.O...

•i / •>
lUlV "?!..,

August 4

August 6

August 7

August 14
August 16

hollows of the soil, so that time is allowed for direct penetration
downward. Nevertheless, it must be remembered that, owing to the
relatively low permeability of these soils, a large proportion of the
water which falls in the heavy rains fails to soak into the puddled sur-
face and finds its way to the mesa below, where it rapidly drained
away to the Santa Cruz.

But, from the facts presented in the discussion of the water con-
tent of this soil it is clear that what water does attain to the depth of
half a meter or more is well protected from soil evaporation and will

24 THE RELATION OF DESERT PLANTS TO

not be likely to escape into the air to any great extent, excepting
through the transpiration of plants. A discussion of the conservation
of moisture by this soil will be given under the succeeding heading.

ATMOSPHERE STUDIES.
GENERAL PROBLEM.

Aside from the ravages of animals, desert conditions in the locality
under discussion are brought about mainly by two different, though
related, factors— dry ness of the soil and excessive evaporating power of
the air. The former factor offers resistance to the absorption of water
by plant roots, and the latter accelerates water loss by transpiration
from the leaves and stems, so that both factors work together to bring
about the extreme xerophytism so manifest everywhere in the aspect
of the vegetation.

The soil conditions have been discussed in the previous section.
There will now be presented the results of some measurements of the
evaporating power of the air. This depends upon two conditions— rela-
tive humidity and air movements. Temperature variations affect the
evaporation rate through changes in relative humidity. Relative
humidity acts directly through alterations in the vapor tension of water.
As is well known, a wind increases the rate of evaporation very mark-
edly by furnishing a constantly renewed air layer against the evapo-
rating surface and thus preventing, to a greater or less degree, the local
rise in relative humidity which would otherwise occur. Both of these
factors are of the utmost importance in influencing the transpiration of
plants, and the latter deserves more attention than has heretofore been
given to it by most plant physiologists. The only important investi-
gations of the effects of air currents upon the transpiration rate are
those of Wiesner (1887), who has shown that, while in certain cases
wind causes a closing of the stomata, it usually does not have this effect,
but causes a marked rise in the rate of water loss. Eberdt (1889) has
corroborated these results of Wiesner.

The data obtained in regard to the evaporating power of the air will
be given under two headings: 'Evaporation from a water surface,"
and ' ' Evaporation from the soil. ' '

EVAPORATION FROM A WATER SURFACE — A NEW FORM OF

EVAPORIMETER.

Measurements of evaporation are usually made by direct determina-
tion of water loss (in terms of either volume or weight) from some sort
of vessel of water, the upper surface of which is open to the air. Even
though comparatively small readings can be taken, this method is not

SOIL MOISTURE AND TO EVAPORATION. 25

sensitive to slight variations from hour to hour, due to air currents, etc. ,
for, as soon as the water surface falls below the upper edge of the con-
taining vessel, this surface is protected, to some extent at least, from
the full action of the wind. In attempting to relate plant transpira-
tion to physical evaporation it became necessary to devise a form of
evaporimeter which should, if possible, be as sensitive both to vari-
ations in air currents and to those in relative humidity as the plant
itself. At the same time it should be capable of giving readings for
short periods of time, so that changes in the rate of evaporation from
minute to minute and from hour to hour might be studied.

Happily, a method was hit upon, which, while it gives practically
perfect results, is exceedingly easy of operation and requires a minimum
of time and care. The apparatus consists essentially of one of the
unglazed porcelain cylinders described on page 20, closed by a rubber
stopper carrying a glass tube, the opposite end of which is connected
with the outlet of a burette. When the cylinder is placed considerably
above the level of the top of the burette and the whole apparatus is
filled with water, the pressure of the air is entirely removed from the
water in the cylinder, since the water films across the capillary pores
of this porcelain will support at least one atmosphere of air pressure,
and thus the liquid fails to flow down into the burette. At the same
time, evaporation of water from the surface of the moist porcelain is
constantly accompanied by a corresponding outward seepage from within,
and therefore also by a corresponding withdrawal from the burette.
Evaporation from the meniscus of the burette column is prevented either
by an oil layer, as shown in figure 4, or by a nearly air-tight closure of
the top of the burette, using an inverted test tube or a cork stopper
with a small opening cut in one side. The last method is most satis-
factory. Readings are taken from time to time of the contents of the
burette, and the difference between any two readings gives the volume
of water lost from the evaporimeter surface for the period of time
intervening between these readings. Only distilled water should be
used, since the gradual accumulation of salts within and on the walls of
the cylinder alters appreciably the rate of evaporation and thereby
introduces an error into the record.

For ease in filling the burette its inlet tube was connected with an
elevated separatory funnel, so that it could be refilled at will by simply
opening the cock at the base of the latter. The whole apparatus may be
mounted on a ring-stand so as to be easily portable (see fig. 4), or the
evaporimeter tube may be fixed permanently out of doors and the
burette and reservoir may stand in a room, the two parts of the
instrument being connected by a tube which passes through the wall.

THE RELATION OF DESERT PLANTS TO

For most of the work at the Desert Laboratory the fixed form of
evaporimeter was used. The porcelain cylinder was supported verti-
cally, with the stopper uppermost, on a wooden
arm reaching out from a window-casing on the
north side of the building. The center of the
cylinder was 50 cm. from the stone wall of
the Laboratory and 2 meters above the ground.
It was well under the projecting eaves of the
building and was thus protected to a great
extent from rain. The sun shone upon it for
a few hours in the early morning and again
in the late afternoon. The connecting tube,
partly of glass and partly of rubber, passed
into the building through a hole bored in the
window-casing. A burette of 100 cc. capacity
was used and stood on a support inside the
window, at such height that its upper end was
several centimeters below the base of the por-
celain cylinder outside. The window was kept
closed, excepting when momentarily opened to
obtain data for other experiments carried on
in the same place, which will be described
farther on. An air thermometer, graduated
in degrees Fahrenheit, was placed outside the
building near the evaporimeter and readings
upon it were taken whenever the burette was

read.

For absolute measuremements of evapora-
tion it is necessary to calibrate each evapori-
meter by exposing for some time and in the
same place an open vessel of water with a
known area of exposed surface, and weighing
this vessel whenever a reading is taken on the
evaporimeter. From data thus obtained a co-
efficient is easily derived by which to multiply
any increment of loss from the evaporimeter
in order to obtain the rate of evaporation for
the same period from any assumed standard
area of free water surface. After such cali-
bration has been accomplished, the evapori-
meter may be operated indefinitely, care being,
of course, taken never to allow air to enter the
cylinder. In this work the precaution was taken to wipe off the porce-

FIG. 4.— Evaporimeter, consist-
ing of porous clay cylinder,
burette, and water reservoir,
the latter in the form of a
separatory funnel.

SOIL MOISTURE AND TO EVAPORATION. 27

lain evaporating surface from time to time with a moist cloth, to
remove dust which was observed to accumulate thereon, especially dur-
ing the dust storms so frequent in the desert.*

It was thought at the outset that the length of the water column to
be lifted by evaporation might influence the rate, so that an error
would be introduced by the gradual increase in the height of this column
as water was removed from the burette, but this was found by actual
tests not to be true. Apparently the tensile strength of the capillary
films in the porcelain is so great that their curvature is not appreciably
altered by changes of a meter or less in the height of the water column.
It was found, however, that if the top of the water column in the
burette was above the evaporating cylinder, water was slowly forced
out of the latter and appeared as dew upon its surface. Therefore the
cylinder was placed, as stated, well above the level of the top of the
burette. Had the height of water column appeared to exert any
influence upon the rate of evaporation the burette might have been
refilled after each reading, but thorough preliminary tests showed this
to be unnecessary.

The calibration figures for the fixed evaporimeter above described
will now be given. For measuring the loss from a free water surface,
a cylindrical glass crystallizing dish of 118.82 sq. cm. cross section
and about 5 cm. high, filled with distilled water, was used. This was
suspended by means of wires from a wooden arm similar to the one sup-
porting the evaporimeter cylinder, projecting from the other side of
the same window out of which the evaporimeter was placed. The dish
was so arranged that its upper surface was at the same height from
the ground as the center of the porcelain tube, and also the same dis-
tance from the Laboratory wall, thus occupying a position corresponding
to that of the evaporimeter cylinder, but on the opposite side of the
window, about a meter distant. At hourly intervals this dish was
weighed and returned to its position, a reading of the evaporimeter
burette being taken at the same time. The first column of Table IV
presents the hourly losses from the burette, for the period from 8 a.m.
to 7 p.m., July 28. The second column presents the corresponding
losses from the crystallizing dish, while the third column gives the ratio

*In the spring of the present year the author was able to test the feasibility of
obtaining automatic records on such an evaporimeter as the one above described by
means of the Ganong (1905) transpirimeter, manufactured by the Bausch & Lomb
Optical Company. A perfectly satisfactory record was obtained of the varying inter-
vals at which a gram of water was lost during several days. The instrument is well
adapted to this work, but could be greatly improved by being so arranged as to oper-
ate without attention for a week at a time.

THE RELATION OF DESERT PLANTS TO

of the evaporimeter loss to that of 1 sq. cm. of the water surface. In
the heading for this column a signifies the area of the dish, 118.82
sq. cm. The different items are for the several hour periods, from
8 a. m. to 7 p. m. , on July 28.

TABLE IV.— Calibration Data for Evaporimeter.

Loss from
evaporation

Loss from

Ratio
/ ea\

Loss from
evaporation

Loss from

Ratio
(ea\

(e).

dish (d).

V d )

(e).

\ d )

4.0

5-3

89.6

8-3

9-97

98.9

S-4

6.625

96.9

5-4

6-7

95-9

5-5

6-455

IOI.2

6.2

6.44

114.4

6.5

8-17

94.6

6.6

7.00

II2.O

7-2

9.25

92-5

2.4

2.82

IOI.2

ill

15 68

8d 7

Average

QQ.26

"I-/

The fluctuations in the ratio are probably in large part due to the
failure of slight air currents to accelerate evaporation from the dish
as much as they hastened that from the porcelain cylinder; at the begin-
ning of the test the upper surface of the water in the dish was about 7
mm. from the upper edge of the lateral walls. The average ratio for
the whole series of observation is 99.26. Other determinations gave
coefficients which closely approximated this one. Therefore, in order
to save computation, it may be assumed that this evaporimeter lost
water approximately 100 times as fast as would a centimeter of free
water surface in the same position. In other words, the actual loss of
the evaporimeter is taken to be equivalent to the loss from a water sur-
face of one square decimeter.

On the instrument above described readings were taken at conven-
ient intervals from July 24 to August 16. Unfortunately the instru-
ment was not installed until after the beginning of the rainy season,
so that a rate of evaporation approaching the maximum for the year
was probably not observed. Since reliable records of evaporation are
exceedingly rare, and especially on account of the fact that the pres-
ent observations were made at a station whose atmospheric phenomena
are especially interesting to botanists, a table of the daily increments
of water loss for the above-named period is worthy of its space here.

Table V presents the daily evaporation and also the rainfall from
July 25 to August 22. In the first column are given the dates; in the
second, the actual losses of the evaporimeter in cubic centimeters; in
the third, centimeters of evaporation, being the loss from a single
square centimeter of free water surface, derived from the evapori-
meter losses by means of the coefficient 100 above derived; and in the

SOIL MOISTURE AND TO EVAPORATION.

fourth, the same in inches. The last three columns give the rainfall, in
centimeters and inches, and the average temperature (given in degrees
Fahrenheit). The latter was obtained by averaging the readings for
the day and night separately and taking the mean of these.

TABLE V. — Evaporation, Precipitation, and Temperatures, Summer of 1904.

Date.

Evapori-
meter loss.

Evaporation.

Rainfall.

Tempera-
ture.

Tulv 2?

cc.
129.3
95.86
73.60
116.64
96.40
44.50
42.12
78.08
104.72
101.58
80.60

73-65
77-15
40.40
85.90

71-34
105.26
105.10
85.60
81.00
71.40

99-75
77-78

67.35
48.75
67.60

57-35
46.65
38.25

cm.

1.293

•959
-736
1.166
.964

•455
.421
.781
1.047
1.016
.806

•737
.772
.404
.859

•713
I-°53
1.051

.857
.8zo

•7U
.998
.778
.674
.488
.676

-574
.467

.383

Inches.

0.504

•374

287

•455
•376
.174
.164

•305
.408

•396

•314

.287
.301
.158

•335
.278

.411
.410

•334
.316
.278
•389
-303
.263

.190
.262
.230
.182
.149

cm.
0.038
.038
.000
.000
.OOO

2-383
1.230
.000
.OOO
.OOO
.589
.OOO
.871
.179
.000
.OOO
.000
.OOO
.OOO
.OOO
Trace
.000
1.461
.076
.000
1.512
.000
.000
.000

Incites.

0.015
.015
.000
.000
.000

•93°
.480
.000
.000
.000
.230
.000

•34°
.070
.000
.000
.000
.000
.000
.000
Trace
.000
•570
.030
.000
•59°

.000

°F.

87.0
80.7

79-4
88.0
83.1
80.4

75-1
77-8
81.5

83-9
80.7

81.5
83.2

75-i
83-7
81.2

27...

-v

T.O...

Aug. i

7..

4...

Q...

I I

85.2

1 1 .

82.0
81.2
80.5

77-9
74.6

77-8
78.6

77-8
77.0
77.0

* J

I c

,g ::::

17...

1 V
2O

Total, July 25-

22.642

8.830

8-377

3.270

It is interesting to note that from July 25 to August 22 the total
evaporation was about 2. 33 times the rainfall, notwithstanding the fact
that this represents the period of summer rains, and the total rainfall
noted is considerably over one-fourth of the average annual precipi-
tation here. The latter is 30.10 cm. (11.74 in.), according to Coville
and MacDougal (1903, p. 27).

Observations on the evaporating power of the air can not be made
in terms of relative humidity as determined with the psychrometer,
for this method, of course, leaves entirely out of account the factor of
air currents already mentioned. Perhaps, aside from the evaporimeter

THE RELATION OF DESERT PLANTS TO

itself, the stationary wet and dry bulb thermometer, placed
in an open position where air currents may affect it, is the
most reliable instrument for determining evaporating power.
Relative humidity computed from readings of this instrument
should not be the same as when com-
puted from psychrometer readings, §
but should bear a closer relation to
the losses from the evaporimeter.

The power of air currents to ac-
celerate evaporation was constantly
observed in the progress of the work
at the Desert Laboratory. This pow-
er is noticeable in regard to transpi-
ration from the plant surfaces, as
was observed repeatedly in exper-
iments where weighings of potted
desert plants were made at short
intervals, the plants stand-
ing in the open on a shelf
near the cylinder of the evap-
orimeter so that transpira-
tion rates could be compared
with those of water loss from
the instrument. When the *'
air was quiet the rates of
both transpiration and phys-
ical evaporation were com-
paratively low, while the
rates rose immediately when
even a breeze sprang up and
always reached their max-
ima for any given tempera-
ture during the heavy gales
which often blew over the
hill for hours. It therefore
appeared that the
transpiration figures
obtained by Spalding
(1904) by means of
the bell- jar method
are uniformly far too
low to represent nat-

81 ^

06 <6

66 Q.

—)

FIG. 5.— Curves of temperature and rate of evaporation, July 24-26, 1904.

SOIL MOISTURE AND TO EVAPORATION.

f»

40 percent

i-.ooa.m.

9:OO "

io.-oe •'

11:00 »

iz-oonoon
i-oo p m.
£:OO "
3:OO "
•f-.OO «
5:OO »
6:OO "

• r.ooa.m.

too ••»

s-oo

/*.oo noon

3:oop.ir>.

FIG. 6.— Curve of rate of evaporation, July 28-29, 1904.

FIQ. 7.— Curves of rates of evaporation from soils containing different amounts of water.

32 THE RELATION OF DESERT PLANTS TO

ural transpiration; the air about the Desert Laboratory is seldom per-
fectly at rest and then only for short periods.

Curves of temperatures and of evaporation rates were constructed
for the period during which the evaporimeter was observed, and these
two curves, when brought together upon the same sheet, bring out this
point very clearly. During dry gales the curve of evaporation lies
much higher in relation to the curve of temperature than when the air
was more nearly quiet. Several examples of such rises in rate of
evaporation are shown in the portions of these curves given in figs. 5 and
6 (pp. 30, 31) . The first of these is for the period from 3 p. m. , July 24 to
2h30m p.m., July 25, the second from 8"30m a.m., July 28 to 7 p.m., July
29. The evaporimeter curve is constructed by plotting rates per hour
as ordinates, with time intervals plotted as abscissas. The actual loss
for each period is divided by the number of hours, and the resulting
average rate per hour is plotted at the middle of the period. Thus
different abscissas represent, not the actual times of observations, but
the middles of the time periods. The temperature curve is constructed
in a similar way, the mean temperature between two readings being
plotted with the same abscissa as the rate of evaporation for the corre-
sponding period. The scales are merely chosen so as to bring the two
curves into proximity for the whole time of observation. In the figures
the broad line denotes evaporation, the narrow one temperature, and
the numbers placed near the points on the curves denote, in the one
case cubic centimeters per hour and in the other degrees Fahrenheit.

Examination of the curves shows at once that, while in general
they both rise or fall at the same time, there are nevertheless many
periods during which the direction of change is in the opposite direc-
tion in the two, and even where they agree in direction the variations
in the two are often by no means quantitatively identical. While many
of the minor ones of these independent rises and falls in the curve of
evaporation are undoubtedly due to changes in the absolute amount of
water vapor in the air, all of the more pronounced ones are to be traced
to variations in wind. In figure 4 an extremely high evaporation rate
is shown during a violent dust storm which arose about 3h30"' p.m.,
July 24, and continued until 6 p.m. A similar high wind arose the fol-
lowing morning about 7lWa and gradually fell during the day. The
day was cloudy for the most part and a gentle, continuous rain began to
fall about 6h30m p. m. and continued for an hour or more. In figure 5
the effect of a wind storm is shown between 12"30m and 3"30m p.m.,
July 28, and a less violent one on the following day, rising about noon
and ending in the heaviest shower of the season, which lasted from
2h30m to about 3 p. m. With this shower, as is quite usual, the wind

SOIL MOISTURE AND TO EVAPORATION.

ceased with the rain and the evaporation rate continued to fall for
some time after, not only because of the fall in temperature, but also
because of the increase in the moisture content of the air.

Besides the evidence of the curve just presented, the importance of
air currents in determining the rate of evaporation may be illustrated
by the following comparative measurements. The velocity of the air
current produced by an electric fan, when in motion at each of three
different speeds, was taken by means of an air meter at a position
30 cm. in front of the fan. Then the air meter was removed and a
portable evaporimeter was so placed that its porcelain cylinder occupied
the same position. Readings for 5-minute periods were then taken on
the evaporimeter, with the air at rest and with three different veloci-
ties of air current. Each test consisted of several 5-minute periods.
After every test with an air current a test was made in still air, by
merely stopping the fan, so as to make absolutely sure that the rate in
quiet air had not changed appreciably as time passed. The whole
experiment lasted less than two hours and during this time the air
temperature remained at 29° C. (84.2° F.), and the relative humidity
remained at 63 per cent, as determined by the sling psychrometer. The
results are given in Table VI. Air velocities are given in meters and
feet per minute and in kilometers and miles per hour; evaporation
rates are given in cubic centimeters as observed for 5-minute periods
and as calculated for hour periods.

TABLE VI. — Effect of Wind on Evaporation Rate.

Velocity of air current.

Evaporation.

Per minute.

Per hour.

Per 5 minutes.

Per hour.

Meters.

Feet.

Kilos.

Miles.

cc.

0.0

o.oo

O.OO

O.IO

1.2

273.6
364.8

486.4

900
1, 200
1, 600

16.37
21.82

29.10

10.23

13.64
18.19

•35
.50

• 57

4.2

6.0
6.8

This particular evaporimeter was not calibrated by weighing a vessel
of water, but since it was similar in every respect to the fixed one which
was calibrated, its readings may be taken as approximately equal to
the loss from a free water surface of 100 sq. cm. in the same position
and during the same period. It is to be noted from the above data that
a breeze with a velocity of only 16.37 kilometers per hour produces an
acceleration in water loss by evaporation of 250 per cent, and that with
an air current moving at the rate of 29. 1 kilometers per hour, an accelera-

34 THE RELATION OF DESERT PLANTS TO

tion of 470 per cent is produced. No observations of wind velocity on
Tumamoc Hill were made, but the air, as has been remarked, is seldom
at rest, and strong gales of a velocity probably far surpassing 50 kilos
per hour are frequently experienced and often last for hours.*

EVAPORATION FROM THE SOIL.

As has been said already, the surface layers of the soil on Tumamoc
Hill are air-dry during most of the year. After a shower they dry out
rapidly and in so doing shrink in such a way as to be somewhat loosely
porous to a depth of several centimeters. The deep cracks so charac-
teristically produced in many similar soils upon drying from a puddled
condition are not prevalent here. Cracks indeed often form, but these
are small and close together and seldom penetrate more than a few
centimeters below the surface.

The high evaporating power of the desert air removes water from
these surface layers much more rapidly than the movement in the soil
films can supply it from below, and this soon results in the air-dry con-
dition just noted. Thus the evaporating surface retreats farther and
farther into the soil, evaporation being hindered more and more by the
thickness of the nearly air-dry layer through which the water vapor
must diffuse upward, and finally an equilibrium must be reached where
the rate of upward movement of water in the soil films will equal the
rate of evaporation. This point is attained in the rock-bound pockets
of the Laboratory hill at a depth of less than a meter, as is shown
by the actual amounts of water noted in the dry season, and possibly
also by the position of the caliche layer, which may mark roughly the
position of the average evaporating surface throughout many centuries.

Thus the surprisingly large amounts of water found comparatively
near the soil surface even at the end of the dry season are undoubtedly
due, as has been already remarked, to the presence of a thick layer of
air-dry soil, acting like the dust mulch of the agriculturists. If we
suppose a soil to be saturated and supplied with water from below, and
if it be supposed to be losing water by evaporation at its upper surface,
whether or not a dry mulch will be formed will depend upon the rate of
water loss as related to the rate of water movement through the soil.
With a sufficiently low rate of evaporation water will be supplied from
below as fast as it leaves the upper surface, and therefore during a
long period of drought much more water should be lost, and this from a
much greater depth, under these conditions, than would be the case if
the evaporation rate were high enough to far exceed the rate of water

*0n the influence of wind velocity upon the rate of evaporation, see Hondaille
(1892, 1 and 2), Russell (1895), and Davis (1900).

SOIL MOISTURE AND TO EVAPORATION. 35

movement through the soil films, thus producing a protecting air-dry
layer at the surface. The maximum rate of movement of liquid water
through a soil layer depends, first, upon the dimensions of the capillary
spaces of the soil, and, second, upon the amount of moisture contained
therein.

The comparatively high water content of the humid East or of the
Great Lakes region, even during such periods of drought as occur in
these regions, produces a comparatively low rate of evaporation, and
hence a removal of water from relatively great depths in the soil. Thus,
after several weeks of dry weather the soil of the humid East, where
exposed, is probably nearly as dry as the soils of the arid West. Cam-
eron (1901) and Means (1901) have called attention to the occurrence
of true alkali spots in the East, which are evidence of such a condi-
tion. This subject was discussed by Hilgard (1902). The present
author was able to get other evidence in the same direction from the
soils of Northern Michigan at almost the same time that the present
work was begun. About June 16, 1904, some two weeks previous to the
beginning of the studies of desert soil, a number of soil samples were
collected in Kalkaska and Roscommon counties, Michigan, at a depth of
about 25 cm. from the surface, and the water content of these was
determined. The highest water content observed at this depth was
15 per cent by volume, in the case of a heavy clay soil covered by a
forest of beech, maple, elm, etc., the lighter soils ranging from 2.7 per
cent in the case of the sandy jackpine (Pinus Banksiana) plains to 10.3
per cent in the case of several loamy soils covered by Norway and white
pine (P. resinosa and P. strobus). Thus these soils had at that time
a moisture content which closely approached that of the clay of
Tumamoc Hill at about twice as great a depth and at the end of the
spring dry season. Of course it is to be remembered that, while the
desert soil remains at a low moisture content for many months at
a time, the content of the Michigan soils must often rise far above these
figures after the comparatively frequent rains. But the evidence is
clear that, with the high humidity of the latter region and the accom-
panying slower rate of evaporation, the soil is subjected to a more rapid
drying at relatively great depths than occurs in the arid regions.

Determination was made of the comparative rates of evaporation
from the surfaces of several samples of clay from Tumamoc Hill with
different water contents. Only 100 cc. of soil were used in each case, so
that the experiment lacks accuracy. The samples were made up to
contain 10, 20, 30, and 40 per cent of moisture by volume, and were
placed in Stender dishes 5.5 centimeters in diameter, being tamped into
place as uniformly as possible. Thus the general soil surface exposed

36 THE RELATION OF DESERT PLANTS TO

was circular and had an area of 23.76 sq. cm. The surface was some-
what below the edge of the dish, but this distance was the same in all
cases. The consistency of the 40 per cent sample was about that required
for modeling clay, perhaps somewhat too moist for such use; the 30 per
cent could still have been used for modeling; the 20 per cent sample was
cohesive under great pressure, while the 10 per cent sample was hardly
cohesive at all. The dishes stood in the laboratory and were weighed
at frequent intervals, readings being simultaneously taken on an
evaporimeter standing beside them. In order to eliminate the effects
of variations in the humidity of the air and of such slight air currents as
might occur in the room, the rates of water loss have been calculated
in terms of the evaporimeter rate for the same period. These rates
are presented in the form of curves in figure 7 (p. 31), the actual
quantities given being the quotients of the rate per hour of evaporation
from the soil divided by the corresponding rate per hour of the evapori-
meter. These ratios are plotted, as in other cases, at the middle of the
time periods which they represent.

An inspection of these curves shows a curious initial behavior of
the 10 per cent soil. Its rate at the start was exceedingly high, but it
fell to a position below the other soils within the first three hours.
This is probably due to the fact that in this soil there was not enough
water present to even approximately fill the spaces, so that the actual
evaporating surface was very large, extending down into the soil for
some distance. As the surface soil dried out the checking of water
loss by the dry soil above became apparent in the rapid fall of the rate,
which continued to fall more and more gradually as the air-dry layer
increased in thickness. At the end of the experiment 6.95 grams of
water had been lost, or 69.5 per cent of the whole amount originally
present in the sample.

In the case of the 20 per cent sample no such excessively high rate
was observed at the start, there being apparently sufficient water pres-
ent to close the spaces which were filled with air in the 10 per cent
sample. From the behavior of this curve it appears that with this
water content the soil can transmit water at a rate not very markedly
below the evaporation rate which prevailed at the time, and hence the
air-dry surface layer was very slow in forming. However, it did
gradually form, and after 22 hours this curve is seen to fall more rapidly.
At the end of the experiment 12.62 grams of water had been lost, or
63 per cent of the amount originally present.

The 30 and 40 per cent samples show little tendency to form air-dry
layers; their curves do not descend markedly, and the rate of water
loss at the end of the experiment is approximately as great as at the

SOIL MOISTURE AND TO EVAPORATION. 37

beginning. This must be interpreted to mean that both of these soils
were able to supply water from below as rapidly as it was lost at the
surface. During the experiment the amount of water lost by these
samples was 17.69 grams for the 30 per cent soil and 19.58 grams for
the 40 per cent soil, or 59 and 49 per cent of the original moisture con-
tent, respectively.

A similar vessel, containing at the start 100 cc. of water, was
included in the same series with these soils. Its curve in general follows
very closely that of the 40 per cent soil and it is omitted from the figure
for the sake of simplicity. The fact above pointed out that the two
soils with greater moisture content can supply water as rapidly as it is
lost by evaporation is again clearly indicated by the observation that the
curve of loss from the water surface is practically coincident with that
of the 40 per cent soil. The actual evaporating surface of the soil films
is probably larger than that of the water, but this difference is practi-
cally overcome by the slower diffusion of the water vapor as soon as the
evaporating surface penetrates at all below the surface of the soil.

The average hourly rate of evaporation during this experiment, for
each square centimeter of general soil surface, was 0.0055 cc. for the
20 per cent soil and 0.0077 cc. for the 30 per cent soil. Of these two
soils the one with the greater moisture content was able to transmit
water at a rate at least as great as 0. 0077 gram per hour for each square
centimeter, while the drier soil could not transmit water at a rate as great
as 0.0055 gram per hour, since the latter soil was unable to maintain its
average rate, but showed a rate which fell continuously. This point is
interesting in connection with the power of the soil to deliver water to
plant roots.

38 THE RELATION OF DESERT PLANTS TO

PLANT STUDIES.
INTRODUCTORY.

As has been already pointed out, the main physical factor which
determines the nature of the vegetation on Tumamoc Hill is the
water relation. Except during the rainy seasons, this soil is far too dry
for most plants and only those forms can live here which are adapted
to dry soils and high evaporation rate. In the studies to be here recorded
an attempt was made to determine some facts in regard to the minimum
water supply with which desert plants can thrive. Studies of the
minimum water supply for germination of seeds were also made.

Since it is next to impossible to make accurate measurements of
transpiration and water supply in the case of plants growing in the
ground, small plants were grown for the experiments in cylinders of
tinned sheet iron, perforated at the bottom to facilitate drainage.
Condensed-cream cans, holding from 250 to 300 cc., were found to
serve admirably for this purpose. Some cultures were made in Stender
dishes of the form used by microscopists for holding stains, but these
lacked drainage and were not as satisfactory as the tins. Only rain
water or distilled water was used for watering the cultures, since the
water from the supply tank contains much dissolved salt and the rapid
evaporation soon produced a sufficiently high concentration in the soil
to injure the plants.

On account of the voracity of the desert animals— insects, birds,
and small mammals— it was soon found necessary to protect the cul-
tures by wire netting. A cage was therefore constructed for this
purpose about a meter long, 40 cm. wide, and 50 cm. high, raised about
40 cm. above the level of the ground. Ordinary mosquito screen of
about 3 mm. mesh was used for this purpose. This cage stood in the
open sunshine about 4 meters from the wall of the Laboratory and was
thus subjected to uniform weather conditions with plants growing in
the ground nearby.

Several different plant forms were chosen for the work, some being
extreme xerophytes, others more mesophytic in their nature. The
fact that all work of this kind necessitates potted plants restricted the
choice of forms. It is almost impossible to lift from the soil and pot
mature specimens of those desert plants which live through the dry
seasons; their roots penetrate far into the soil, through openings
between the rock fragments, and can not be removed without injury.

Of a number of the smaller forms with which transplanting was
attempted, only a few survived and produced new roots. One of these
was a small plant of Euphorbia capitellata. This is a form with small,

SOIL MOISTURE AND TO EVAPORATION. 39

more or less nyctitropic leaves, and stems which extend upward and
outward for several centimeters from the summit of a long, woody
primary root. It is seldom possible to excavate deeply enough to dis-
cover the lateral roots of this plant. It grows and flowers in the driest
of situations, is very resistant, and, at most, loses some of its older
leaves at times of greatest drought. The small leaves are thick and
leathery and do not show the phenomenon of wilting to any considerable
degree. When death ensues they simply dry up and still retain their
positions along the basal portions of the stems until broken off by
external agencies. The plant used lost the main portion of its root
system in transplanting, but after about three weeks, during which
time the soil was kept well watered, growth had been renewed and the
plant appeared quite normal.

Other plants which were transplanted from the ground for these
experiments were taken in the seedling condition. At the advent of the
summer rains the ground everywhere suddenly becomes almost covered
with seedlings of a great group of annual plants which complete
their generation in a single rainy season and pass the dry season in
the form of seeds. These plants are not especially xerophytic in their
structure and appear to be very much like the smaller annuals of more
humid regions. Immediately upon germination they send out a long
primary root which grows rapidly into the deeper layers of the soil. It
is not uncommon to find, a few days after a shower, seedlings of these
forms with no development of plumule and only the cotyledons and
perhaps a centimeter of stem above ground, while the main root is 10
or 20 cm. in length, still unbranched and growing rapidly downward.
It thus comes to be possible for such seedlings to start in the moist
soil following a rain and to penetrate within a short period to such a
great depth that they are not injured by the rapid and almost com-
plete drying to which the upper few centimeters of the soil are soon apt
to be subjected.

The forms which were transplanted to cylinders in the very early
stages of their development were a species of Boerhama, about 20 cm.
high at maturity, and a Tribulus brachystylis, and a single specimen
of Allionia incarnata. Besides these plants transplanted from the
ground, seedlings of Fouquieria were grown directly from the seed.
Seeds of this plant germinate readily, the two cotyledons becoming
green as soon as they reach the light. The hypocotyl elongates rapidly
until about 2 cm. long, when this growth ceases and a slow thickening
begins. This growth of the hypocotyl continues for two or three
weeks, this organ often reaching a diameter of 3 mm. before any
development of the plumule occurs. This transverse enlargement is

40 THE RELATION OF DESERT PLANTS TO

accompanied by marked hardening of the tissues and by the formation
of a true bark. In the meantime the primary root grows directly
downward without branching, probably attaining a length in the open
soil of many decimeters. In the seedlings grown in pots the roots
extended around the base of the pot and finally branched profusely in
their distal portions. Not until the root has obtained a remarkable
length and the hypocotyl has become enormously thickened and very
woody, does elongation of this organ begin again. The plumule, which
has been dormant up to this time, then begins slowly to elongate, the
first true leaves being produced as much as a full month after the first
appearance of the cotyledons.

Several cultivated plants of the more humid regions, such as squash,
beans, etc., were also grown from the seed and used for purposes of
comparison.

Growth of all these forms, excepting the aerial portions of Fouquieria,
was exceedingly rapid at this season of the year. The Boerhavia and
Tribulus plants were in full bloom within four or five weeks after their
cotyledons appeared. This, it is to be remembered, was during the
hottest season. The high temperatures which prevailed seemed to
have no deleterious effect upon any of the native plants, nor upon the
cultivated plants experimented with, so long as an ample supply of
water was provided for the roots, thus allowing the excessively high
transpiration to be kept up.

WATER REQUIREMENT FOR GERMINATION.

Seeds of Fouquieria splendens were planted in Stender dishes con-
taining soil of several different water contents and note made of their
germination. In soils containing 5 and 10 per cent of water by volume
the seeds failed to germinate. In the latter soil the wings and outer
layers of the seed coat softened and became somewhat like moist paper,
but in the former such signs of absorption were hardly perceptible.
In a soil containing 15 per cent of water the seeds germinated at last,
although germination occurred much sooner in the 20 per cent sample.
They germinated earlier in moister soils up to 40 per cent, but were
soon destroyed by fungi in 30 per cent and above. It thus becomes
evident that, at the temperatures of the summer rainy season, Fouquieria
seeds require for germination a moisture content in the soil of about 15
per cent, while they germinate and develop well in soils of higher
moisture content up to about 25 per cent.

Seeds of Cereus giganteus were found to germinate well in 15 per
cent soil and with higher moisture content, but soon died with apparent

SOIL MOISTURE AND TO EVAPORATION. 41

damping off in soils of 25 per cent or above. It was often noticed that
the soil of Tumamoc Hill is full of spores of fungi and bacteria,
which develop very rapidly as soon as sufficient moisture is present.

For comparison, a number of seeds of cultivated plants were tested
in the same way. Mexican beans (Phaseolus) and wheat (Triticum
vulgare) germinated in 15 per cent and more vigorously in 20 per cent
soil. The cultivated balsam (Impatiens) germinated slightly in 20 per
cent but much better in 25 per cent. Radish (Raphanus sativus) failed
to germinate in drier soil than 20 per cent. Red clover (Trifolium
pratense) failed to germinate until a moisture content of 25 per cent
had been reached. Thus it appears that of these plants the bean and
wheat are able to germinate with as scanty water supply as can
Fouquieria. Balsam and radish require more water than these, and
clover still more. It is probable that the seeds of typically desert plants
possess no greater power to germinate in dry soil than many plants of
the humid regions. Adaptation to arid climate does not appear to be
well marked as far as germination is concerned.

TRANSPIRATION OF DESERT PLANTS.
THE GENERAL PROBLEM.

Whether transpiration is a directly necessary function in plants may
be regarded as an unsettled question. By some it is considered as
essential in the transport of dissolved salts from the roots where they
are absorbed to the upper growing regions, and also in the cooling of
green parts when exposed to bright sunlight. By others transpiration
is considered as only a necessary evil, an evil because it increases so
greatly the amount of water necessary for plant life, and necessary
because in order to absorb carbon dioxide from the air, wet membranes
must be exposed. This must allow evaporation and thus necessitate a
renewal of water to the absorbing surfaces within the leaves. Notwith-
standing the emphatic denial by Burgerstein (1904) that there is any
reason in the position of Reinitzer (1881), Oels (1902), and Haberlandt
(1892), who have expressed themselves more or less definitely as favor-
ing the second of the hypotheses outlined above, the question must not
be regarded as settled without conclusive experimental evidence, which
Burgerstein is noticeably unable to adduce. So far it seems practically
impossible to check transpiration absolutely by inclosing the plant in
supposedly saturated air under bell jars and the like, on account of the
fact that the absorption of heat by the green leaves must usually raise
their temperature slightly above that of the surrounding air. There-
fore the only method of experimentation which is available for study-
ing this problem is that of increasing or decreasing transpiration and

42 THE RELATION OF DESERT PLANTS TO

determining whether such treatment accelerates or retards growth and
the absorption of salts. As far as the writer is aware, no experiments
have been carried out with sufficient accuracy to make their results
applicable here in more than a general way.

While a certain amount of transpiration may be necessary for plant
life in general, it is evident that this does not need to be very great,
first, from the fact that the most luxuriant vegetation occurs in the
humid tropics and in greenhouses, where transpiration is relatively low,
a point brought out by Reinitzer, Haberlandt, and others, and second,
from the mere fact of existence of the xerophilous types in which, as
is well known, the amount of transpiration is kept very low by struc-
tural modifications. It is probably safe to assume that by far the
greater portion of the transpiration of desert plants is only a neces-
sary evil. The forms here found are so adapted to xerophytic condi-
tions that their transpiration is reduced to as low a figure as is com-
patible with the exposure of sufficient surface of moist membranes to
secure the necessary carbon dioxide for photosynthesis.

The conditions affecting transpiration in any given plant are, of
course, the evaporating power of the air, the supply of water available
to the roots, and, to some extent, physiological responses of the
leaves, such as the stomatal responses, to changes of light, tempera-
ture, etc., and the nyctitropic movements of the leaves themselves.
Since the water relation is of paramount importance in all plants, and
especially so, as has been already noted, in the forms inhabiting the
desert, transpiration becomes probably the most important phenomenon
in determining the nature of the vegetation in these regions. There-
fore, attention was largely confined during these investigations to
measurements of the effect of the three factors mentioned above as
controlling transpiration. The results will be given under the two
headings, ' 'Measurements of transpiration" (including some discussion
of the effect of nyctitropic movements and regulatory phenomena),
and ' 'Wat er requirements. ' '

SOME MEASUREMENTS OF TRANSPIRATION; A NEW METHOD FOR STUDYING THE
PHYSIOLOGICAL REGULATION OF THIS FUNCTION.

As has been emphasized above, in order to obtain measurements
of the transpiration rate which will most nearly approximate the condi-
tions in naturally growing plants it is necessary to take these measure-
ments in the open air, without inclosing the plant in a chamber. This
is to take account of the effect of air currents which have been shown,
especially by linger (1861), to exert great influence on evaporation
and transpiration. It is further necessary not to injure the plant in

SOIL MOISTURE AND TO EVAPORATION. 43

any way, since the effect of wound stimulus is sometimes great and is
always an unknown factor until carefully studied. Thus the potometer
commonly used in transpiration measurements is at least of doubtful
value until it is tested for each form experimented upon by some other
method which does not involve mutilating the plant. On this point see
also Curtis (1902).

Furthermore, if the subject of stomatal or other physiological regu-
lation of water loss is to be studied, it is essential that the rate of
merely physical evaporation from a uniform water surface be ascer-
tained simultaneously and for the same place with the transpiration
measurements. The evaporimeter devised for this purpose has already
been described.

The only method which fulfills all the conditions is that of weighing
potted plants, the soil of which is so inclosed as to lose no water except-
ing through transpiration. This method was adopted for the work.
Plants which had been lifted from the ground or had come from seed
sown in the pots were allowed to grow in the plant cage already
described, the soil being kept moist by frequent waterings, until they
appeared perfectly healthy and vigorous and had attained a convenient
size. Then the pots were sealed up so as to prevent water loss except-
ing through the plant, and the cultures thus treated were weighed at
intervals, readings on an evaporimeter which stood beside them being
made simultaneously with the weighings.

For sealing up the pots the composite modeling clay used by sculp-
tors was found to answer very well. It is of about the consistency of
putty, adheres with an air-tight joint to all dry solids, hardens very
little with age, is readily removed with a knife or spatula when the
experiment is finished, and, most important of all, can be applied cold to
plant surfaces and has no injurious effect. In short, it is an ideal soft
sealing-wax for use in all cases where air-tight and water-tight joints
of any kind are to be made and where it is not necessary that the
joint bear much pressure. Its cheapness and the fact that it can be
obtained from any dealer in artists' supplies, together with the ease
with which it can be removed when it has served its purpose, make it
much more satisfactory than any of the soft waxes prepared with
Venice turpentine, beeswax, etc., with which the author is acquainted.

During the time of the experiment the plant received no addition
of water. The soil, of course, became gradually drier and many of
the plants finally wilted or their leaves began to wither, showing that
they were suffering from lack of water.

At the end of most of the experiments the leaves were removed from
the stems and dried in a press. After the writer's return to Chicago the

44 THE RELATION OF DESERT PLANTS TO

area of these leaves was determined by making photographic prints of
them by contact, on the developing paper known as "velox," cutting
out the white portion representing the leaves and calculating the area
of this portion from its weight and the area and weight of the sheet.
The area thus obtained is, of course, that of one side of the leaves only
and must be doubled for the total area. For a full description of this
method and data on the uniformity in weight of "velox" paper, see
Livingston (1905).

Time was lacking for the determination of these areas at Tucson or
they would have been obtained without first drying the leaves. The
surface shrinkage, upon drying in the press, of leaves which are not
fleshy is, however very slight, not amounting to as much as 10 per cent
of the original area in the case of wheat, as the writer has had oppor-
tunity to observe. None of the leaves here worked with were of the
fleshy type, and thus the error here introduced is probably small. Also,
no account was taken of the area of the stems, from which a small
amount of evaporation must have taken place. On this point see Bur-
gerstein (1904, p. 27).

In the following paragraphs will be presented the data from the
several experiments. These sets of data are numbered in Roman
numerals, merely for convenience of reference.

Owing to the difficulty experienced in obtaining suitable pot cultures
of those plants which persist in vigorous vegetative condition during
the driest parts of the year, only three examples can be given of this
class. A single plant of the extremely xerophytic Euphorbia already
mentioned was available, and the only other plants of the hardier forms
which were used were two cultures of Fouquieria seedlings, two plants
constituting a culture, and each having at the time of the experiment
only four or five leaves. All of the other cultures of desert plants were
of annuals which appeared only about August 1 and which had prac-
tically all ripened their seeds and died by September 7.

In the experiments which are to follow the plants stood either on a
shelf near the stationary evaporimeter already described, and were
thus mostly in the shade, or on the uncovered portion of the south
porch, about 2 meters from the wall of the building, where they had
direct sunshine during the day. In the latter case a special evapori-
meter stood near them. The evaporation data given in the different
experiments are from the appropriate evaporimeter. The plants were
taken inside during showers.

Experiment L —The subject of this experiment was a thrifty plant
of Euphorbia, in flower at the time. It had been potted several weeks
and had apparently entirely recovered from injuries received in trans-

SOIL MOISTURE AND TO EVAPORATION.

planting. The pot was sealed and the experiment was begun at 12''30m
p.m., August 17, the plant standing in bright sunshine. Weighings
and readings of the evaporimeter were made from time to time until
6 p.m., August 19. No wilting or drying of the leaves had yet taken
place when the record was discontinued. The data are tabulated in
Table VII. In the first column are given the times of observation.
Column It gives the increment of water loss during the time period
just ending, column Rt gives the rate of water loss in grams per hour
for that period, and column Rta gives the same rate per square centi-

T ABLE VII . — Data from Experiment /. — Euphorbia .
[Total leaf area 398.4 sq. cm.]

Transpiration.

Evaporation.

Time of

Incre-

Grams per hour.

Incre-

Grams per hour.

Ratio.

Date.

observation.

ment

Rta

(grams).

Total.

Per

(grams).

Total.

Per

Rea

sq. cm.
St.

sq. cm.

August 1 6

12 iom p.m.

3 3°

2.2

0-73

o. 018

18.3

6.1

0.061

0.030

6 30

•4

•13

.0003

14.1

4-7

•047

.007

9 3°

.07

.0002

8.6

2.9

.029

.OO6

ii 30

.05

.000 1

2.O

I.O

.010

.013

August 17

5 30 a.m.

•3

.05

.0001

4-7

0.8

.008

.Ol6

8 3°

•7

•23

.0005

4.6

T-5

.015

.038

ii 30

2.9

•97

.0024

10.6

3-5

•°35

.069

3 3° P-m-

3-3

•83

.0021

23-9

6.0

,060

•°35

6 30

•4

,13

.0004

9-5

3-2

.032

.OIO

9 3°

.07

.0002

6.1

2.O

.020

.OO9

August 18

7 oo a.m.

•5

.05

.000 1

9.4

I.O

.010

.013

10 oo

2.9

I.OO

.0025

19.8

3-6

.036

.O7O

2 oo p.m.

5-4

1.38

.0045

19.8

5.0

.050

.069

6 oo

1.6

•53

.0014

17-3

5-8

.058

.023

10 oo

•4

.08

.0002

13.2

2.6

.026

.008

August 19

6 oo a.m.

•4

•°5

.0001

9.2

1.2

.012

.Oil

II 00

4-7

•94

.OO24

18.6

3-7

•°37

.064

6 oo p.m.

6.0

.09

.OOO2

31-5

4-5

.045

.005

meter of leaf surface. Thus Rt is It divided by the number of hours in
the period, while Rta is Rt divided by the total leaf area. Column Ie
gives the increment of water loss from the evaporimeter (approximately
equivalent to 100 sq. cm. of free water surface) for the period, column
Re gives the rate per hour, and column Rca gives the same rate per
square centimeter of free water surface. In the last column the figures
denote the ratio between the rate per hour for unit leaf surface and
the same rate for unit water surface. In other words, this ratio shows
the fractional part of a square centimeter of water surface which would
be required to give off as much water as would evaporate during the

46 THE RELATION OF DESERT PLANTS TO

same period and in the same position from a single square centimeter
of leaf surface of this plant. This ratio will be termed the rate of
relative transpiration, the term being used to denote that this ratio
shows the relation of transpiration to evaporation.

For the whole period of the experiment, from 12h30m p.m., August 16,
to 6 p.m., August 19, the average hourly rate of transpiration for the
entire plant was 0. 420 gram. , and the same rate per square centimeter
of leaf surface was 0.00105 gram. In order to bring out clearly the
manner in which the rates per hour vary during the day, curves have
been constructed for them and for their ratio, and these are presented
in figure 8. The curves are marked at the left with the symbols which
head the corresponding columns in Table VII. Abscissas denote time,
dates and two-hour periods being marked on the lower horizontal axis,
which is drawn as a broad line for the night periods, from 6 p.m. to
6 a.m. The ordinates are the figures from the table and are placed
directly upon the curves. They are plotted at the middle of their
respective periods. The two rate curves are plotted on the same
horizontal axis and on the same scale for the abscissas. In order to get
the curve of evaporation rate into the space allowed, the ordinates for
this curve are plotted on a scale only one-fourth as great as that used
in the curve of transpiration rate. The horizontal axis for the ratio
curve is placed above the other two curves in order to avoid intersec-
tions. The scale of the abscissas for this curve is identical with that
for the other curves, but the scale for the ordinates is merely one of
convenience. A curve of temperatures, arranged by plotting the
average temperature for each partial time period at the middle of that
period is given with the curve of evaporation rate, this curve being
marked T. Since weather records are usually made with the Fahrenheit
scale a thermometer of this type was used for these observations. The
data are given without reduction to the centigrade scale.

It is to be noted at once that the rate of transpiration rises to a
maximum in the day period and falls to a minimum in the night, and
that the rate of evaporation has similar maxima and minima. This
illustrates the commonly observed phenomenon that the rate of
transpiration is higher in the day than in the night, and points to the
fact that this is largely due to variations in the evaporating power of
the air and not mainly, at least, to physiological regulation. It is plain,
however, that the two sets of ordinates do not vary at the same rate.
This is brought out clearly in the ratio curve, which shows that the
rate of transpiration approaches most nearly that of evaporation in
the day time and departs farthest from it in the night, although the
periods do not coincide exactly with those of light and darkness. This

SOIL MOISTURE AND TO EVAPORATION.

FIG. 8.— Curve of relative transpiration for a plant of Euphorbia, August 16-19, 1904. The scale
for the orrtlnates of curve Rta is four times that used for the ordinates of curve RM.

48 THE RELATION OF DESERT PLANTS TO

phenomenon may be brought about to some extent through the action
of the green chlorophyl in absorbing heat and thus increasing evapora-
tion from the leaves. This is probably not an important factor, how-
ever, since such rises in temperature can not be very marked. It is
probably brought about mainly by some physiological change in the
plant, effective during certain hours, which reduces transpiration to a
greater degree than would be brought about by the night conditions of
lower temperature and absence of light, as these affect mere physical
evaporation.

This physiological activity of the plant is perhaps mainly the
response of the stomatal mechanism.* In this plant it may also be due
in part to the nyctitropic movements of the leaves, which, during the
hours of darkness or of weak light, fold up closely against the stem
and overlap one another so as to decidedly reduce the exposed surface.
Lastly, it is possible that the physiological retardation of transpira-
tion may be due to some periodic change in the permeability to water
of the protoplasm of the plant tissues. This might occur in the roots,
which, from the experiments of many authors on the subject of root
pressure, seem to show a periodicity in absorptive rate, or it might per-
haps occur in the mesophyl of the leaves themselves. No evidence is
at hand regarding either of these suppositions.

In order to facilitate the study of these periods of high and low
rates of relative transpiration, the average ratio for the whole period
of the experiment has been found and has been plotted on the ratio
curve as a horizontal line with a constant ordinate equal to the average
ratio, which is 0.027. The average ratio was obtained by merely
summing the partial surfaces which are included in the quadrilat-
erals bounded by each pair of adjacent ordinates, the curve and the axis
of abscissas, and then dividing this total area or integral of the curve
by the last abscissa, which represents the entire time period of the
experiment. The points of intersection of this line of the average ratio
with the ratio curve itself are to be considered as the limits of the
physiological periods just noted. Since no withering of the leaves
occurred while these observations were being taken, it follows that the
plant did not suffer from lack of water during the period of the experi-

*Burgerstein (1904, p. 32) agrees with previous writers that the condition of the
stomata, whether open or closed, etc. , may usually be judged by measurements of the
rate of water loss, ' 'denn ist bei einem Blatte die epidermoidale Transpiration gegring,
so wird die Grosse der Gesamtverdunstung, die in diesem Falle hauptsachlich auf
Rechnung der stomataren Transpiration kommt, bis zu einem gewissen Grade propor-
tional sein dem Offnungszustand der Spaltoffnungen, so dass man bei relativ hohem
(durch Wagung ermittelten) Transpirationswert, auf Offnung, bei sehr geringer tran-
spiratorischer Leistung auf eine mehr oder wenger vollkommene Clausur der Stomata
schliessen kann. "

SOIL MOISTURE AND TO EVAPORATION.

ment. This fact is shown also by the uniformity of the ratio curve
itself, and it makes possible the use of this method for determining the
average rate of relative transpiration.

The physiological periods cut off by the average line are seen to be
fairly regular. They do not, however, as has been already noted,
coincide with the periods of solar day and night, but terminate in the
vicinity of the hours 3 a.m. and 3 p.m. The period of high rate of
relative transpiration falls mainly in the day and that of low rate
mainly in the night.

The average ratio for each of the partial periods just described was
determined in the same manner as was that for the whole period of
the experiment. These ratios are given in Table VIII and are shown
on the ratio curve by horizontal lines extending within the limits of
the time period which they represent, and having the average ratio
for constant ordinate. In the last line of the table are given second
averages of the three night periods and of the two complete day periods.
Inspection of these data makes it evident that in this case relative trans-
piration was, in round numbers, three times as great for the day
periods as for those of the night.

TABLE VIII. — Average Ratio — Experiment I.

Low periods.

Average
ratio.

High periods.

Average
ratio.

(i) Aug. 16, 2 p. m. to Aug.
17 4h 10™ am..

O.OI4

(2) Aug. 17, 4h30m a. m. to
2h3o'n p.m

o 04°

(3) Aug. 17, 2h3om p. m. to
Aus: 18, ih-iom a.m...

.on.

(4) Aug. 1 8, 3h30m a. m. to
i p.m...

.CK4

(5) Aug. 1 8, 3 p. m. to Aug.
IQ, 4h a.m..

.01 T,

(6) Aug. 19, 4 a. m. to i2h
•?om p.m..

•'•' _)H
.OdC

Average

.01"?

Average

.O47

A comparison of the rate of evaporation from a free water surface
with the transpiration rate from an equal leaf surface was long ago
made by linger (1861), who even went so far as to determine the
ratio between the two daily rates, showing that this ratio for Digitalis
purpurea varied in value, under different weather conditions, from 1 : 7
to 5:7. This writer observed the existence of a daily periodicity of
absolute transpiration and called attention to the fact that the varia-
tions in the rate of transpiration do not follow exactly the variations in
the evaporation rate. He regarded transpiration (p. 368), as "ein physi-
kalischer durch die Beschaffenheit der Pflanze modificirter Process. "

The only other experimenter who has studied the ratio of the trans-
piration rate to that of evaporation is Masure (1880), who obtained,

50 THE RELATION OF DESERT PLANTS TO

by the weighing method, the ratio of water loss from three vessels,
one containing free water, the second containing moist soil, and the
third similar to the second but with growing plants of Xeranthemum
bracteatum. The amount of transpiration was obtained by subtracting
the decrease in weight of the second dish from that of the third, the
assumption being made that the soil would lose water at the same rate
whether with or without plants. This writer's periods of observation
were so long, being about a week, that his results failed to bring out
the variations in the rate of relative transpiration with which we are
chiefly interested here.

A study of the relation of external factors to this physiological
periodicity will be instructive. In the first place, it was noticed at
once that these periods do not coincide at all with the periods of nycti-
tropic movement. The leaves were observed to take their nocturnal
position between 3''30m and 5h30ra in the afternoon and to return to their
diurnal position at about the same time in the morning, while relative
transpiration began to decrease, and even to decrease rapidly, several
hours earlier in the day than there was any evidence of leaf folding.
Thus the leaf movements are shown to be of comparatively little im-
portance in determining the rate of relative transpiration, and stomatal
or internal adjustments appear to be the probable controlling factor.
This plant transpires mainly from the lower surface of the leaves, and
the closing of these organs does not affect evaporation from this surface.

To facilitate the study of the relations existing between external
conditions and these variations in relative transpiration, the minimum
and maximum points on the curve of the latter have been designated in
figure 8 (p. 47) by heavy vertical lines. The lighter vertical lines merely
divide the day from the night periods.

A study of the points where the heavy lines intersect the other
curves and the axis of abscissas brings out certain interesting facts.
Obviously the maximum for the first day of the experiment is not
shown. The first minimum is at 8 p.m. Following down the line from
this point to the other curves, we find that increase in relative trans-
piration began at a time when the air temperature was 75° F., and
when the evaporating power of the air was such as to produce evapora-
tion from unit water surface at the rate of 0.029 gram per hour. De-
termining these data for each maximum and for each minimum point
throughout the curve of relative transpiration, we arrive at the facts
presented in Table IX. This table gives the hour, temperature, and
hourly rate of evaporation from unit water surface, together with the
maxima and minima of relative transpiration with which they are coin-
cident in time.

SOIL MOISTURE AND TO EVAPORATION.

TABLE IX. — Relation of Transpiration to Temperature and Evaporation Rate —

Experiment L

Minima.

Maxima.

Hour,
p.m.

Relative
transpira-
tion rate.

Temper-
ature.

Evapora- !
lion rate.

Hour,
a.m.

Relative
transpira-
tion rate.

Temper-
ature.

Evapora-
tion rate.

°F.

8hoom

0.006

0.029

I0h 00m

0.069

79-5

0.035

8 oo

.009

.020

8 30

.070

.036

7 3°

.008

,026

8 30

.064

.037

Average.

.0077

76.7

.025

Average.

.0677

79.17

.036

From these data it appears that the hours at which sudden changes
occurred in the general direction of the transpiration curve are not
nearly as uniform as are the simultaneous evaporation rates and tem-
peratures. This seems to indicate that either temperature or intensity
of evaporation is to be considered as probably the controlling factor in
the regulation of transpiration in this plant. From the fact that the
hours of the above table fail to show uniformity, it appears that this
regulation is not to be related to changes in light intensity nor to any
form of chronometric rhythm which the plant might be supposed to
possess. Since intensity of evaporation follows temperature rather
closely, it is impossible to distinguish between these two factors by the
data at hand. Whatever may be its cause, the observed regulation
is seen to cause a variation in relative transpiration from a maximum
of about 0.068 to a minimum of about 0.008, or from unity to about
one-ninth.

The foregoing method promises to be of very great value in studies
of the rate of transpiration and of the factors which cause this rate to
vary. Indeed, it is the only method so far devised which can give
direct evidence in regard to the physiological regulation of transpiration
rate. The time necessarily devoted to other lines of research limited
the taking of data regarding the relation of temperature and intensity
of evaporation to transpiration, so that those here given are necessarily
only of a preliminary nature. In the description of the following
experiments this matter will be reverted to whenever the data are
sufficient to warrant it.

Experiments II and III. —These were brief experiments, carried on
in duplicate from 7h30m p.m., September 4, to l''30m p.m., September 5.
Each culture consisted of two seedlings of Fouquieria splendens. They

THE RELATION OF DESERT PLANTS TO

stood in the shade near the fixed evaporimeter. Withering of leaves
was beginning to be manifest at the time of the second weighing;
therefore no more weighings were made. The data for both experiments
are given in Table X.

The symbols Rt, etc., of this table have the same significance as in
Experiment I. It will be noticed that the hourly rate of transpiration
from these plants was 0.839 or 0.83 mg. for unit leaf surface. No data
were obtained for the comparative rates of night and day periods.*

TABLE X.—Data for Experiment II and III.

Experiment
11.

Experiment
III.

Total leaf area

Transpiration, average rate per hour
Transpiration, average rate per hour

sq. cm...
(/?,). .gram
per sq. cm.
gram

18.339

.0077

.000839
3-58

.0358
.0234

14.697
.0061

.000830
3-58

.0358
.0234

Evaporation, average rate per hour (7
Evaporation, average rate per hour
(R ) .

? ) gram.. ..

per sq. cm.
...gram...

R.elative transpiration \ — — (

•*^ €(t

Experiment IV. —The subject of this experiment was a thrifty plant
of Tribulus brachystylis standing in the shade on the north side of the
building. It was sealed and the record was begun at 9"30ma.m., August
13, and was continued until 8h30m a.m., August 15. The total leaf area
was 39.69 sq. cm.

This plant has a more marked nyctitropic movement than the
Euphorbia of Experiment I. The movement consists in the rising of
the leaflets of the pinnately compound leaves until their upper surfaces
approximate each other, after the manner of the similar movement in
Gleditschia triacanthos.

The table of fundamental data will be omitted in this and the follow-
ing experiments, the essential points being brought out clearly by the
curves. The curve of relative transpiration for this experiment,
together with those of temperature and rate of evaporation, are given in
figure 9. These are constructed on the same plan as those of Experi-
ment I.

An inspection of figure 9 shows that during the progress of the
experiment the rate of relative transpiration, while showing something

*Prof. F. E. Lloyd was studying the transpiration of this species especially with
reference to night and day rates, while the present work was in progress. His paper
on this subject has not yet appeared.

SOIL MOISTURE AND TO EVAPORATION.

of the periodic rise and fall already described for Euphorbia, was, on
the whole, gradually decreasing. This is undoubtedly due to the grad-
ually diminishing supply of water in the soil. Wilting did not occur
during the experiment, but probably would have been evident had the
record been continued an additional day. The average hourly rate of
transpiration for the last 24 hours of the record, from 8''30m a.m.,
August 14, to the same hour on August 15, was 0.0723 gram for the
whole plant and 0.0018 gram per square centimeter of leaf surface.

\.Z63

~~"

88\

.063

043

7:30 p. m.

\ 76 *•

\ „--

w —

\.OI2 .O/3

•• so p.m.
u%. 13

O2O

/.033

.013

.034

.O/3
t£:3Op.m.

.04-3

6:30 a.m.

.04 8

9:00 p.m.

.02 6

.0/5

6:30 p.m.

77.5

6:30 a.m.
Aug. 15

FIG. 9.— Curve of relative transpiration for a plant of Tribulus brachystylis,

August 13-15, 1904.

As is noted above, this plant exhibits a physiological regulation of
the rate of transpiration which is very similar to that of Euphorbia.
Two maxima and two minima of relative transpiration are shown
within the period of the experiment. The comparative data for these
points on the curve are presented in Table XI, which is arranged in the
same manner as Table X.

THE RELATION OF DESERT PLANTS TO

An inspection of these data makes it appear that here, as in Euphor-
bia, the hours of the day at which the critical points occur seem to show
no uniformity, and therefore the time factor can not be considered as
controlling the regulative response. The early hour of the maxima
seems again to preclude light intensity. Also, the evaporation rates
for the two minima and also for the two maxima are very far from being
alike, while the corresponding temperatures are almost identical in each
case. Thus it appears that air temperature is the most probable con-
trolling condition governing the regulative response. It is to be noted
that the temperature for the maximum in relative transpiration, i. e.,
the temperature at which the physiological checking of transpiration
begins to be manifest, is 10 degrees higher than that which corresponds
to the minimum. The latter, is, of course, the temperature at which
the check is removed and transpiration begins to increase again.

TABLE XL — Relation of Transpiration to Temperature and Evaporation Rate-
Experiment IV.

Minima.

Maxima.

Hour.

Relative
transpi-
ration.

Temper-
ature.

Evapora-
tion rate.

Hour.

Relative
transpi-
ration.

Temper-
ature.

Evapora-
tion rate.

7h3Om p.m.
9 oo p.m.

Average....

0.008
.010

Grain.
0.043
.026

uhoom a.m.
12 30 p.m.

Average...

0.263
•123

°F.
89
90

Gram.

0.032
.048

.009

79-5

•193

89-5

The leaves of this plant were observed to close between 4h30"' a.m.
and 6 p. m. and to open between 3 and 4 a. m. , thus making it
apparent that, while the leaf movement undoubtedly has considerable
effect in the regulation under consideration, this movement is not
the controlling means by which the regulation is accomplished. Again,
it appears that the stomata may be the organs mainly effective in this
regard or that some internal adjustment is operative.

Experiment V. — The subject of this experiment was another plant
of Tribuliis, similar to the one used in the last experiment and standing
in the same place. The plant was sealed at 2"50m p.m., August 13, and
the record was continued till 8"30m a.m., August 15. Wilting occurred
six hours after the end of the record. The total leaf area was 121.34
sq. cm. The average hourly rate of transpiration during the last 24
hours, from 8h30m a.m., August 13, to the same hour August 14, was,
for the whole plant, 0.3346 gram, or 0.0028 gram per square centimeter
of leaf surface.

The curve of the rates of relative transpiration, together with those
of temperatures and of evaporation rates, are presented in the same

SOIL MOISTURE AND TO EVAPORATION.

manner as that used in the previous experiment, in figure 10. As in
Experiments I and IV, a periodic fluctuation in the rate of relative trans-
piration is shown, the low period being in the night and the high
period in the day. During the record of the experiment only a single
unquestionable maximum is shown. This has a rate of relative trans-
piration of 0.237 at 6''30m a.m., and occurs with a temperature of 79° F.

FIG. 10.— Curve of relative transpiration for a second plant of Tribulus,

August 13-15, 1904.

and an evaporation rate of 0.013 gram. Two minima are shown on the
curve, one indeterminate from the curve's form, but probably to be
considered as in the vicinity of 9h30ra p.m., with a relative transpiration
rate of 0.038, a temperature of 75° F., and an evaporation rate of
0.012 gram, the other at 9 p.m., with relative transpiration rate of
0.029, temperature of 79° F., and evaporation rate of 0.026 gram. It is
to be noticed that in the night of August 13-14 the minimum tern-

THE RELATION OF DESERT PLANTS TO

.78S

.t09\

79.5
'.046

.609

perature, as far as the observations show, occurred very early, at 9h30m
p.m. The temperatures at which the two minima occur are much more
nearly in agreement than are the corresponding evaporation rates.
Obviously light intensity can play no important role in the response.

On the whole this curve of transpiration agrees fairly well with that
of Experiment IV, and points to the same general conclusion. The
variation in relative transpiration in the middle portion of the period
of this experiment was from a maximum of 0.237 to a minimum of

0.029, or from unity to about
one-eighth, which is the same
as in the other specimen of this
form used in the last experi-
ment. The present plant had
more young leaves than did the
previous one, and this fact may
explain why its hourly rate of
water loss per unit area was
higher than in the former case.
Experiment VI. — The sub-
ject was a plant of Allionia
incarnata, consisting of three
shoots, each about 15 cm. long,
coming from a single root.
Flowers were opening during
the experiment. The plant was
sealed at ll"30m a.m., August
15, and the record of weighings
was continued until 3"30"'p.m.,
August 17, when wilting en-
sued. The plant was exposed
to bright sunshine during the
daytime. The evaporimeter
for use in sunshine was not
available until 3"30m p. m.,
August 16, so that relative
transpiration was not obtained
till after that time. The total
leaf area was 119.44 sq. cm.,
and the average hourly rate of

transpiration from the entire plant for the whole period of the experiment
was 0. 8396 gram, or 0. 007 gram per unit of leaf surface. Since the aver-
age daily rate of transpiration remains practically uniform throughout
the experiment, and does not fall toward its end, this hourly rate may

\I38

73.!.

.O/O

.008

6:30 p.m.

79.5.

'.0/5

,137

/O:00a.m.

84
/.O6O

'.O35

6:3O
Aug.,

a.m

FIG. 11.— Curve of relative transpiration for a. plant
of Allionia incarnata, August 16-17, 1904.

SOIL MOISTURE AND TO EVAPORATION.

be taken to represent the conditions just preceding the wilting of the
plant.

The curve for rates of relative transpiration and those for tempera-
tures and evaporation rates, for the period following 3h30"' p.m., August
16, are given in, the usual manner in figure 11. Relative transpiration
is seen to vary from a minimum of 0.029, at 8 p.m., August 16, to a

.256

.^^o

.OS 5

/.•oop.m.

J2/

.IO9

027

12:30 p.m.

04-8

.024

9:00 p.m.

06^

.058

.032

.0/5

!2:3Op.m,

90.5/

93 93

88\

86.5

.06 3

.OS
'.O44-

0361

.03^

043

/.?--- 76

\.OI2 .012

'.0/3

.042

.021

.OI5

6:30 p.m

Aug./ 3

: 3d a.m.

6:3 p.m.

6:30 a.m.
Aug./ 5

FIG. 12.— Curve of relative transpiration for three plants of Boerhavia, August 13-15, 1904.

maximum of 0.371 at 10 a.m., August 17. The minimum occurred with
an air temperature of 75° F. and an evaporation rate of 0.029, while
the maximum occurred with a temperature of 79.5° F. and an evapora-
tion rate of 0.035 gram. It will be seen that the regulation of water
loss causes a variation in relative transpiration from unity as a maximum
to about one-twelfth as a minimum.

THE RELATION OF DESERT PLANTS TO

The period of this experiment was so short that it is impossible to
draw any general conclusions therefrom concerning the causal factors
which govern the relative transpiration rate. It may be noted simply
that the temperature at which the maximum relative transpiration
occurred is several degrees higher than that at which the minimum
occurred. This plant has no nyctitropic movement and the well-marked
regulation of transpiration which is unequivocally shown in the curves
is probably due to the stomatal or some internal mechanism.

Experiment VII.— Three Boerhavia seedlings were used in this case.
They were about 10 cm. high, in bloom at the beginning of the record.
The experiment extended from 9"30m a.m., August 13, to 3"30mp.m.,
August 15, when wilting occurred. The total leaf area of this culture
was not determined. The hourly rate of transpiration for the last 24
hours was 0. 14 gram for the entire plant.

TABLE XII. — Relation of Transpiration to Temperature and Evaporation Rate-
Experiment VII.

Minima.

Maxima.

Hour.

Relative
transpi-
ration.

Temper-
ature.

Evapora-
tion rate.

Hour.

Relative
transpi-
ration.

Temper-
ature.

Evapora-
tion rate.

7h 30™ p.m.
9 oo p.m.

0.028
.024

So
79

Gram.

0.043
.026

ih oom p.m.
12 30 p.m.
10 oo a.m.

0.256
.109
.058

93
90

Gram.

0.044
.048
.042

In default of the leaf area a curve of the ratios of the hourly rates
of water loss from the whole plant to the hourly rates of evaporation
from the whole evaporimeter surface is given in figure 12 (p. 57) . This
is accompanied by the usual curves of evaporation rates and of tem-
peratures. The ratio curve here given shows, of course, the same varia-
tions as would the curve of rates of relative transpiration. The latter
curve would be obtained from the ratios of the given curve by dividing
each of those ratios by the leaf area and multiplying the quotient by 100,
the standard water surface represented by the evaporimeter.

It is apparent from the curve that the rate of relative transpiration
decreased on the whole throughout the period of the experiment.
Three maxima and two minima are clearly shown upon the curve of
relative transpiration. The hours, temperatures, and evaporation rates
for each of these are shown in Table XII.

From the data for the two minima it appears that air temperature
is probably the external condition which causes the regulative mechan-

SOIL MOISTURE AND TO EVAPORATION.

.37/

.295

.(755V

.108

ism to act. But from the data for the maxima it appears that evapora-
tion rate is more uniform than temperature and therefore that this is
probably the controlling condition for the regulating response. Thus
the evidence here is conflicting, much as in the case of Experiment I.
Light intensity is again seen to be assuredly not the controlling con-
dition.

This plant has no definite nyctitropic movement, so that here, as in the

case of Allionia, the variations
in relative transpiration are not
due to such movement. In the
middle portion of the period of
this experiment relative trans-
piration varied from a maximum
of 0.121 to a minimum of 0.024.
The regulative activity is thus
shown to be able to reduce relative
transpiration from unity to about
one-sixth. It is thus only about
one-half as effective in these
plants of Boerhavia as it was in
the two specimens of Tribulus and
in the Allionia.

Experiment WIT".— The plant
was a Boerhavia standing in
bright sunshine during the day.
The experiment lasted from 10"30m
a. m., August 16, to 3h30m p. m.,
August 17, when wilting occurred.
The total leaf area was 47.5 sq.
cm. For the last 24 hours before
wilting the entire plant transpired
at the average hourly rate of
0.2877 gram, or 0.0061 gram per
square centimeter of leaf surface.
The usual curves are given in
figure 13. That of relative trans-
piration is seen to be very similar
to the same curve for the Allionia of Experiment VI. The first point is
probably a maximum, so that here we have to consider two maxima and
one minimum. The first maximum is 0.785 and occurs at Ilh30m a.m.
with a temperature of 79.5° F. and an evaporation rate of 0.046 gram;
the second is 0.609 and occurs at 10 a.m., with the same temperature as

.071

,010

79.5.

/ovoa.m.

.035

e:3op.m

6:303. m

FIG. 13.— Curve of relative transpiration for a
plant of Boerhavia, August 16-17, 1904.

THE RELATION OF DESERT PLANTS TO

the other and an evaporation rate of 0.35 gram. The minimum is 0.054
and occurs at 8 p.m., with a temperature of 75° F. and an evaporation
rate of 0.029 gram.

It appears here again that temperature rather than intensity of evap-
oration is possibly the controlling factor in the regulation of relative
transpiration, and that the light intensity is not important. The tem-
peratures for the maxima are again about 10 degrees higher than those
for the minima. The variation in rate of relative transpiration due to

.OSJ

90.S/

89/

86. S

.048

036,
\J
.032

.130

.065

i:oop.m.

93 93

-\
\
\
\

88"

.063

.05.
.044

043

.OI4-

7. -30 p.m.

£V--"76

6:30 p.m.

,084

.069

(0/3

6:30 a.m.

.079

.043

84
\

030

009

9:oop.m.

.Of 6

.015

6:30 p.

7.5

OZI

AuftS

FIG. 14. — Curve of relative transpiration for a plant of Boerhavia,

August 13-15, 1904.

physiological action amounts here to the difference between 0.054, the
minimum rate, and 0.609, the maximum. Thus the regulative mech-
anism is able to reduce relative transpiration from unity to about one-
twelfth.

Experiment IX.— This experiment was carried on with another
mature Boerhavia plant, sealed at 9h30m a.m., August 13. The record
was continued until 8h30m a.m., August 15, the plant standing in the
shade on the north side of the building. The leaf area was not

SOIL MOISTURE AND TO EVAPORATION.

determined, but the ratio curve derived from the rates of transpiration
for the entire plant is given in figure 14, together with the curve of
temperatures and that of evaporation rates for the period. The aver-
age hourly rate of actual water loss from the plant for the 24 hours
from 8h30ma.m., August 14, to the same hour August 15, was 0. 1340 gram.
Wilting did not occur within the time of the experiment.

The curve of relative transpiration shows two maxima and two
minima. The hours, temperatures, and evaporation rates for these are
shown in Table XIII.

Apparently here relative transpiration is again governed by tem-
perature, and the turning points in its curve are at about 90° and 80° F.,
the higher temperature once more corresponding to the maximum and
the lower to the minimum. The response is effective in reducing rela-
tive transpiration from 0.084 to 0.009, or from unity to about one-ninth.

TABLE XIII. — Relation of Transpiration to Temperature and Evaporation Rate —

Experiment IX.

Minima.

Maxima.

Hour,
p.m.

Relative
transpira-
tion.

Temper-
ature.

Evapora-
tion rate.

Hour,
a.m.

Relative
transpira-
tion.

Temper-
ature.

Evapora-
tion rate.

7h30m
9 oo

0.015
.009

OJJT
80

Gram,.

0.043
.026

iboom

12 30

0.130
.079

ojr

Gram.
0.044
.048

Experiment X.— The subject was another flowering plant of Boer-
havia, standing in the shade. The pot was sealed at 12h30mp.m.,
August 16, and the record was continued until 7 a.m., August 18. No
wilting occurred. The leaf area was not determined. The average
transpiration rate per hour for the entire plant from 9h30m a.m., August
16, to 9h30m a.m., August 17, was 0.1729 gram. Curves for this experi-
ment are given in figure 15 (p. 62) , following the plan of Experiments
VII and IX.

The curve of relative transpiration includes a single minimum of
0.015 at 8 p.m., with a temperature of 75° F. and an evaporation rate
of 0.029 gram; and a single maximum of 0.118 at Ih30ra p.m., with tem-
perature 84° F. and evaporation rate of 0.041 gram.

The effect of the regulative response amounts in this case to a differ-
ence between a relative transpiration rate of 0.015 at the minimum
point and about 0. 121 at the maximum. Relative transpiration is thus
reduced from unity to about one-eighth.

THE RELATION OF DESERT PLANTS TO

.121

.118

051

.109

.04-1

l:3Op.m.

83 .065

Experiment XL —This experiment was carried out with three Boer-
havias just coming into flower. The record was begun at 6h30ra p.m.,
August 16, and discontinued at 9h30ra p.m., August 17. The total leaf
area of the three plants was 82.6 sq. cm. The average rate of water
loss from all three plants for the whole period was 0.7052 gram, or
0. 0085 gram per square centimeter of leaf surface. The usual curves are
given in figure 16. The period of the experiment was not long enough
to warrant any discussion further than to state that a single maximum
is shown at about 10 a.m., with a temperature of 79.5° F. and an

evaporation rate of 0.035
gram. Minima are not
definitely shown within
the period.

Experiment XII. —The
subject was a single flow-
ering plant of Boerhavia
in a Stender dish standing
in the shade. The record
extended from 6h30m p.m.,
August 16, to 9h30'u p.m.,
August 17. The leaf area
was not obtained. The
hourly rate of water loss
from the entire plant for
the whole period was
0.1926 gram.

"The ratio curve for this
plant need not be pre-
sented. It shows a max-
imum having a rate of
relative transpiration of

0.158 at 7p.m., with a temperature of 73.5° F., and an evaporation rate
of 0.011 gram. Minima are not certainly indicated by the curve.

Experiment XIII.— The subjects were three flowering Boerhavia
plants standing in the shade. The record extended from 10 p.m.,
August 18, to S'^O"1 a.m., August 21, when wilting occurred. The total
area was 63.193 sq. cm. From 6 p.m. August 19 to the end of the
experiment the entire culture transpired at the average rate of 0.5860
gram per hour, or 0.0093 gram per hour per unit leaf surface. Only a
few weighings were made and a curve could not be constructed.

Experiment XIV. — - The subject was a single plant of Boerhavia
standing in bright sunshine. The record extended from 6 a.m. to

/75

/.02Z

'.OH

.04 1

[\
.020

6: 30 p.m.
. 16

6. :3O a.m.

Aug. /7

6:3Op-m.
Jug. 17

PiG. 15. — Curve of relative transpiration for a plant
of Boerhavia, August 16-17, 1904.

SOIL MOISTURE AND TO EVAPORATION.

11 a.m., August 19. The plant began to wilt at the last-named hour.
The total leaf area was 50.376 sq. cm. During the entire period the
plant transpired at the average rate of 1.3620 grams per hour, or 0.027
gram per unit leaf surface. Owing to the short period no curve was
constructed for this plant.

Experiment XV. — This test was performed with three seedling
squash plants (Cucurbita pepo), each having two leaves besides the

cotyledons. The soil about them
had been kept moist since germi-
nation and they had grown with
exceedingly great rapidity. The
three plants together possessed a
total leaf surf ace of 238. 2 sq. cm.
They were sealed August 16 at
12"30m p.m., and had begun to
wilt at 3"30m p.m. During this
period of three hours they trans-
pired at the rate of 1.9 grams per
hour, or 0.008 gram per square
centimeter of leaf surface. For
this period of three hours their
rate of relative transpiration was
0.131.

GENERALIZATIONS FROM THE
EXPERIMENTS.

It appears from the data just
presented that Euphorbia, Trib-
ulus, Allionia, and Boerhavia all
show a periodic fluctuation in
their relative transpiration. The
highest relative transpiration ob-
served was 0.785 (Experiment
VIII) and the lowest was 0.008
(Experiment IV) . They all have
some form of regulative response
whereby transpiration begins to
be checked between 6h30m a.m.
and 1 p.m., the check being generally removed between 6 and 8 p.m.
It also appears that in all three forms stomatal or some internal foliar
responses probably play the most important role in this regulation of
water loss, these being aided perhaps by nyctitropic movements in the
first two forms mentioned. As far as the limited data at hand can be

FIG. 16.— Curve of relative transpiration for
three plants of Boerhavia, August 16-17, 1904.

64 THE RELATION OF DESERT PLANTS TO

trusted, the temperature of the surrounding air seems to be the control-
ling condition which governs this regulative response. It seems that
when the temperature reaches a certain point in its daily rise the checking
of transpiration begins to be effective, and that the check is removed
when the air temperature has passed its daily maximum and again
decreased to a certain point. The latter point seems, in most cases, to
lie somewhat below the point at which the checking response begins.
The physiological maximum, at which transpiration begins to be checked,
lies, for the forms studied, between 79° and 90° F., and the corresponding
minimum, at which the check is removed, occurs between 75° and 80° F.
There seems to be no evidence from these experiments for supposing
light intensity to be the controlling condition for this regulation, as it
is commonly taken to be for most plants,* for the checking of transpi-
ration begins to be noticed too early in the day to be due to diminished
light intensity. It is of course possible that with high intensity of illu-
mination the checking of water loss occurs and that this check is
removed with the coming on of the nocturnal darkness, but this suppo-
sition is the direct opposite of the prevalent idea regarding this regula-
tion. More data are necessary for a test of this point.

There is some evidence that intensity of evaporation is the con-
trolling factor, in some cases at least, but this is not as consistent as
the evidence for air temperature. There is practically no evidence from
these experiments that the response is due to some chronometric rhythm
within the plant. The data at hand do not bear upon the question as to
whether this regulation is in any way connected with photosynthesis.

Table XIV presents in tabular form the data obtained as to the rela-
tive efficiency of the regulative response. In the first two columns are
given the number and subject of the experiment, in the next two the
maximum and minimum rate of relative transpiration, as nearly as
these can be ascertained. The fifth column gives the efficiency of the
regulation of transpiration, being denoted by the ratio of the minimum
to the maximum, the former being considered as unity and the latter
expressed in round numbers. Thus, in the case of Experiment I, the
symbol 1/9 means that the minimum of relative transpiration is approxi-
mately one-ninth of the maximum. In the last column the external
conditions which apparently control the response are stated, T referring
to air temperature and E to intensity of evaporation. It appears from

*The experimental evidence in regard to stomatal movements and their cause is
not very conclusive. For a presentation of the whole subject of the effect of light,
temperature, wind, etc., upon the absolute transpiration rate, the reader is referred
to Burgerstein (1904). The preliminary character of the present results renders a
thorough discussion of the literature unnecessary.

SOIL MOISTURE AND TO EVAPORATION.

this table that the efficiency of the regulative response varies from one-
sixth to one-twelfth, and this without apparent relation to nyctitropic
movement; for one plant of Tribulus, with marked nyctitropic move-
ment, shows an efficiency of one-twelfth and the other an efficiency of
one-eighth, and Boerhavia, without appreciable nyctitropic movement,
shows a variation in efficiency of from one-sixth to one-twelfth.
Further work will need to be done in this field of inquiry before any
definite conclusion can be reached.

TABLE XIV. — Summary of Transpiration Experiments.

Experiment
number.

Subject.

Relative transpiration.

Efficiency of
regulation.

Apparent con-
trolling factor.

Maximum.

Minimum.

IV
V
VI
VII
VIII
IX
X

Euphorbia

0.068

.193

.237
•371

0.008
.009
.029
.029

i/9

I/I2

1/8

1/12

*i/6

1/12

*i/9

*i/8

T. or E.
T.
T.

Tribulus

Allionia

Boerhavia

T. or E.
T.
T.

.609

.054

*These efficiencies are obtained from the maxima and minima ratios derived from the rate
of transpiration from the entire plant and that of evaporation from the whole evaporimeter
surface. See the discussions of the experiments.

A table of the rates of water loss from these plants will be given in
the following section, together with data concerning the moisture con-
tent of the soil at the end of the experiment.

WATER REQUIREMENT OF CERTAIN DESERT PLANTS.

In the present section will be presented what data were obtained
bearing upon the amount of water needed in the soil in order that
plants may live in the desert. This problem was attacked directly, by
determining the water content of the soil samples in which the plants
for the foregoing transpiration measurements had been growing. This
was the sole end in view when the first of these experiments were
started, the intention being merely to relate the moisture content of the
soil to the rate of transpiration both for the entire plant and for unit leaf
surface. The data on regulation of water loss, presented in the last
section, developed as a secondary consideration in the course of the work.

The results of the moisture determinations of the soils are pre-
sented in Table XV. In this table the first two columns again give
the numbers and subjects of the experiments. In the two following
columns are given average hourly rates of transpiration for entire plant
and for one square centimeter of leaf surface, these being calculated

THE RELATION OF DESERT PLANTS TO

from the last 24 hours of the experiment, or for as nearly that period
as was possible from the data at hand. The rates marked with an
asterisk (*) in the third column are for plants which had begun to
wilt at the end of the transpiration record; in the last column is given
the water content of the soil at the end of the experiment, in per cent
of its volume under water. These moisture determinations all cor-
respond to soils in which incipient wilting had just occurred.

TABLE XV. — Relation of Transpiration to Moisture Content of the Soil.

Experiment
number.

Subject-

Average hourly rate of
water loss.

Moisture con-
tent of soil,
per cent of
wet volume.

For entire
plant.

Per sq. cm.
of leaf
surface.

I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV

Euphorbia

Oram.
0.4618
.0077*
.0061*
.0723
•3346
.8396*
.1400*
.2877*
.1340
.1729
.7052*
.1926
.5860*
1.3620*
1 .9000*

Oram.
0.00115
.00084
.00083
.0018
.0028
.0070

9-13

5.50

7.70

IO.OO

9.70
9.50
9.05

10.72

Fouquieria

Tribulus

Allionia

Boerhavia

.0061

do
do

.0085

13-70

8.59

9.20

9.90

12.20

do
do

.0093
.0270
.0080

Cucurbita

It appears from the table that the wilting point for these plants, in
terms of moisture content of the soil, lies between 5.5 and 13.7 per
cent. It was lowest for Fouquieria, intermediate for Allionia and
Boerhavia, and highest for the squash plants of Experiment XV.
Judging from the other experiments with Boerhavia, the high moisture
content of the soil in Experiment XI is probably erratic. This general
arrangement of the different plants in regard to their power to with-
stand a dry soil is what should have been anticipated from their char-
acters. Fouquieria and Euphorbia are extreme xerophytes, while the
squash is a mesophyte. Allionia and Boerhavia, although they are
desert forms, are not active during the dry season, are not markedly
xerophytic in their structures.

As has been noted, all of the determinations of moisture content
correspond to incipient wilting on the part of the plants involved. In
Experiment I this did not occur until 15 days later than the termination
of the transpiration record, so that the rate of transpiration given in the
table is probably somewhat too high to correspond to the last 24 hours

SOIL MOISTURE AND TO EVAPORATION.

before wilting. In Experiment IV wilting occurred on the day follow-
ing the end of the record and the rate is probably not far from correct,
while in Experiment V the plant began to wilt only 6 hours after the
weighings were discontinued and therefore the rate is very nearly cor-
rect in this case. In Experiment XII wilting was manifest 3 days
after the end of the record, so that here again the rate given in the table
is probably too high. The data at hand do not represent a sufficient
number of plants to warrant a critical study of the relations existing
between the transpiration rate and the amount of moisture in the
soil at the time of wilting. This is a field for another investigation.
The problem involves not only the tenacity with which the soil withholds
its water from the plant, but also the rate of water movement through
the soil from one region to another.

TABLE XVI. — Moisture Contents of Soils in which Mesophytes Wilted.

Number.

Name of plant.

Moisture
content, by
volume.

Vicia faba

Per cent.

IO 71

IO. 17

-i

Phaseolus multiflorus

10.65

IO.4O

1 1.62

1 1 30

Helianthus annuus

I S.22

11. SO

• J' jw

Several other determinations of the moisture content of the soil
when wilting occurred were made without transpiration records. Cab-
bage seedlings wilted July 18 with a moisture content in the soil of
11.10 per cent. Three different soil samples taken August 15 from the
root systems of Boerhavia plants which were beginning to wilt in the
open showed moisture contents of 6.40, 6, and 6.74 per cent, while
another sample taken from the root system of a plant which was still
vigorous showed a moisture content of 13.6 per cent.

In February, 1905, a number of well-grown potted plants were taken
from the greenhouse at the Hull Botanical Laboratory of the University
of Chicago and placed in one of the laboratory rooms, where they were
allowed to stand without addition of water to the pots until wilting
occurred. When this occurrence was noticed soil samples were taken
from the midst of the root systems and their moisture contents were
determined. The names of the plants and the moisture contents which
corresponded with the incipient wilting are given in Table XVI. The
soil was a sandy garden soil, containing considerable humus. As

68 THE RELATION OP DESERT PLANTS TO

regards the relation of weight to volume in this case, 45.25 grams of dry
soil occupied, when allowed to settle under water, 43 cc.

These data are in very good agreement with those obtained for cab-
bage and squash at Tucson, and probably approximate an average
wilting point for most mesophytes.

Of all the determinations made for the wilting point of desert plants
only two soils surpassed 10 per cent in water content, these being in
the cases of Experiments VIII and XL It was pointed out on page 11
that the soil of Tumamoc Hill at the end of the spring dry season
contained from 5 to 10 per cent of moisture at a depth of only 10 or 12
cm., while, as far as evidence is at hand, it appears that from 12 to 15
per cent of moisture occurred at depths not exceeding 40 cm. Thus it
is seen that even in the driest part of the year the moisture content of
the soil at a depth of not over 30 or 40 cm. is probably high enough to
readily supply such plants as Fouquieria, Euphorbia, Tribulus, and
even, perhaps, Allionia and Boerhavia, with transpiration water. As
far as the first three forms are concerned (Allionia and Boerhavia
are not commonly seen here excepting in the rainy season), it seems
that these soils are not excessively dry even at the end of the dry sea-
son. Such plants as Brassica, Cucurbita, Vicia, Phaseolus, and Helian-
thus must succumb to drought conditions somewhat sooner. It was
observed in growing the seedlings of squash and cabbage that they
required watering several times a day in order to keep them in health,
while Euphorbia and Fouquieria could not only live, but thrive for many
days, in a similar vessel of the same soil without watering.

The roots of seedling Fouquierias were often observed to have pene-
trated to a depth of 10 cm. or more within 48 hours after the first ap-
pearance of the cotyledons, * and when it is remembered that the deeper
layers of this soil dry out very slowly after being wet by rain, it is
easily seen how such seedlings, germinating in the rainy reason, may
attain to a depth where they will have a permanent and adequate water
supply before the upper layers of the soil have dried out sufficiently to
produce death. Seedlings of Boerhavia and Tribulus are also very
active in the elongation of their primary roots, and all of the desert
plants studied were characterized by very long tap roots without lateral
branches. Although Boerhavia thrives only in the rainy season, it was
found impossible to lift seedlings of this form more than two or three
days after the cotyledons appeared without cutting off their roots.
These organs penetrate into the crevices between the rock fragments,
so that it is extremely difficult to remove them to a depth greater than
from 15 to 30 cm. It appears that plants whose habitats are in the more

*Covillea has the same habit in germination as has Fouquieria when the soil is
rather dry. See Spalding's figure 3, in the paper (1904) already cited.

SOIL MOISTURE AND TO EVAPORATION. 69

humid regions are uniformly not quite so resistant to drought as the
desert forms studied. The difference is not very marked, however, and
in explaining the existence of desert plants emphasis is apparently to be
laid, not upon the greater resisting power of such forms to paucity of
soil water, but upon the facts that there seems to be always considerable
moisture in the soil under discussion, that this moisture is conserved by
comparatively slow transpiration, and that most non-storage forms of
the desert root very deeply.

The general conclusions from these studies of the moisture require-
ment for the desert forms experimented with are: (1) Those plants
which exist throughout the dry season can withstand a somewhat drier
soil than those which appear only in the rainy season, and even these
latter may often resist wilting in a drier soil than can such non-desert
plants as squash, cabbage, etc. (2) There is sufficient moisture in the
soil of Tumamoc Hill, and this is near enough to the surface, to
supply the transpiration needs of such plants as Euphorbia and seedlings
of Fouquieria. The larger plants of Fouquieria, as well as the other
shrubs, must be considered as having a root system well enough dis-
tributed through the soil to correspond to their comparatively large
transpiration surfaces. They probably root very deeply in rock crevices.
(3) The roots of seedling Fouquierias elongate directly downward so
rapidly as to make it appear possible for them to reach a permanent
and adequate water supply before the soil, wet thoroughly by the fre-
quent showers of the rainy season, can produce injury through condi-
tions of drought. After their roots have reached a depth of 30 cm.
the plants are probably safe on the hill in most seasons.

The open formation of desert vegetation doubtless makes it possible
for the plants to draw upon a very large volume of soil for their water
supply. The noticeable scarcity of seedling or even young plants of the
more typical desert forms, even in the rainy season, would seem to indi-
cate that conditions other than those of available moisture are effective
to reduce the number of these. It may be that in most years the sur-
face layers of the soil do not remain moist long enough after each
shower to allow the seedlings to obtain a foothold. It seems more prob-
able, however, that the depredations of animal life, especially of insects
and the smaller mammals, are the most important factor in preventing
the growth of seedlings. As has been stated, when young plants are
left exposed in the early part of the rainy season, before the desert has
assumed the semimesophytic aspect of this season, they are almost sure
to be cut off by animals within a day or two. The importance of animal
life in determining the nature of desert vegetation is well substantiated
by the patent observation that plants which succeed well in arid regions
are generally well protected from animals in one way or another.

THE RELATION OF DESERT PLANTS TO

OSMOTIC PRESSURE OF CACTUS JUICES.

Attempts to express the juices from Boerhavia plants and determine
their osmotic pressure met with only indifferent success. The sap of
these plants is small in amount and very much thickened with slime-
like material, so that to express it in adequate amount for the determi-
nations was well-nigh impossible with the available apparatus.

Better success attended similar attempts to determine the osmotic
conditions of the juices from the storage tissues of Echinocactus and
Cereus. The storage tissue was cut out in masses, chopped into small
pieces, mashed with a mallet, and then strained free from cells and
tissue fragments by means of a cloth filter. The extract thus obtained
was subjected to freezing-point determinations by means of the
apparatus of Beckmann. * The results of this determination are given in
Table XVII. Two tests of the freezing-point were made in each case
and their averages are used in the calculation of the pressures.

TABLE XVII. — Freezing-points of Cactus Juices.

Pressure,

calculated

for 25° C.

Juice of—

First test.

Second
test.

Average.

Atm.

Cm. Hg.

Cereus

°O.
o 420

°C.

O.422

°a

O.42 I

c.C4

421.62

0.248

Echinocactus

.296

.102

O.2QQ

_>• j-t

-Z.Q4

200.44

.177

In the table, j denotes the lowering of the freezing-point, and the
calculated osmotic pressures at 25° C. are given in terms of atmos-
pheres, centimeters of a mercury column, and the pressure of a molec-
ular solution of a non-electrolyte, this being taken as 22.3 atmospheres
and denoted by M.

A test of Echinocactus juice by the boiling-point method gave an
elevation of 0.08° C., and a calculated pressure at 25° C. of 3.6 atmos-
pheres, or 274.2 cm. of mercury, which is in very good agreement with
the results obtained from the freezing-point.

The osmotic pressure of the cell sap of the cortex of Cereus was
determined also by the method, commonly used for such purposes, of
partial plasmolysis and variation in turgor tension. The epidermis and
the underlying storage tissue to a depth of about 5 mm. was removed
and cut into strips about 10 cm. long and 5 mm. wide. Owing to the
tissue tensions these immediately became concave on the epidermal
side, and the curvature was recorded by laying them upon paper and

*For a description of the methods of freezing and boiling points here used, see
Livingston (1903), and references there given, or any book on physical chemistry.

SOIL MOISTURE AND TO EVAPORATION. 71

tracing the contour with a pencil. They were then placed in solutions
of potassium nitrate of different concentrations and left for half an
hour. At the end of this period they were removed and again placed
on the tracings which represented their original contour, note being
taken as to whether the effect of the salt solution had been to increase
or decrease their curvature or to leave it practically the same as at the
beginning. Since the epidermal layer is practically nonabsorptive for
water and also gives it up with great difficulty, while the cut surfaces of
the storage tissue absorb and give out water very readily, an increase in
curvature denotes an absorption by the latter tissue and a decrease
denotes an extraction of water by the external solution. Thus those
solutions which caused no change in curvature are to be regarded as
isotonic with the cell sap of the cortex, those in which curvature
increased are of lower concentration than this sap, and those in which
curvature decreased are of higher concentration. Of course, this method
is based upon the general assumption that potassium nitrate fails to
penetrate the protoplasmic membranes of these cells.

A large number of tests of the form just described were carried out
with several different individual plants, and the results showed that
the cell sap of the storage tissues just beneath the epidermal layers has
a concentration which is equivalent to that of a potassium nitrate solu-
tion having a strength of from n/9 to n/5. That is, this sap has an
osmotic pressure of from 3.9 to 7 atmospheres. The middle point
between these extremes of pressures is very close to the value obtained
by the method of the freezing-point, 5.38 atmospheres, so that the two
methods are in fair agreement.

Similar tests were made with strips from the flattened internodes of
Opuntia Engelmannii, and gave n/6 as the approximate concentration
of potassium nitrate which is isotonic with the sap of their storage
tissues. This is equivalent to about 5.9 atmospheres and is seen to be
approximately the same as the pressure found in the case of Cereits,
but somewhat greater than that found in Echinocactus. The osmotic
pressures exhibited by these plants are not markedly higher than the
author has often observed in the cortex of scapes of Taraxacum and
stems of Ricinus seedlings. It is not nearly as high as that observed
by Sutherst (1901) with the freezing-point method in the case of a num-
ber of common agricultural plants. This author found, for instance,
that the sap of the green stalks and leaves of celery have a pressure of
1,284.25 cm. of mercury. (See in this regard Livingston (1903), p. 85).

All of the cactus juices experimented with contained considerable
amounts of mucilaginous material. While such substances do not alter
the freezing-point of the solution and probably have no effect upon the

72 THE RELATION OF DESERT PLANTS TO

osmotic pressure, they undoubtedly decrease the rate of evaporation.
It has been suggested by Aubert (1892) and others that the low trans-
piration rate observed in the case of the cacti is in part due to the
presence of large amounts of organic acids, gums, and slimes in the cell
sap of such plants. How important the latter substances may be in
Cereus, Opuntia, and Echinocactus should be well worth a determination.

CONCLUSION.

The most important results of the three lines of investigation already
discussed separately will now be brought together. Probably the most
interesting fact discovered through these studies is that the deeper
lying soil layers of Tumamoc Hill contain at the end of the spring
dry season, and therefore probably at all times, a relatively large water
content. During the two weeks just preceding the beginning of the
summer rains, tests indicated that the soil contained from 12 to 15 per
cent of moisture at a depth of not over 40 cm.

This surprisingly large water content of the lower soil layers is
probably largely due to the fact that the evaporation rate from the soil
surface far exceeds the rate of movement of soil water, thus causing
the true surface of evaporation to lie some distance below the soil surface,
the water lost finding its way to the air in the form of vapor, which
diffuses upward very slowly through the air-dry layers. In this way
the deeper portions of the soil are to a great extent protected from loss
of moisture by a layer of dry surface soil resembling a dust mulch.
The deeper soil layers are doubtless also protected by the presence of
numerous rock fragments and by the hard-pan of caliche, which is very
slowly permeable to water.

Downward penetration of precipitation water, while it takes place
slowly through the soil itself, is on the whole comparatively rapid
on account of the oblique rock surfaces, along which movement is not
markedly checked.

The amount of soil moisture at a depth of half a meter or less is
sufficient to supply the transpiration needs of such typically desert
plants as were experimented upon (Euphorbia and Fouquieria) , and is
probably also adequate for Tribulus and Allionia, and perhaps even for
Boerhavia, the most mesophytic desert form studied. These annuals,
however, may not root deeply enough to avail themselves to any great
extent of this water.

Seeds of Fouquieria and Cereus fail to germinate in soils containing
less than 15 per cent of moisture by volume, not differing in this respect
from Phaseolus and Triticum. It is thus apparent that Fouquieria

SOIL MOISTURE AND TO EVAPORATION. 73

and Tribulus exhibit no special adaptation to the arid climate of the
desert, as far as germination is concerned.

As soon as germination occurs, in Cereus, Fouquieria, Covillea,
Tribulus, and Boerhavia, a very rapid elongation of the primary root
sends the tip of this organ far into the soil. While this is taking place
the aerial parts grow but slowly. In the case of Fouquieria and Cereus
the cotyledons are the only leaves for many days and even weeks. In
Fouquieria a curious transverse thickening of the hypocotyl accom-
panies the rapid root growth, so that after two or three weeks the stem
of the seedling is exceedingly thick and woody and is covered with a
corky layer, while the root may be still unbranched and may have
extended many decimeters into the soil.

This habit of growth is well adapted to desert conditions. During
the rainy season the soil is often sufficiently moist for germination, and
by the end of the summer the perpetually moist soil of the deeper
layers is continuous upward to within a few centimeters of the surface,
so that seedlings which exhibit the phenomenon of growth just de-
scribed should find themselves well rooted in perpetually moist soil long
before the drying out of the upper layers could result in their death.
It seems that moisture conditions alone can not account for the notice-
able lack of seedlings and young plants in the desert, but that the rav-
ages of animal life must play an important part in restricting vegetation.

The clay soil of Tumamoc Hill has a high moisture-retaining
power, being able to hold water to an amount about equal to 41 per
cent of its wet volume. While this prevents rapid percolation of pre-
cipitation water from the surface layers to those more deeply seated,
thus keeping much of the water of the first rains of the summer
near the surface and thus poorly protected from evaporation, this phe-
nomenon favors water absorption by those storage plants which take
moisture mainly from the surface layers of the soil. Mrs. Spalding has
noted that when the ground about a Cereus plant is moistened, either
artificially or by rain, absorption begins almost immediately, long before
the water could have reached the deeper soil layers. This must mean
that these plants, and probably also the other cacti of the region, absorb
water very rapidly from the wet surface soil directly after the rains.
Thus the high retaining power of the clay gives to such plants practi-
cally all of the water which falls in their vicinity, excepting what is lost
by evaporation before they have time to absorb it.

The saps of Cereus, Echinocactus, and Opuntia exhibit osmotic
pressures no higher than those commonly found in plants of the humid
regions. Therefore, for these cacti at least, adaptation to desert con-
ditions is not manifest in increased concentration of the cell sap.

74 THE RELATION OF DESERT PLANTS TO

Experimental data are presented upon the effect of air currents in
increasing the rates of evaporation and transpiration, the relative
humidity of the air remaining constant. This effect is so marked that
methods of transpiration measurements involving the placing of plants
in closed chambers, while valuable in studying the physiological condi-
tion of the transpiring tissues, must be regarded as giving no clue to
the actual amount of transpiration occurring in the open air.

Transpiration studies showed that the rate of water loss per unit
of leaf surface is relatively low in the most xerophytic forms studied
and somewhat higher in the semimesophytic forms which appear only
in the rainy season. A comparison was made between the rate of trans-
piration and the rate of evaporation from a water surface, with the
result that a physiological regulation of the former rate was unquestion-
ably shown to exist. By means of a newly devised form of evapori-
meter the hourly rate of evaporation from unit water surface was ob-
tained simultaneously with the hourly rate of transpiration from several
different plant forms, for different periods throughout the day and
night, and curves were constructed showing the variations in the ratio
of transpiration rate to evaporation rate. This ratio has been termed
the rate of relative transpiration, and denotes the number of square
centimeters of leaf surface necessary to exhibit as great a water loss as
was observed, for the same time and place, from a single square centi-
meter of free water surface.

From the curves constructed for Euphorbia, Tribulus, Allionia, and
Boerhavia, relative transpiration was found to vary from a minimum
occurring about 8 p. m. to a maximum between 6h30m a. m. and 1 p. m.
The highest relative transpiration observed in the experiments was
0.785 and the lowest 0.008. The physiological regulation which this
variation shows to exist is not mainly related to nyctitropic movements
of the leaves, although these movements may have some auxiliary
effect in those forms in which the leaves are nyctitropic. There is
slight evidence that the regulatory response is related to evaporation
rate, and no evidence at all that the checking of transpiration occurs
with diminished intensity of illumination, as is commonly supposed. It
is barely possible to explain the phenomenon observed on the sup-
position that the checking of the transpiration begins when increasing
light intensity reaches a certain point and that the check is removed
with the removal of light altogether in the early evening; but this sup-
position is highly improbable and the data at hand are not sufficient to
test the question. The supposition that the variation in relative trans-
piration is due to some chronometric rhythm in the protoplasmic activ-
ities of the plant receives absolutely no support from the evidence at hand.

SOIL MOISTURE AND TO EVAPORATION. 75

The experimental evidence is very consistently in favor of the idea
that air temperature is the controlling factor for the regulatory response
in question. It appears that with the rising temperature of the morn-
ing hours a physiological maximum is reached at which the rate of
relative transpiration begins to be checked, and that this response is
reversed and relative transpiration begins again to increase when the
air temperature has passed its daily maximum and has decreased to
another point which seems to be a physiological minimum. The latter
temperature appears to be somewhat lower than the physiological
maximum at which the check is imposed. This maximum occurs
between 79° and 90° F. , while the corresponding minimum occurs between
75° and 80° F.

The regulative response produces a reduction in relative transpira-
tion from unity in the high periods to from one-twelfth to one-sixth in
the low periods.

SUMMARY.

The main results of these experimental studies may be briefly stated
as follows:

(1) The deeper soil layers of Tumamoc Hill contain, at the end
of the spring dry season, and thus probably at all times, a water con-
tent adequate to the needs of those desert plants which are active
throughout the months of drought.

(2) This conservation of soil moisture is largely due to the high
rate of evaporation and the consequent formation of a dust mulch. It
is partly due to the presence of rock fragments and of the hard-pan for-
mation called caliche.

(3) Desert forms show an adaptation to existence in dry soil,
being able to exist in soils somewhat drier than those needed by plants
of the humid regions, but this adaptation is comparatively slight and
can not be considered of prime importance.

(4) The downward penetration of precipitation water is slow
through the soil itself, but comparatively rapid on the whole, on ac-
count of the presence of numerous oblique rock surfaces along which
the flow is not markedly impeded.

(5) By the middle of the summer rainy season all of the soil
excepting the first few centimeters is sufficiently moist to allow germi-
nation and growth of most plants. The surface itself is often wet for
several days at a time during the period of summer rains.

(6) Seeds of Fouquieria splendens and of Cereus giganteus fail to
show any special adaptation to germination in soils drier than those
needed by the seeds of such mesophytes as Triticum and Phaseolus.

76 RELATION OF DESERT PLANTS TO SOIL MOISTURE AND EVAPORATION.

(7) Immediately following germination, the seedlings of desert
plants exhibit a slow aerial growth, but an exceedingly rapid downward
elongation of the primary roots, so that these should soon attain to
depths where moisture is always present in adequate amount for growth.

(8) The high moisture-retaining power possessed by the soil of
Tumamoc Hill holds near the surface much of the water received
from single showers and offers excellent opportunity for the rapid
absorption of this by such shallow rooting forms as the cacti.

(9) The sap of Cereus, Echinocactus, and Opuntia exhibit osmotic
pressures no higher than those commonly found in plants of the humid
regions.

(10) The effect of air currents in increasing evaporation and trans-
piration rates is so great that measurements of natural transpiration
can not be made in closed chambers.

(11) By means of a new method involving a newly devised evapo-
rimeter, a physiological regulation of the rate of transpiration was
unquestionably shown to exist in the forms studied. The mechanism
of this regulation has not been studied.

(12) The regulation of transpiration seems to be controlled by air
temperature, the checking of water loss beginning to be effective
between 79° and 90° F., and the check being removed between 75° and
80° F.

(13) The ratio of transpiration rate per unit leaf surface to evap-
oration rate per unit water surface is termed relative transpiration.
Relative transpiration is reduced by the regulatory response from unity
in the high periods to from one-twelfth to one-sixth in the low periods.

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