158 
south-facing than on north-facing slopes. Instru- 
mentation has been earried on at six stations, 
separated by vertical intervals of 1,000 feet. The 
data have furnished curves showing the altitudinal 
change of temperature, rainfall, soil moisture, 
humidity, evaporation and soil temperature. Men- 
suration has been carried on to determine the 
density and volume of the stands of characteristic 
trees at the different altitudes. Ratios of soil 
moisture to evaporation have been worked out 
from the instrumental data, showing the conditions 
which control the water intake and water loss of 
plants to be nine times as severe at 3,000 feet as 
at 8,000. The duration of the most critical season, 
as respects the water relation, is three times as 
long at 3,000 feet as at 8,000, making the desert 
conditions actually twenty-seven times as severe 
as those of the forested summits. The factors of 
soil moisture supply and atmospheric aridity, 
modified by slope-exposure, determine the lower 
limits of forest and the trees of the chaparral 
zone. The factors of winter cold, modified by 
topography through the operation of cold-air 
drainage, determine the upper limits of the char- 
acteristic desert species. 
A Possible Mutant of the Sugar Maple: A. F. 
BuAKESLEE, Connecticut Agricultural College. 
A single tree discovered in a roadside row of 
sugar maples is distinguished from the normal 
type by its peculiar habit of growth. The limbs 
are slender and branch profusely to form a close 
thicket of slender branchlets which end at a uni- 
form distance from the trunk and produce an 
appearance similar to that of a symmetrical arbor 
vite. 
A Balancing Method for Differentiating between 
Absorption and Transpiration: LyMan J. 
Brices and H. L. SHantz, U. S. Department of 
Agriculture. 
In this method the potted plant is suspended 
upon knife-edges in a horizontal position, so that 
the soil part of the system is on one side of the 
knife-edge and the plant on the other. The system 
is balanced by counter-weights, and a center of 
gravity so adjusted as to secure the required 
sensibility. As water is lost through transpira- 
tion, the soil end of the system becomes lighter 
and rises in consequence. If the moisture content 
of the plant has not changed during this process, 
the system will be restored to its zero position by 
the addition to the soil centroid of a weight equal 
to the total loss of water from the plant during 
the observation period. If the weight which must 
SCIENCE 
[N.S. Vou. XXXV. No. 891 
be added to the soil centroid to balance the sys- 
tem is less than the total loss of weight during the 
period, then the amount absorbed by the plant has 
been less than the amount transpired. If the 
weight required to restore the balance is greater 
than the loss in weight of the system, then the 
absorption during the period has exceeded the 
transpiration. 
If the distances of the soil and plant centroids 
from the axis are known, the difference between 
transpiration and absorption during any period 
can be quantitatively determined. The location of 
the soil centroid can be found by observing the 
loss of weight, after the plant has been kept for 
a time under uniform conditions, so that the 
transpiration is equal to the absorption for the 
observed period. The plant centroid is determined 
from measurements of the area and the distance 
of the leaves from the axis. 
This method is also adapted to the determina- 
tion of the wilting coefficient for plants, which, 
owing to structural peculiarities, do not wilt when 
the moisture content of the leaves is reduced. 
The Application of Wilting Coefficient Determina- 
tions to Field Work in Ecology and Phyto- 
geography: LiyMAN J. Brices and H. L. 
SHantz, U. S. Department of Agriculture. 
The wilting coefficient is defined as the moisture 
content of the soil (expressed as a percentage of 
the dry weight) at the time when the leaves of 
the plant growing in that soil first undergo a 
permanent reduction in the moisture content as 
the result of a deficiency in the soil moisture 
supply. 
. The results of 1,300 determinations have shown 
that species differ only slightly in their wilting 
coefficients when grown in the same soil. The type 
of soil has a great influence upon the wilting co- 
efficient, which ranges from less than 1 per cent. 
in sand to over 30 per cent. in the heaviest type 
of clay. It is therefore evident that soil moisture 
determinations for the purpose of establishing, 
available moisture in the soil are valueless unless 
the wilting coefficient of the same sample used in 
the moisture determination is also known. 
The desirability of a rapid determination of the 
wilting coefficient of samples taken in connection 
with field work led to an investigation to deter- 
mine whether it could be computed from physical 
measurements of the moisture retentivity of the 
soil. Several methods have been developed, the 
relationships established being expressed in the 
following equations: 
