WATER RELATIONS OF DESERT PLANTS. 83 
THE WATER RELATIONS OF DESERT PLANTS. 
During recent years it has become increasingly apparent that data 
obtained by the measurement of the activities of plants under laboratory 
conditions are oftentimes misleading, especially when dealing with forms 
fitted for special conditions, as illustrated by the water relations of desert 
plants. One of the earliest series of experiments in the field to remedy 
this defect was carried on at Turkey Tanks, on the western edge of the 
Malpais Desert, near the Little Colorado River, east of Flagstaff, Arizona, 
at an elevation of 6,500 feet, in July, 1898. Attention was given to 
transpiration and temperatures and the results are given below. 
Measurements of transpiration were made by means of a potometer of 
the form described in the Botanical Gazette (vol. 24,110,1897). Thisappara- 
tus consists of a long calibrated tube of small internal diameter supported 
in a horizontal position and fitted with a Y extension at one end. The 
tube is filled with water and the excised shoot of a plant fitted to one end 
of the Y by means of a tightly wired section of rubber tubing. The other 
end of the Y is closed by a stopcock, which may be opened to admit water 
when necessary. The rate at which water is taken into the shoot is noted 
by the progress of an air bubble in the horizontal portion of the tube. It 
is to be borne in mind that the rate at which water may be absorbed by 
the basal portion of an excised shoot in contact with water may not, and 
probably does not, represent the exact rateat which transpiration actually 
takes place, but it offers a very valuable method of comparison of the 
capacities of shoots of various types to take up and throw off water under 
similar conditions. 
Experiment 1.—Mentzelia pumila is a representative of a class of 
planis which, annually growing from seeds, produce flowers and seed 
during the season of greatest humidity, and then die, the species surviv- 
ing through the resting season in the form of seeds. It isa marked exam- 
ple of the xerophytic species which have a weakly developed root-system 
consisting of a number of thin branching fibrous roots which extend 
chiefly laterally through the upper layers of soil and do not penetrate 
beyond a depth of a few inches. The aerial shoot has a roughened cylin- 
drical stem about 16 inches long and a number of lateral branches of equal 
length, giving the entire leafy shoot a globoid form, a form characteris- 
tic of many desert plants. The specimen used was furnished with 18 
branches and bore about 900 irregular narrow roughened leaves and 200 
yellow flowers. The entire surface of the portion of the plant exposed to 
the air might be estimated at about 800 square inches. The plant was 
taken from the soil after the above facts had been ascertained, and the 
root-system was cut away from the base of the stem before attachment 
tothe potometer asabove. Several minutes were allowed to elapse before 
observations were taken to allow the plant to recover from the shock of 
handling, to which it had been subjected. 
