86 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
nection with the amount available in the soil. Thus it was found that a 
small plant of Opuntia versicolor gave off water at a rate of 129 milligrams 
per hour in April, 27.5 milligrams in June, and 26.1 milligrams in July, 
after the ground around the roots had been thoroughly wetted. A 
renewal of growth following the summer rains brought about an accelera- 
tion, due in part to the activity of the minute leaves, which are highly 
functional in transpiration. 
It is not to be taken for granted that the measurements given above 
represent absolute capacities, for not only is the rate of transpiration 
affected by a wide variety of conditions, but also a great individual diver- 
sity results from environmental differences. Prof. V. M. Spalding (Bio- 
logical Relations of Certain Desert Shrubs, Botanical Gazette, vol. 38, p. 122) 
found that a plant of Covillea grown with a plentiful supply gave off water 
3.7 times as rapidly as one reared under more arid conditions. 
The facts brought to light by Dr. W. A. Cannon (On the Water Con- 
ducting Systems of Some Desert Plants, Botanical Gazette, vol. 39, p. 
397, 1905) also show a lack of correlation between the amount of develop- 
ment of conducting tissue in stems in general, and the water-supply. It 
does seem probable, however, that this development is influenced largely 
by the water-supply available at the exact time when the conductive 
cells are in a formative condition. 
In all of these investigations it was found possible to make use of a 
method of measuring transpiration available in dry climates, in which the 
test-plant was inclosed in a bell-jar of known capacity and the amount 
of water thrown off estimated from the changes in relative humidity 
indicated by a hair hygrometer inclosed with the plant. In test-periods 
of a few minutes nothing superior to this method has been found. 
Further work upon the subject by Professor Lloyd has demonstrated 
that the rate of transpiration exhibits noclose relation withthe movements 
of stomata, and that these minute organs have but little of the adaptive 
capacity with which they have heretofore been accredited. In fact it is 
found that the variations in the daily rate of transpiration are capable 
of rational interpretation only by the strictest comparison with the condi- 
tions of relative humidity and when taken, as described by Dr. B. E. 
Livingston, as relative transpiration (Relation of Desert Plants to Soil 
Moisture, Carnegie Institution of Washington Publication No. 50, 1906), 
governed largely by purely physical factors, although subject to physio- 
logical regulation the mechanism of which is not understood. Thus it 
was found that this regulatory action was exerted to check water-loss 
at temperatures between 79° and go° F. in the plants examined, and that 
checking action disappeared at temperatures ranging from 80° down 
toore? dh 
With regard to any possible capacity of desert shrubs to absorb 
moisture from the air during periods of high relative humidity, the inves- 
