THE DAILY MARCH OF TRANSPIRATION IN A DESERT 
PERENNIAL. 
INTRODUCTION. 
A consideration of the various types of perennial plants indigenous to the 
vicinity of the Desert Laboratory at Tucson, Arizona, brings to view some 
striking differences between the species concerned, in respect to their total 
annual water-losses. The perennials fall into the general types, succulent 
and non-succulent, the latter being easily classified further into three physio- 
logical groups, namely: (1) small plants whose perennial parts are confined 
to roots or underground stems; (2) plants continuously in leaf; (8) tropo- 
phytic plants. Examples of the second class are Covillea tridentata, Lycium 
berlandieri, Celtis crassifolia, Encelia farinosa, Hyptis emoryi; examples of 
the third class are Prosopis velutina, Acacia greggii, Fouquieria splendens, 
Jatropha cardiophylla, and Parkinsonia microphylla. The tropophytic 
plants are alike in that they all drop their leaves more or less quickly in 
times of drought, but Parkinsonia microphylla differs fromthe others in hav- 
ing a continuous covering of epidermis over all of its parts, even the limbs 
over 100 years old having an active chlorphyll layer covered with an 
unbroken living epidermis. Thus, although the trees are without leaves 
during times of drought, they must still lose a large quantity of water. 
Some of the evergreen plants have a continuous epidermis, but these are all 
_much smaller than Parkinsonia and hence expose a smaller evaporating 
surface. In spite of this necessity for a large loss of water, Spalding* 
calls Parkinsonia a “highly successful desert species, growing in abundance 
and equally well on slopes of all exposures.”” Shrevef found a high death- 
rate in seedlings of Parkinsonia, but reached the conclusion that the critical 
period in the life of the plant is over after the first two years. 
The abundance of Parkinsonia on hillslopesinan arid region is an index of 
its apparent success in spite of the exposure of a large evaporating surface 
during the entire year and the difficulty its seedlings have during the first 
two years. In consequence of the ability of Parkinsonia to overcome 
these adverse conditions, it was selected as the subject of transpiration- 
behavior studies. Ageneral study of transpiration behavior ofseedlingsand 
adult plants during different seasons of the year is in progress, but certain 
fluctuations in the daily transpiration rate make a separate publication on 
this phase of the work advisable. Only such description of the plant, its 
habitat, and its anatomical features will be given as seems to apply either 
directly or indirectly to the phase of transpiration mentioned. 
*Spalding, V. M., Distribution and movements of desert plants, Carn. Inst. Wash. 
Pub. 113, 1909. 
{Shreve, F., Establishment behavior of palo verde, Plant World, vol. xrv, p. 294, 1911. 
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