The Daily March of Transpiration in a Desert Perennial (Classic Reprint)

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Excerpt from The Daily March of Transpiration in a Desert Perennial
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 physiological groups, namely: (1) small plants whose perennial parts are confined to roots or underground stems; (2) plants continuously in leaf; (3) tropophytic 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 from the others in having 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." Shreve found a high deathrate 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 hill slopes in an 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. A general study of transpiration behavior of seedlings and 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.
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