52 BOTANY OF THE DEATH VALLEY EXPEDITION. 



A resinous exudate of doubtful origin is sometimes seen on the 

 leaves of AmpMaehyris fremontii, but in most cases sucli a substance, 

 when it occurs in desert plants, is definitely associated with conspicu- 

 ous glands. Seven of the shrubs in our list are provided with glands 

 situated upon the surfaee of the epidermis or partly imbedded in the 

 tissues. These shrubs are Aplopappus interior, A. monactis, Bigelovia 

 teretifolia, DaJea fremontii,, Dalea polyadenia, Feueephyllum schottii, 

 and Thamnosma montana. In the species last named the glands are 

 confined almost entirely to the stem and branches, and are found only 

 very sparingly on the leaves. A similar condition of things exists in 

 Dalca polyadenia. In the two species of Aplopappus, the Bigelovia, 

 and Peucepliyllum the glands are confined to the leaves. These two 

 types of the distribution of glands are seen at once to be correlated 

 Avith the functions of stem and leaf. Plants which rely principally 

 upon their leaves for transpiration, have these organs more glandular 

 than their stems, while plants in which the leaves drop off early, and 

 which, therefore, are forced to transpire from their stems, have precisely 

 the opposite provision. In three of these seven glanduliferous species, 

 namely, in Thamnosma and the two species of Dalca, the contents of the 

 glands do not exude over the surface of the adjacent tissues, and there- 

 fore only a portion of the surfaee is protected by the exudation. This 

 fact suggests strongly the idea that in such cases some other function 

 than the mere mechanical sheltering of the trail spiratory surface must 

 be ascribed to these glands. The elucidation of the problem is likely 

 to be attained only by direct experiment. 



Fourteen of our forty-one shrubs have a conspicuously developed hairy 

 coating of the leaves or stems. They are Cassia armata, Dalea polya- 

 denia, the four species of Atriptex, Encclia farinosa, Eurotia lanata, 

 Franseria dumosa, Krameria parvifolia, K. canescens, Salazaria mexi- 

 cana, Tetradymia comosa, and T. spinosa. A minute examination of 

 the various types of hairs represented in these plants, while interesting 

 and instructive from a purely histological standpoint, is not desirable 

 here. The form of the individual hairs in most of the species is 

 described in the list above, but the general important fact is that 

 from almost any form of trichome there may be developed, under a 

 desert environment, a close hairy covering, so constructed as to 

 greatly reduce the amount of heat transmitted from the air to the 

 plant. In general, the individual hairs of such a covering have no 

 moist cell-contents, but are minute sacks or tubes of cellulose filled 

 with air and closely felted together. The air contained in the cavities 

 of the hairs and in the spaces between them constitutes an excellent 

 nonconductor of heat. It is undoubtedly true also that the circula- 

 tion of air through the interstices between the hairs is comparatively 

 sluggish, and the extremely dry atmosphere is therefore admitted very 

 slowly to the stomata and through them to the moist interior of the leaf. 

 The preponderance of the coating on either the leaves or the stem is 



