FUNCTION OF RESINOUS COVERINGS. 51 



mc.vicana, and Thamnosma montana nearly all the leaves drop away 

 in early summer, soon after the long drought has begun to cut oft' the 

 customary rain supply of the spring months. The most interesting 

 feature of this phase of adaptation is the fact that in none of these four 

 plants do the leaves possess the character of either a hairy, scaly, or 

 resinous epidermis, nontranspiratory modifications to be described 

 below. It seems that these leaves, pushing out from the bud with the 

 spring rains, carry on for a few weeks a rapid transpiration; but at the 

 advent of arid conditions they become powerless to continue such a 

 rate of transpiration, and not being adapted for the minimum rate de- 

 manded by their new surroundings they dry up and drop away. It is 

 probable that the few other smooth-leaved shrubs in the list have the 

 same summer history, but I had no opportunity of observing them. In 

 Tetradymia glabrata the axillary-fasciculate leaves, evidently early 

 deciduous, are glabrous, while the primary, persistent leaves are per- 

 manently covered with arachnoid hairs. In the case of species whose 

 leaves are deciduous, transpiration is carried on in summer through the 

 epidermis of the stem, and necessarily there is in the hitter organ an 

 abundant supply of chlorophyll to permit the assimilation of food. 

 Cassia armata, Salazaria mexicana, and Thamnosma montana are con- 

 spicuously thus provided with chlorophyll. 



In the majority of plants, however, the leaves remain on tire stems 

 during the greater part of the summer, carrying on their functions. 

 To confine transpiration to that minimum which alone it is possible 

 for a plant in such environment to support, the surfaces of the leaves 

 are protected either by a resinous exudate or by a close covering oi 

 dry hairs. 



In Larrea tridentata is found the apparently simplest form of resinous 

 coating. The leaves and small twigs are thinly spread with a covering 

 that closely resembles in appearance ordinary shellac. To the abundance 

 of this resinous matter the plant's popular name, creosote bush, is due, 

 for in burning the green wood and leaves of Larrea a pnngent odor is 

 detected and a dense smoke arises. That the function of the coating 

 is to minimize transpiration there can be no doubt, but the i>recise 

 method by whicli this is brought about has not been ascertained. Ii 

 it were simply by the complete mechanical varnishing of the leaf-sur- 

 face, all transpiration would cease. It should be pointed out here that 

 in winter, when we first became familiar with the creosote bush, its 

 leaves were thoroughly varnished; but in June, when the spring growth 

 had nearly ceased, the leaves appeared to have very little of the 

 coating. There is in this fact an evident correlation between rapid 

 transpiration and absence of resinous covering, and a similar cor- 

 relation between slow transpiration and the presence of such a cover- 

 ing. Herbarium specimens of this plant seldom show conspicuously 

 the resinous coating, since they are usually collected in the flowering 

 season, apparently before the resin has fully developed. 



