42 BOTANY OF THE DEATH VALLEY EXPEDITION. 

 GENERAL ADAPTATIONS. 



Before examining the adaptive peculiarities of the desert flora, it is 

 necessary to expunge those species which are not subjected to true desert 

 conditions. Aquatic and paludose plants, by their situation in constant 

 proximity to water, have not been compelled to modify their structures 

 conspicuously along the same lines as the arid vegetation of the desert, 

 and do not represent its true flora. We shall therefore exclude all 

 such plants from consideration unless specific reference is made to 

 them, and confine the discussion to the dry-land, or true terrestrial, 

 vegetation. 



The absence of trees is the most conspicuous general feature of the 

 vegetation. On some of the higher desert ranges, as the Charleston, 

 Panamint, and Inyo, occur caps of coniferous timber, which have an 

 abrupt lower limit. When seen from a distance of many miles, the line 

 appears quite horizontal and clearly denned ; above it there are forests, 

 below it the naked mountain slopes. The winter storms afford a sug- 

 gestive fact in explanation. Three or four times in the winter of 1891, 

 when the air cleared after along rain and the clouds broke away about 

 the mountain tops, the new fall of snow could be seen lying along the 

 mouutain crest with a lower limit as level as if laid down along a 

 straight-edge, the line of snow corresponding with the lower limit of 

 trees. Below this altitude the precipitation was in the form of rain, 

 and could not therefore be stored for the future needs of forest vegeta- 

 tion. Under extraordinary conditions snow may fall at lower altitudes, 

 but it always melts away after a few hours' exposure to sunlight. The 

 general statement may be made that in the desert mountains timber 

 does not grow below the winter snow line, a condition clearly due to 

 lack of moisture. 



These conditions of treelessness and scant moisture exist throughout 

 the desert, the former resulting from the latter. It is probable that 

 the combined amount of foliage produced by all the shrubs on an acre 

 of desert surface is not greater than that afforded by a single tree of 

 the ordinary humid-region type. If the moisture absorbed by all these 

 plants could be concentrated for the use of one tree it could undoubt- 

 edly live. But the support of a root surface sufficiently great to absorb 

 all the moisture from such an area of ground would be an unbearable 

 tax upon the vital resources of any tree, and therefore none, of the 

 ordinary type, have succeeded in adapting themselves to this region. 



The tree yucca, Yucca arbor escens, is the nearest approach to a real 

 tree that the desert affords, often attaining a height of G to 7.5 meters, 

 with a trunk 0.5 meter in diameter. Even this arborescent plant and 

 its relative, Yucca macrocarpa, are, however, confined to the high 

 altitudes of the desert, just below the lower limit of coniferous trees. 



It is in the shrubby vegetation of the desert that the arid character 



