44 BOTANY OF THE DEATH VALLEY EXPEDITION. 



continued existence. An individual of Larrea ) however, never dies un- 

 less by some accident. As each one of the several stems grows older it 

 bends forward toward the ground and finally dies, being replaced by a 

 younger vigorous stem in the center of the group. The relation be- 

 tween these two types of shrub is precisely analogous to that which 

 exists between herbaceous annuals and perennials. 



Perennial herbs are fewer in number than either shrubs or annuals. 

 Of the thirty recorded in the list, only three, Cucurhita pahnata, C. 

 fwtidisv ima'.md Rumcx hymeno8epalu8,h&ve stems that die to the ground 

 each year; three are grasses that remain more or less green throughout 

 the winter ; and the remainder are suffrutescent at the base. This latter 

 character is one of the most important in the history of desert peren- 

 nials. In a humid temperate climate the typical perennial has a stem 

 and leaves of soft herbaceous texture, which, at the approach of 

 winter, die to the ground. The underground portions, protected from 

 cold and not subjected to drought, are well adapted to carry the life of 

 the plant through the winter. In regions of somewhat less aridity 

 than the desert, such as the plains of western Kansas, eastern Colorado, 

 and central California, the typical perennial is one with greatly en- 

 larged underground parts, capable of withstanding a long season of 

 drought. To this type belong the three plants that are mentioned 

 above, Cucurhita palmata, C. fmtidissima and liumcx hymenosepalus, and 

 many liliaceous plants, such as Chlorogalum, Brodima, and CalochortWy 

 common in central California. But even such a provision against the 

 effects of aridity appears to be insufficient for most plants in the Mo- 

 have Desert, and the adaptation generally successful is the maintenance, 

 throughout the dry season, of a slow rate of transpiration. This is 

 effected through the suffrutescent living lower portion of the stem. In 

 none of the perennials of this type listed above is there a conspicuous 

 enlargement of the roots for the storage of water. The adaptation for 

 a minimum but continued transpiration seems to be entirely adequate for 

 the needs of the case. It is to be remarked, therefore, that these suft'ru- 

 tescent perennials are functionally shrubs, as distinguished from true 

 perennials and from annuals. With the exception of the three peren- 

 nials already mentioned and the three grasses, whose status is some- 

 what uncertain, the arid vegetation of the Mohave Desert region may 

 be treated as made up of shrubs and annuals. 



The list of annuals is large, and in their types of for in they do not ap- 

 pear peculiar. A few are prostrate, as Dalca mollis and A cliyr onychia 

 cooperi; many diffusely branched, like Nemacladus ramosissimus and 

 Cleomclla obtusijolia ; and many others erect and little branched, for 

 example Encelia eriocephala and Oenothera hrevipes. An annual is well 

 adapted for life in this desert, because it confines its period of growth 

 to the rainy season, when the climate is not extraordinarily arid, and 

 at the approach of summer dies, only the ripened seeds living through 

 the summer and winter. 



