250 THE PLANT WORLD 



plants that will materially modify some of the most important general- 

 izations current in botany at the present time. 



A brief sketch of investigations dealing with desert vegetation has 

 recently been published in the report of the Advisory Board of the 

 Desert Laboratory, together with a bibliographical appendix which in- 

 cludes the principal papers bearing upon the subject.* 



It may be profitable to point out some of the striking features of the 

 more prominent types of plants indigenous to arid regions, and to call 

 attention to phases of their life-history needing investigation, at the 

 outset of the activity of the Laboratory. 



A general inspection of the southwestern deserts during the active 

 vegetative season and during the resting period shows that the following 

 groups of plants may be distinguished upon the basis of seasonal habit 

 and general composition of the shoot and root. In citing these groups 

 it is by no means intended that the divisions in question are to be taken 

 to include the entire flora. The ideas set forth in this crude classification 

 however may serve as a point of departure for an exhaustive analysis of 

 the flora upon a more exact basis of habit and habitat : 



I. Herbaceous annuals which start into activity from seeds soon after 

 the beginning of the rainy or favorable season, and quickly develop a 

 complete flowering shoot. — The formation of flowers and the maturation 

 of seeds ensues under conditions not widely different from those en- 

 countered by species living in the same latitude in moister regions. The 

 roots of such plants are not distinguishable by any special characteristics, 

 and do not exhibit any unusual capacity for the penetration of the soil, 

 horizontally or vertically. The greater number of forms of this group 

 however show a comparatively greater development of the lateral roots 

 through the upper layers of the soil in a manner that places these organs 

 in a position to make use of the scant rainfall before it sinks deeply into 

 the soil. The shoots do not present marked xerophytic adaptations 

 beyond a heavy cuticle, and the usual regulatory devices for controlling 

 the action of stomata. The total leaf surface of the shoot may be very 

 great, and the whole plant so delicate that it quickly wilts when uprooted. 

 The more marked structural features are to be found in the seeds, which 

 are provided with extremely resistant coats ; for it is the seeds of such 

 species that must endure the rigors of drought and extremes of temper- 

 ature of the desert. The seeds of many forms are so well protected by 

 impervious coatings that they may be soaked in strong acids and other 

 corrosive solutions for some time without injury to the embryo or the 

 storage material within. 



II. Perennials with woody, bulbous or tuberous stems, which are 

 chiefly subterranean, or which lie closely on the surface of the soil. — 



* Desert Botanical I,aboratory of the Carnegie Institution. Origin and Site. F. V. Coville and 

 D. T. MacDougal, Publication 6, Carnegie Institution. Washington, 1903. 



