38 BOTANY OF THE DEATH VALLEY EXPEDITION. 



day at Bennett Wells was from 73° to 31°, a difference of 42°. Such 1 

 a range is undoubtedly not infrequent in Death Valley. That plants 

 must be especially adapted to endure such wide variations in tempera- 

 ture, has been pointed out by Dr. J. T. Bothrock; 1 but these modifica- 

 tions are so associated with adaptations to lessen transpiration that 

 the two cannot, with our present knowledge, be clearly distinguished. 



SEASONS. 



In the temperate zope, as for example in the latitude of New York, 

 the year is divided into two well-defined seasons, summer and winter, 

 characterized respectively by a high and a low degree of temperature. 

 The behavior of the accompanying flora is directly related to these 

 seasonal changes. In summer the vegetation is luxuriant, in winter 

 dormant. In many tropical regions the seasons are characterized by 

 only comparatively slight differences in temperature, the cold being at 

 no season so great as to stop the growth of vegetation. The period of 

 growth in such a region usually follows the season of rainfall, as dis- 

 tinguished from the season of drought. 



In Death Valley and the adjacent deserts occurs a combination of 

 temperate and tropical seasons which, although not so unvarying as the 

 winter and summer of a northern latitude, have sufficient individuality 

 to admit of fair characterization. They are (1) a period of combined 

 moisture and heat from February to May; (2) a period of drought from 

 June to November; (3) a period of cold during December and January. 

 During the first season, which we may call spring, the principal growth 

 of vegetation takes place; during the second season, or summer, the 

 vegetation assumes a state of rest necessitated by the extreme dryness 

 of the atmosphere and soil; and during the third or winter season, al- 

 though sufficient moisture is often present, growth is minimized on ac- 

 count of the severity of the cold. 



CLASSIFIED LIST OF DESERT PLANTS. 



A reference list has been prepared which contains only the plants found 

 actually in the Lower Sonoran zone of the desert. Weeds are omitted. 



The whole number of species has been divided into two classes: first, 

 those of humid habitat, which are subdivided into (1) aquatic, growing 

 in water, and (2) palurfoae, growing in wet soil; and secondly, those of 

 ARID habitat, which also are subdivided into (1) rupestrine, growing in 

 canons and on rock- sheltered slopes, and (2) campestrine, growing on 

 mesas, washes, sand-wastes, and clayey valley -bottoms. In each of the 

 four groups thus constituted the species are set down in four columns 

 containing respectively trees, shrubs, perennials, and annuals. All 

 herbaceous plants which live through more than one summer are here 

 included among the perennials. 



1 Report upon United States Geographical Surveys West of the One Hundredth 

 Meridian, in Charge of First Lieut. Geo. M. Wheeler. Vol. vi., Botany, 1878, p. 4. 



