46 BOTANY OF THE DEATH VALLEY EXPEDITION. 



climate. Modifications similar to those enumerated are found also 

 among the annuals. 



Hibernation is, in many types of vegetation and under many condi- 

 tions of life, a period that demands the most perfect preparation. The 

 death of the stem and the storage of food-material in underground 

 structures in a typical perennial, and the resting of seeds before ger- 

 mination in annuals, are the two ordinary methods of herbaceous hiber- 

 nation in humid temperate climates. The first provision is nearly un- 

 known in the desert, because of the almost entire want of true peren- 

 nials, while the second is, of course, the common and only method for 

 the wintering of annuals. The hibernation of desert shrubs presents 

 many remarkable features, but as it is intimately connected with the 

 phenomena of growth it will be discussed under that head. 



The germination of seeds in a desert is a subject that presumably 

 presents some unusual features, yet the probability that germination 

 takes place almost exclusively at rainy periods simplifies the matter 

 very materially. It was, however, impossible to make observations in 

 this line. 



The problem of ordinary vegetative growth of desert shrubs must, 

 from the nature of things, be more serious than any other in their life- 

 history. Light and food are abundant, but moisture, both of soil and 

 air, is far too little for the usual needs of vegetation. 



A plant absorbs moisture from the soil through its roots, carries it 

 along its stem, and transpires it by evaporation from the stomata of 

 the leaves. Transpiration is an absolute necessity in the growth of a 

 plant, for upon it depends directly the performance of several of the 

 vital functions. If a plant of ordinary structure, such for example as 

 rod clover, were exposed to the climatic conditions of the desert it 

 would wilt, dry up, and die. To speak in physiological terms, the hot, 

 dry air has caused more water to be transpired from the leaves than 

 the roots can supply, the soft tissues have lost their turgescence, and 

 the dependent vital functions have ceased. The first theoretical neces- 

 sity of the plant is that the water it absorbs from the ground shall 

 practically equal in amount that which it transpires. Desert shrubs 

 accomplish this by the reduction of transpiration and by the increase 

 of means for absorption. 



A specimen of Opuntia cchinocarpa, growing on the open mesa in the 

 vicinity of Daggett, was examined to ascertain to what extent its root 

 system had developed. The main root divided itself, close to the base 

 of the stem, into horizontal branches. One of these, 8 mm. in diam- 

 eter at the base, was traced to the distance of 2.64 meters when it 

 broke at a diameter of 1.5 mm. Its distance beneath the surface varied 

 from 5 to 10 cm., and many smaller roots branched off from it. The 

 plant had eight roots as large or larger than this one, two of them 1.6 cm. 

 in diameter at the base, and several smaller ones. The aerial part of the 

 plant consisted of two branching stems, each 48 cm. high. The root 



