

36 BOTANY OF THE DEATH VALLEY EXPEDITION. 



moisture. In a long- continued storm or a series of storms, the procesl 

 may go on almost indefinitely, so that the effect perhaps reaches to 1 

 the depth of several feet. But as soon as the period of rain ceases, the ; 

 hot desert air dries the surface of the ground, and the capillary currents 

 are reversed. At the same time the seeds of annual plants germinate, 

 and absorption begins in the roots of all the vegetation. Nearly all 

 the moisture thus absorbed is evaporated from the surface of the leaves 

 and stems, a small proportion only, together with the nutritive mate- 

 rial in solution, being retained for use in the construction of the new 

 tissues of the plant. This double process of reevaporation goes on for 

 days or even weeks, until the capillary moisture of the soil is exhausted, 

 or in other words until the soil has become air-dried. This period 

 coincides in time with the active annual growth in the vegetation, and 

 is the direct cause of it; and on the other hand the cessation of growth 

 is an accurate index of the exhaustion of capillary moisture. 



Why the spring or the summer, or whatever seasonal name is applied 

 to the period of growth, is so variable both in the time of its beginning 

 and in its duration, may now be easily understood. The fall rains ex- 

 plain the frequent phenomenon of a well-defined autumn blooming of 

 desert plants, and the varying amount of the early rains decides the 

 varying luxuriance of the spring growth in different years. A few 

 weeks after the great February storm of 1891, Mr. C. R. Orcutt found 

 in the Colorado Desert specimens of an Amarantm '10 feet' high. 

 A year later, when the rains were very scant, he collected mature 

 specimens of this plant at the same place only 10 cm. (about 4 inches)., 

 high. This is the common experience of botanists in the desert, and to 

 this fact, more than to the actual failure of certain species to germi- 

 nate, is due the apparent scarcity of annual vegetation in a dry year. 



When an annual plant is unable to obtain sufficient moisture for 

 further vigorous growth, even if it has attained but a small proportion 

 of its normal size, it flowers and begins to mature its fruit. If the 

 same conditions continue, it accomplishes the final act of life by trans- 

 ferring its food supply to its seeds, and then perishes. 



In the upper part of Surprise Canon, Panamint Mountains, were 

 found mature specimens of LepiMum lasiocarpum 11 mm. high, Mvnulus 

 rubellus 17 mm., Braba caroUniana mierantha 10 mm., Piptocalyx eir- 

 cumcis.sus 7 mm., and Stylocline micropoides 9 mm. No special effort was 

 made to collect the smallest. 



In a humid climate an important form of conserved moisture is 

 dew, which is deposited upon the surface of the ground and rapidly 

 absorbed into it. When we crossed the Mohave River, at Daggett, on 

 January 0, 1891, early in the morning, frost was seen over a strip of 

 ground a few rods wide along the stream. During the remainder of 

 the expedition, neither dew nor frost was seen in the desert, although 

 in the next two mouths perhaps one-half the nights were cold enough 

 to freeze any dew that might have fallen. The weather observer in 

 Death Valley, too, recorded no dew. 



