DESERT VEGEJ^ATION. $69 



water is absorbed and held by capillarity of the soil for a depth of 

 perhaps several feet. 



1057. The rainy-season flora thus appears in later winter 

 and early spring. The seeds germinate along about February. 

 When the rainy season closes the water held in capillarity by 

 the soil begins to evaporate under the influence of the dry air and 

 hot sun. The loss of this water is increased by transpiration of 

 most of the water absorbed by the vegetation which has sprung 

 up under the influence of the rains and warm spring. Sometimes 

 in April the capillary water in the soil has all been withdrawn. 

 The annual plants in the meantime, feeling the lessening amount 

 of moisture in the soil, mature and ripen their seed, and then 

 die, while the perennials prepare for the period of rest, by the loss 

 of their leaves or the death of their aerial parts. They are sus- 

 tained through the dry hot summer and autumn solely (Coville 

 suggests) by hygroscopic moisture, i.e., that which the now air- 

 dry soil absorbs during the night from the very small amount of 

 moisture in the air. There are occasional summer storms 

 (amount to about ^cm water), but usually so slight that the pre- 

 cipitation is immediately re-evaporated into the dry air. As the 

 rains begin again in December a few of the perennials bloom, but 

 the season is then too cold for most of the vegetation to flourish. 



The annuals are the best adapted of any plants for the life in 

 the desert, since the growth period is confined to the rainy season 

 and the seed carries the life over the dry period and the cold of 

 winter. The seed is practically the only adaptation for tiding 

 the plants over the severe dry period, since the leaves are usually 

 soft and thin and none are succulent. The adaptation is the same 

 as many annuals in the cool temperate regions have for passing 

 the winter. 



1058. Sand-dunes in the desert. In the Tularosa district of 

 the desert in New Mexico, these drifting sand-dunes occupy 

 an area of 10 by 40 square miles, and the sand here is composed 

 of gypsum. These dunes reach a maximum height of 60 feet. 

 The most characteristic plant of the dunes is the three-leaf sumac 

 (Rhus trilobata). The cover afforded by the mat of branches and 



