. 



54 BOTANY OF THE DEATH VALLEY EXPEDITION. 



is required to protect the plant from excessive transpiration, the same 

 structure is made to answer both purposes, and no differentiation of 

 bud-scales is required. 



A few genera of desert plants, Ephedra, Cercus, and Echinocactus, carry 

 on transpiration tli rough their steins only, and Are protected by neither 

 glands, resinous exudate, nor hairy covering. In Ephedra transpira- 

 tion is reduced undoubtedly by a thickened and extremely impervious 

 cuticle, aided by the mechanism of the stomata. In the other three 

 genera, all belonging to the order Cactaceae, there is likewise a marked 

 thickening of the cuticle, together with a special modification of the 

 interior tissues of the stem to retain water. If a leaf or stem of any 

 plant not containing these water reservoirs be split, the organ is speedily 

 dessicated, since the soft tissues exposed by the cutting are not adapted 

 to resist the drying effect of the air. But if an Echinoeactus stem be 

 cut open, the outer layers of cells on the raw surface become dry and 

 form an artificial coating. With this slight protection the interior 

 tissues are capable of retaining their moisture, even in the plant press, 

 for several weeks. 



A few other adaptive characteristics of desert shrubs may be men- 

 tioned before the subject is dismissed. The prevailing type of wood 

 is of a hard, close grained texture, such as that of Larrea or Sareo- 

 hatus. There appears to be a general tendency for shrubs of slow 

 growth to form harder, liner wood than those of rapid growth. In 

 Salazaria mexieana and Cassia, armata, however, we find as exceptions 

 to this rule a large pith surrounded by a thin, soft layer of wood. These 

 two plants and others like them I take to be species which, in a region 

 with humid climate and cold winter, would develop herbaceous stems. 

 In the arid climate of the desert they have followed the general ten- 

 dency of the vegetation to persist alive through the winter, but they 

 have still retained some of the structural characteristics of herbaceous 

 stems. 



The bark of those desert shrubs which transpire principally through 

 their leaves is thickened, dry, and spongy, while that of transpiratory 

 stems, such as Thamnosma Montana, is usually reduced to an epidermis 

 covering a green, chlorophyllose, assimilative tissue. These two types 

 of bark are well adapted tor their respective functions. 



One function of hairy coverings must be passed with the barest 

 mention, namely, prevention of extreme or rapid radiation. On account 

 of the clearness and dryness of the desert atmosphere, unusually great 

 radiation ensues after sunset, and the temperature of all objects on the 

 surface of the earth is rapidly reduced. Often on the desert in winter, 

 alter working during a sunny afternoon in a warm and comfortable tent) 

 we found ourselves within a few minutes after sunset chilled and 

 shivering. We observed a frequent daily fall of temperature from 70° 

 F. to a few degrees below the freezing point. In summer a similar daily 

 range occurs, but with higher extremes. Such abrupt changes in tern- 



