

TRANSPIRATION OF DESERT SHRUBS. 47 



surface of this plant, it may be seen, is remarkably large in comparison 

 with the height of the stem. It should be remembered, too, that the 

 steins and branches of this plant are cylindrical, about 2.5 cm. in diam- 

 eter, and, except for the minute leaves that are present for a few 

 weeks in spring, devoid of foliage. Furthermore, the surface of the 

 branches is conspicuously cuticularized. The transpiiatory system of 

 the plant is exceedingly restricted, compared with the absorptive 

 surface of the root. In this particular case the roots, from their hori- 

 zontal position near the surface of the ground, are evidently adapted 

 to take up surface moisture. Such a provision does not appear, how- 

 ever, to be general among the plants of the Mohave Desert, in which 

 dew rarely or never falls, although it is reported to be a common 

 adaptation in the deserts of Arabia, where dewy nights are frequent. 1 



The roots of Prosopis juliftora grow to enormous lengths. In the 

 dry bed of the Amargosa River, between Salt Wells and Saratoga 

 Springs, Death Valley, was seen a root of this tree, which had been 

 washed out by a torrent, 15.8 meters in length. Its diameter at the 

 smaller end was about 1.5 cm., at the larger about 3 cm., and it bore 

 no lateral branches. A cortex of unusual thickness is present in the 

 roots of many of the desert shrubs. 



During the season of drought the largest amount of moisture that the 

 roots can by any possibility absorb is comparatively small, and the 

 greatest burden of modification must fall on the transpiratory system. 



Before presenting a general consideration of the methods of reducing 

 transpiration we shall examine several shrubs, to find what particular 

 modifications each has undergone to this end. 



Aeamptopappua sphwrocephahts.— -The stem is of slow growth and the bark white, 

 thin, and spongy. The leaves are oblaneeolate-linear, reaching 1.5 cm. in length, 

 thick, green, with glabrous or sometimes scabrous surfaces. The short, stiff hairs 

 that are sometimes present appear in their young state to he slightly resiuiferous. 



Amphiachyris J'remontii. — A stem 2.5 mm. in diameter, in addition to the hark, 

 shows six annual rings. The wood is hard, the hark about 1 mm. thick and composed 

 of soft, dirty-white, pith-like tissue. The leaves, which are deciduous, are from 5 to 

 10 mm. long, green ki color, and scabrous with short hairs, some pyramidal-acumi- 

 nate and pointed, others gland-tipped. 



Aplopappns interior.— One stem 6 mm. thick is 5 years old. The bark when old 

 becomes dark gray, and has a spongy, slightly fibrous structure. The linear-ob- 

 lanceolate leaves are thick, glabrous, and impressed-punetate with numerous resin- 

 glands. Three resin-passages run lengthwise through the loaf, as in Bigelovia tereti- 

 folia. 



Aplopappns monaetis. — The bark is pale brown and more fibrous than in A. interior, 

 while the wood lias about the same texture and rate of growth as in that plant. 

 Each year's growth has a length of from 6 to 10 cm. The leaves are similar in form 

 and structure to those of A. interior, but they bear in addition scattered, short but 

 flexuous hairs, each consisting of a single filiform cell attached to the epidermis 

 through the medium of another short, constricted cell. 



Aster mohareush. — The bark is similar in color, texture, and structure to that of 

 Amphiachyris, but often 1 mm. thick. The annual rings of wood are difficult to count. 



l Yolkcns: Die Flora der ^gyptisck-Arabischen Wiiste, p. 24 (1887). 



