224 PLANT RELATIONS. 



of water, and not to lack of proper materials in the soil, is 

 shown by the fact that where water does occur oases are 

 developed, in which luxuriant vegetation is found. 



The desert which extends from Egypt across Arabia may 

 be regarded as a typical one. It is to be noted that the 

 vegetation is so scanty that the soil is the conspicuous 

 feature, and really gives the characteristic physiognomy 

 (see Fig. 191). Accordingly the appearance of the deserts 

 will depend upon whether the desert soil is rocky, or of 

 small stones, or gravel (as in the Desert of Sahara), or of 

 red clay, or of the dune type. As is to be expected, such 

 vegetation as does occur is of the tuft and bunch type, as' 

 developed by certain grasses, or of the low irregular bush 

 type (see Fig. 190). 



In the South African deserts certain remarkable plants 

 have been noted which have attained a certain amount of 

 protectioji through mimicry, rather than by means of armor, 

 as in the case of the cactus forms. Some of these plants 

 resemble the ordinary stones lying about upon the desert. 

 With the tropical deserts should not be confused such 

 areas as those about the Dead Sea, or in the Death's A'alley 

 in Southern California, as the barrenness of these areas is 

 due to the strongly alkaline soils, and therefore they belong 

 to the halophyte areas. 



lo9. Thickets. — The xerophyte thicket is the most 

 strongly developed of all thicket growths, ilention has 

 been made of willow and alder thickets in hydrophyte con- 

 ditions, but these are not to be compared in real thicket 

 characters with the xerophyte thickets. These thickets 

 ai-e especially developed in the tropics and subtropics, and 

 may be described as growths which are scraggy, thorny, 

 and impenetrable. Warming speaks of those thickets as 

 "the unsuccessful attempt of Kature to form a forest." 

 Evidently the conditions are not quite favorable for for- 

 est development, and an extensive thicket is the result. 

 Such thickets are well developed in Texas, where tlu-y are 



