346 



Introduction to Botany. 



can continent. They have a rainfall of less than 30 centi- 

 meters annually, and, for the most part, are habitable only 

 to xerophytes ; and even these plants, which are constructed 

 specially to defy drought, occur as more or less isolated 

 individuals. 



For most of this vast desert region, the sole cause of 



the scant vegeta- 

 tion is lack of 

 water, the soil be- 

 ing rich enough to 

 support a luxuriant 

 plant population. 

 Schimper says of 

 the Sahara desert 

 near Biskra: "The 

 firm and mostly 

 clayey soil between 

 the oases presented 

 the appearance of 

 a very scantily 

 planted and pecul- 

 iar garden where 

 the individual 

 plants were sepa- 

 rated by barren 

 spaces a meter or 

 more in breadth. Most of the plants were small, rounded, 

 dense shrubs which appeared at first sight so nearly alike 

 that it was a surprise to find on closer examination that 

 they possessed different kinds of leaves or flowers." (Fig- 

 ures 187, 1 88, and 189 represent plants of this character.) 

 In the same region occurred a plant of quite different char- 

 acter belonging to the gourd family, whose thick succulent 



FIG. 189. 



Sarcobatus Baileyi. From western North America 

 a desert plant. After COVILLE. 



