48 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



of drought. It can be pointed out here that plants having hypogeous water- 

 storage organs have a very different relation to the climate of the desert 

 than such as have such organs above ground. The inclosing soil is a pro- 

 tection, nearly perfect, against drying. Where the soil is removed the 

 protected parts quickly become dry.* Given two species, both having 

 water-balance, but one with the storage epigeous and the other hypogeous, 

 other conditions being equal, the latter should survive under more arid 

 conditions than the former. It is rather surprising, therefore, that there 

 are not more of the latter type in the Sahara. 



A few measurements of the root of a typical Citrullus from the M'Zab 

 Valley will give a concrete idea of the water-storage capacity of the species. 

 A specimen growing in the oued near Ben Isguen was selected for examina- 

 tion (see fig. 37); it had a luxurious shoot and several fruits. The shoot 

 was found to arise from a large and aged root 26 cm. in diameter at the 

 crown, but tapering rapidly, it was only 12 cm. in circumference at a point 

 14 cm. beneath the surface, where it forked, giving off one branch which 

 was 1.7 cm. and another 1.8 cm. in diameter; several smaller ones took 

 their origin close to the surface of the ground. In its general appearance 

 the main root of Citrullus is very like that of Cucurbita digitata, which 

 grows on the domain of the Desert Laboratory and in a similar habitat. 

 ' Up to this point the plants whose roots have been described were found 

 growing where the soil was favorable to a fairly normal development, but the 

 soil of the valley is not all of this character, and where hardpan is present 

 the substratum is often extremely hard and, one would think from inspection, 

 impermeable to water as well as impenetrable to roots. In such habitats 

 the number of individuals and also of species is very naturally limited. 



About 2 kilometers west of Ghardaia is an area where the hardpan reaches 

 nearly or quite to the surface. Here Peganum harmala is to be found, since 

 its generalized type of root-system is capable of not a little variation, 

 adjusting the species to a variety of soil conditions not otherwise possible. 

 Associated with Peganum was a single specimen, of dwarfed and badly 

 eaten form, growing out of the hardpan itself. The position of the plant 

 was so unusual that its root-system was in part excavated. The form, an 

 undetermined chenepod, was found to have the exceptional form of root, 

 thus proving the rule just suggested, for it had a well-marked tap-root. A 

 gnarled main root was found to take a zigzag course through 27 cm. of 



* In the vicinity of Tucson (see The Root Habits of Desert Plants) is to be found a 

 slender-stemmed Opuntia whose roots are fleshy and are placed within 2 to 4 cm. of 

 the surface of the ground. It has been observed that if the roots are examined in the 

 midst of a dry season, as in June, they are gorged with water, but if the soil is removed 

 for a few hours they become shriveled. A similar habit was seen in another species of 

 the same genus. Two other genera of the cacti from the Tucson region have the water- 

 storage organs wholly or partly protected by the soil. In Cereus greggii the subterra- 

 nean portion forms an organ 15 to 30 cm. in diameter, and in the other form the fleshy 

 subaerial stem is partly drawn under the surface of the soil, so that only the flat upper 

 surface is visible. 



