68 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



are confined to areas where the soil is relatively or actually deep, while 

 species having generalized roots have a local distribution which may be 

 considered the maximum. 



The relation of the superficial type of root-system to the distribution of 

 the species is not so apparent in this relation in plants as with the other 

 types of roots, but is undoubtedly close and possibly definitive. The ab- 

 sorption roots of plants with water-balance mostly lie less than lo cm. 

 beneath the surface and are thus subject to extreme desiccation for the 

 maximum time, or in other words they are exposed to favorable moisture 

 conditions the minimum time. How long the optimum water-absorption 

 time may be for such species is not known, but that it is longer than might 

 be supposed (from the period certain species can survive without water) is 

 highly probable from the facts concerning their distribution. In brief, the 

 best development of the fleshy species in question occurs where the rain- 

 fall is periodic, occurring twice each year, and they are wanting or sparse 

 where the rainfall is uncertain or occurs but once annually. Had these 

 plants a deeply penetrating type of root-system the local as well as the 

 general distribution would be very different from what it now is. Owing 

 to the unfavorable character of the rainfall in southern Algeria, plants 

 with a water-balance are wanting there, just as they are wanting in por- 

 tions of southwestern United States where the amount or the character 

 of the precipitation is likewise imfavorable. 



Besides the characteristics of the root-systems of the desert perennials 

 whose significance has been sketched, there are other features of importance. 

 For example, the roots lying near the surface bear tufts of delicate roots, 

 which are formed during periods of active growth and perish when such 

 seasons cease. By the organization of deciduous rootlets the absorption 

 area of the species is enormously increased, and quickly, without at the 

 same time increasing the distance of water transport. 



There need be mentioned here only one additional feature of the desert 

 plants. It is now known, in brief, that the non-fieshy perennials of the 

 desert, not halophytes, may possess a very dense cell-sap. This fact has 

 been demonstrated in the subaerial parts of several species, and is assumed 

 to hold for the roots also of the same species. As suggested, not all of the 

 desert plants, however, are capable of developing dense juices. Thus, cer- 

 tain fleshy species, and such mesophytes of the desert as have been studied 

 in this connection, do not have more highly concentrated cell-sap than the 

 ordinary plants of the humid regions. Further, it appears that desert 

 species which, under natural conditions and during the dry seasons, form 

 extremely dense juices, lose this capacity when grown under humid condi- 

 tions. So far as is now known, species capable of developing a cell-sap 

 with high osmotic power have generalized roots, although this may be of 

 no especial significance; but the relation of this capacity to survival in an 

 arid substratum is apparent and vital. 



