94 ROOT HABITS OF DESERT PLANTS. 



ROOT HABITS AND PLANT DISTRIBUTION. 



The character of the root-system of desert plants as an ecological factor 

 is often of great importance. The direction taken by the roots as a factor 

 in the causes underlying the distribution of plants may be illustrated in a 

 simple manner, thus : Let a, b, and c represent the three leading root types, 

 namely, those having both laterals and tap root well developed, generalized 

 root-system (a), those with the tap root as the most prominent feature (b), 

 and those with the laterals of special prominence (c). Of plants having a, 

 b, or c forms of roots, other conditions including temperature relations 

 being equal, a will have the most general distribution, b will have the most 

 limited distribution, while plants with c roots are in a measure intermediate 

 in this regard. In plants with the last type of roots, however, it is probable 

 that factors other than the root character exercise the controlling influence. 



Of plants with generalized root-systems Acacia, Celtis, Encelia, Lycium, 

 Parkinsonian and Prosopis may be found to some extent in each habitat, 

 both where the soil is deep and where it is shallow. With these forms 

 should also be included the annuals except those with bulbous roots. 

 Plants with this root type are either evergreen or deciduous, but all have a 

 fairly large transpiring surface, and in certain of them, especially Encelia, 

 the leaves are of good size. 



Plants having a prominent tap root, the b type, include Cotidalia, Penio- 

 cereus greggii. Ephedra, Kcrrberlinia, and Zizyphus. These forms are re- 

 stricted to localities, preferably the flood-plain, where the soil has sufficient 

 depth for the development of the main root. 



The cacti are nearly all provided with the c form of roots, but to them 

 should be added Jatropha and Kranieria, the latter a parasite. The cacti 

 occur both on the bajada and on Tumamoc Hill, and not on the flood-plain 

 of the Santa Cruz or by West Wash to any extent. They are the plants 

 par excellence of the bajada. 



Summing up the relation between the type of root-system and the dis- 

 tribution of plants, according to the notation given above, we have the 

 following : Plants with a type of roots may be found on the bajada, Tuma- 

 moc Hill, and on the flood-plains of West Wash and of the Santa Cruz. 

 Plants having the b type of roots occur only on the flood-plains of the wash 

 and the river. Plants with the c type are chiefly on the bajada, although 

 they are common also on Tumamoc Hill, to which certain of them, notably 

 Carnegiea, are, for the most part, confined. 



The different root types are differently related to the distribution of the 

 plants. The generalized roots (a type) facilitate distribution because of 

 their plasticity, while the specialized roots of the b type restrict the distri- 

 bution because of their rigiditv and the need of considerable depth of soil. 



