8 ROOT HABITS OF DESERT PLANTS. 



On this point Schimper (Plant-geography, Bnghsh edition, p. 612) says: 



This second category of desert plants exhibits its dependence on'subterranean water 

 nearly universally by the immense length of its root-system, which the depth of the 

 level of the subterranean water renders vitally necessary. A considerable length of root 

 is, to a more or less extent, common to all desert plants and has attracted the notice 

 of all travelers. 



This statement is substantiated by a quotation from Volkens, who says 

 (Die Flora der aegyptisch-arabischen Wiiste auf Grundlage anatomisch- 

 physiologischer Forschungen, p. 7) : 



Often as I have tried to dig up old bushes of perennial plants to the extremity of 

 their roots, I have never succeeded in doing so. The most that I could establish was, 

 that the root was thinner at the depth of one or two meters than at the surface of the 

 ground. One can safely assume that, in this case, the length of the subterranean part 

 was at least twenty times that of the epigeous part. 



Volkens then goes on to say that certain species of Acacia were said to 

 have been seen at the time of the digging of the Suez canal, whose roots 

 were found in its bed, although the trees to which they belonged were 

 growing on eminences on its banks. 



Isolated observations indicate that the roots of certain trees of the arid 

 regions of the southwestern part of the United States may, under favorable 

 conditions, form long or deeply penetrating roots. Thus Prof. R. H. 

 Forbes, director of the Arizona Experiment Station, informs me that he 

 has seen roots of the mesquite (Prosopis velutina), by an irrigating ditch, 

 extend very near the surface of the ground as far as 15 meters, and roots 

 of the same species, where exposed by the washing away of river banks, 

 which penetrated as deep as 8 meters. These figures may be taken as 

 probably representing the deepest root penetration in this vicinity, although 

 it is no unusual thing to see mesquite roots 5 meters in length. Dr. V. 

 Havard is quoted as writing that sometimes in the Southwest camps were 

 pitched on the plains where there was no fuel of any sort to be seen. It 

 is there that the frontiersman, armed with a spade, went digging for wood. 

 Speaking of the deep-penetrating roots of mesquite Havard says : 



Of the vertical roots, the tap root is the only long and conspicuous one. It plunges 

 down to a prodigious depth, varying with that at which moisture is obtainable. On 

 the side of gulches one can trace these roots down thirty or forty feet. (American 

 Naturalist, vol. 18, p. 451, 1884.) 



It will appear from facts given in this paper that the root-systems of 

 different perennials growing in the vicinity of the Desert Taboratorv are 

 extremely variable as regards depth of penetration, lateral extent, and 

 other characteristics, and that no one type of root can be said to be the 

 prevalent one. 



The opposite extreme in the position of the roots of desert perennials is 

 to be found in a highly specialized class, the succulents, in which the roots 

 are uniformly near the surface of the ground. Volkens, for example (Die 

 Flora der aegyptisch-arabischen Wiiste, p. 24), states that Enphorbia has 



