PHYSIOLOGICAL KKATl'RES OF ROOT-SYSTEMS. 89 



warmer than the upper soil layers. The growth conditions both of shoot 

 and of root in summer, therefore, are most favorable, with the result that 

 the summer annuals are very luxuriant, the shoots carrying a large leaf 

 surface and the roots being especially well developed. 



The relations of the roots of perennials to soil temperatures are at once 

 similar and unlike the relations of the annuals. They are similar in that 

 the deciduous roots, which are developed on the more superficial laterals, 

 occupy the same soil horizon as the roots of the annuals, are developed at 

 a time when the annuals appear, and die with their passing. The persistent 

 portions of the roots of the perennials are subject to the greatest tempera- 

 ture extremes of the hojizon they occupy, which in the most superficial 

 roots, like certain of the cacti, must be very great, although the exact 

 measure is not known. The peripheral absorption roots of perennials, 

 both of the specialized and the generalized types of root-systems, have 

 still other temperature relations. It is probable that these roots in the 

 generalized root-systems, and in the Ephedra type of the specialized roots, 

 remain active throughout the year, but that in the cactus tvpe of the 

 specialized roots, especially in such forms as Opuntia arbusciila and Echino- 

 cacius wislizcni, they are formed at the time that the deciduous roots appear 

 and die when the latter kind of roots die. The temperature relations of 

 these roots are thus seen to be extremely complicated. 



Most, perhaps all, of the perennials are active during the summer rainy 

 season, but many do not renew growth or come into flower in winter. This 

 is clearly a temperature relation and finds its reaction in the formation 

 of deciduous roots, and probably in other ways, in the root-systems. It 

 has been found impossible, for example, to induce the formation of decidu- 

 ous roots in Opuntia discata in winter, and deciduous roots have not been 

 seen in Fouquiei ia ipUndcns at this season, though when the examination 

 of the roots of Fouqmcria was made, temporary roots were seen in Encclia 

 jarinosa, Lvciuiii andeisonii, and other plants. 



THE RELATION OF ROOTS TO WATER. 



The water relations of the plants in the vicinity of the Desert Labora- 

 tory are verj' complex, owing partly to diversity in the plants themselves 

 and partly to essential differences as regards soil water in their habitats. 

 These differences lie chiefly in the origin of the water supply as well as in 

 the length of time which the water of the soil is available for use. On 

 Tumamoc Hill the entire supply is derived directly from the rains ; on the 

 bajada the water comes in the main directly from the rains, but some is 

 also received as superficial run-off or by seepage from Tumamoc Hill or 

 other higher ground ; on the flood plains, in addition to the rains and the 

 water which comes from Tumamoc Hill and other higher ground, there is a 

 water-table of unknown depth by West Wash, varying from 5 to i 2 meters 

 beneath the surface on the plain by the Santa Cruz. The water-table by the 



