88 ROOT HABITS OP DESERT PLANTS. 



have been impossible for any other plant, perennial or annual, to gain a 

 foothold without encountering the roots of this species. Krameria can- 

 escens, the parasite, however, has a root-system that is scarcely branched 

 beyond the first order. Among the annuals it was observed that the root- 

 system of those appearing in summer was more richly branched than the 

 roots of the winter forms, and that the annuals both of summer and of 

 winter varied greatly, but consistently in the species, so that genera at least 

 could be distinguished merely by the branching habits of the roots. 



The more superficial laterals of the generalized root-systems of perennials 

 and a few roots of the cactus type of the specialized root-systems are pro- 

 vided with filamentous roots, borne in groups, of limited period of activity. 

 These temporary absorption roots, referred to in this paper as deciduous 

 roots, are formed always during the rainy season, either of summer or 

 winter, and persist until the soil about them becomes unbearably dry. 

 The length of this period is not known, but probably is not far from the 

 time which limits the growth of annuals, from three to six weeks, or even 

 more. Similar roots are to be seen in some of the annuals, particularly in 

 Amsinckia among those of winter, and Trianthema of those of summer, but 

 are regularly absent from certain other species. 



THE RELATION OF ROOTS TO SOIL TEMPERATURE. 



The relation of the roots of annuals and of perennials to the temperature 

 of the soil can be presented briefly. As appeared in an earlier section of 

 this paper, the grand course of heat movement finds its maximum just 

 prior to the summer rains, and its minimum in late winter or early spring. 

 With the coming of the rains in summer the soil temperature falls immedi- 

 ately and considerably, but the rains of winter do not appear to have so 

 marked effect on the temperatures of the soil. However, when more com- 

 plete temperature observations are at hand, the winter rains may be found 

 to have a marked and important effect, particularly in the way of altering 

 the temperature of the uppermost soil levels. 



From what is known regarding the temperature of the soil at a depth of 

 15 cm., it appears that the summer annuals are not subjected to very intense 

 heat, in fact not greatly exceeding 80° F., but those of winter may experi- 

 ence temperatures very close to the freezing point. At a depth of 30 cm. 

 the soil is not as cold as at less distance from the surface, but the minimum 

 is reached later in the season than at the higher level, so that the roots of the 

 winter annuals may be subjected to the following conditions : During the 

 warmer davs of winter and early spring the superficial soil may attain a 

 temperature relatively high, while the deeper soil is yet cold. The condi- 

 tions for most favorable water absorption are not present in winter, there- 

 fore, and the effect is a limitation of the development both of root and of 

 shoot. Reversed conditions are to be found in summer, when the lower 

 soil lavers at the time of the rains and the appearance of the annuals are 



