THE AMERICAN NATURALIST 



[Vol. L 



esting possibilities, was observed at the Coastal Laboratory, at 

 Carmel, California, and may be briefly referred to in this place. 

 Among the species growing in the experimental plots at the labo- 

 ratory are Opuntia versicolor and Fouquieria splendens from the 

 vicinity of the Desert Laboratory, Tucson, Arizona. Owing to 

 the usual low temperature of the air, and soil, these species gen- 

 erally make little or no shoot growth at Carmel. When, however, 

 the roots of the plants are kept in soil whose temperature is 

 25-30° C, the shoots remaining in the cool air, not only do the 

 roots grow rapidly, but new shoots and fresh leaves are promptly 

 formed. Without pursuing this phase of the matter further it can 

 be seen that analogous results might occur in nature should the 

 soil conditions, for instance its color or the relation of the soil 

 surface to the incident heat rays, 1 be such as to bring about a 

 relatively warm soil environment. Under such conditions it is 

 clear that.only a study of the soil temperatures, and the responses 

 of the roots to soil temperatures, would provide the key to the 

 solution of the shoot behavior and to all of its accompanying 

 results. 



It is generally recognized that the soil acts as a reservoir for 

 heat, and that the daily course of soil temperature is unlike that 

 of the air immediately above it. Thus, the roots are subjected to 

 temperature conditions which are quite different from those 

 affecting the shoot of the same organisms. The shoot is warmer 

 by day and colder by night than the root and it is improbable 

 whether the roots of most woody plants are often subject to 

 "optimum" temperature conditions, as must frequently be the 

 case of the shoots. An exception to this statement, however, is 

 to be found in the cacti where the most favorable soil tempera- 

 tures are of great importance among those environmental features 

 that may be called definitive. The roots of most cacti of the 

 Tucson region, and possibly elsewhere, lie near the surface of the 

 ground. For the most part they are less than 30 cm. deep. 

 Inasmuch as the rate of root growth of the cacti, as will be shown 

 below, is relatively slow at temperatures much under the 

 "optimum," the importance to these plants of a shallow position 

 of the roots will be apparent. It is only in the upper soil horizon 

 that such favoring temperatures are to be found. It is of inter- 



i Cannon, W. A., "On the Eelation of Root Growth and Development to 

 the Temperature and Aeration of the Soil," American Journal of Botany, 

 Vol. 2, p. 211, 1915. 



