THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 91 



General observation shows that the three principal species of 

 Jasper Ridge, Adenostoma, Quercus, and Arctostaphylos, all possess 

 the dual root system. Its usefulness is apparent. During the winter 

 and spring, when most of the year's growth is accomplished, the 

 abundant superficial roots absorb large quantities of water. The 

 humus in this stratum both increases water-retaining capacity and 

 adds to the supply of other food materials. During the dry summer 

 the deeply penetrating roots supply the plant with the minimum 

 amount of water necessary to tide it over the critical period. It is 

 inevitable, when one recalls the extreme desiccation of the surface 

 soils, that the superficial roots cease functioning during the dry 

 season. The dual system is of course apparent only in plants growing 

 in ordinary soil. Where the rock is at the surface, which is very fre- 

 quently the case, the roots penetrate the crevices wherever they can, 

 without observable system. 



Eriodictyon californicum has a very different root plan (fig. 20). 

 The main roots are few, and they travel close to the surface, often 

 from 1 to 6 cm. below it, and occasionally one turns abruptly down- 



Fig. 20. — Root system of Eriodictyon californicum. For explanation, see fig. 19. 



ward to the deeper soil-layers. From the horizontal portions 

 sucker shoots appear, so that a number of apparently independent 

 plants are often found to be connected below ground. Whenever 

 the roots are exposed, as by erosion, they produce shoots abundantly. 

 The roots of Eriodictyon are useful, therefore, for propagation as well 

 as for absorption. 



The three principal chaparral species of Jasper Ridge, Adenostorna 

 fasciculatum, Quercus durata, and Arctostaphylos tomentosa, have 

 been proved to be mycorhizal. In the excavations no normal root- 

 hairs were seen, but it is hardly reasonable that the most deeply 

 penetrating roots should be associated with fungi. The mycorhizas 



