TREE DISTRIBUTION 



421 



trees, however, the tap-root, or a few laterals, are rather deeply placed, 

 and appear to predominate in numbers. The formation of the more 

 superficial portion of the root-system, therefore, is a response which 

 comes with the aging f the tree. In the case of the blue oak also, 

 there can be traced a very intimate relation between root character of 

 the open stand of forest and the water relation. 



Quercus Douglasii 

 As in the case of the root-system of the roble oak, that of the blue oak 

 is composed of relatively few roots, which are usually coarse. The tap- 

 root is sometimes well developed and there are relatively few laterals. 

 The root-system of the blue oak, however, differs from that of the roble 

 oak in that the roots of the species, in the proper habitat, are confined 

 to the upper soil strata. The tap-root is also shallowly placed. The 

 relation of the roots of the blue oak, as well as the root-system of the 

 two other species, to soil moisture, will be taken up .immediately. 



Koot Vakiatiox 



The brief characterization of the root-systems of the oaks, as just 

 given, does not take into account the possible variation of the roots. 

 Owing to the impracticability of excavating the roots, it was impos- 

 sible to study the variation exactly. For some reasons, however, it 

 does not seem probable that there is marked vaiiation in root type. 



Fig. 3. Superficial Absorbing Roots of Quercus agrifolia Growing on the Lower 

 Slope of the Santa Cruz Mountains near Stanford University. 



