Page:Popular Science Monthly Volume 85.djvu/425

Rh 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 of 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.

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 taproot 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.

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 impossible to study the variation exactly. For some reasons, however, it does not seem probable that there is marked variation in root type.