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It has been pointed out in another place that the depth to perennially moist soil, which is usually regulated by the depth of the water table, undoubtedly plays an important rôle in the distribution of forests as a whole. Thus, where the moist soil lies too deep to be tapped by the roots of the trees, other factors being equal, forests are usually wanting, but where the water table lies near enough to the surface, so that the perennially moist soil above it can be reached by the roots Of the trees, forests are present. In southern Arizona it has been found that, given a practically homogeneous soil, the mesquite assumes a tree habit with the water 35 feet, more or less, beneath the surface, but that where the soil is not homogeneous, and is stratified, so that a portion of the strata are dry, or where the water table is more than 35 to 40 feet beneath the surface, the species has a shrub-like habit. An analogous condition obtains in the coastal plain of Texas and in the treeless middle-west. In the latter region the deciduous forests are almost wholly confined to the flood-plains of streams, while the adjacent upland is treeless. Thus, over a wide area where the climate is arid or semi-arid, the depth to perennial water is an important factor in determining the presence of forests. Where the trees are unable to attain to such moist soil, they usually develop xerophytic characters or special adjustments by which they are enabled to survive. How true this generalization may be found to be can not at present be told, but that it applies to such regions as central California, there can be no doubt. Among the adjustments, as has been pointed out in this note, are to be included those of the root-system by which they are especially adapted to make use of the superficial waters.

 Coastal Laboratory, Carmel, California