ground. The taller, stronger individuals are consequently in a position to assimilate more vigorously, and to become still taller and stronger as a result. Just as these have taken up a position inferior to that of the facies, so the shorter or weaker species must come to occupy a still more subordinate position. This results, not only because the light is primarily weaker nearer the ground, but also because the taller plants interpose as a second screen. The complete working out of this arrangement with reference to light produces typical subordinate association, which finds its characteristic expression in the layering of forests and thickets. Layers tend to appear as soon as open woodland or thicket begins to pass into denser conditions, and up to a certain point, at which they disappear, they become the more numerous and the more marked, the denser the forest. In the Otowanie woods near Lincoln (_Quercus-Hicoria-hylium_), layering usually begins at a light value of .1 (1 = normal sunshine in the open). Thornber[28] has found the same value to obtain in the thickets of the Missouri bluffs. In these, again, layers disappear at a value of .005, the extreme diffuseness making assimilation impossible except for occasional mosses and algae. A number of herbaceous plants are present in the spring, but these are all prevernal or vernal bloomers, which are safely past flowering before shade conditions become extreme. In the _Fraxinus-Catalpa-alsium_, all inferior holophytic vegetation disappears between the light value of .004 and that of .003. The spruce-pine formation (_Picea-Pinus-hylium_) of the Rocky mountains, with a light value of .01, usually contains but a few scattered herbs, mostly evergreen; in some cases there are no subordinate plants other than mosses and hysterophytes. The lodge-pole pine formation (_Pinus murrayana-hylium_), with light values often less than .005, is nearly or quite destitute of all but hysterophytic undergrowth. Such extremely dense formations are examples of coordinate association merely, since the formation is reduced to a single superior layer, in which the individuals of the facies bear the same spatial relation to incident light. In layered formations, in addition to the subordinate relation of other species to the facies, there is, of course, a kind of coordinate association manifested in each layer.