LIGHT ASSOCIATION 19 



practically all the light incident upon the formation, and the 

 shrubs, herbs, fungi and algae of lower habit and inferior 

 position must adapt themselves to the diffuse light which 

 passes through or between the leaves. The same is equally 

 true of dense thicliets and wastes, except that the vertical 

 distance is less, and the diffuseness of the light is corres- 

 pondingly modified. In these formations, the dominant 

 trees, shrubs or herbs, the facies, constitute a primary or 

 superior layer. The degree of subordinate associ- 

 a,ti on{associatio inferior), as a result of which inferior layers 

 will arise, will be entirely determined by the density of the 

 facies. In open woodlands, which are really mixed for- 

 mations of woodland and grassland, the intervals, and 

 usually the spaces beneath the trees, also, are covered with 

 poophytes, showing an absence of subordination due to 

 light. This is the prevailing condition in the pine formation 

 {Pinus ponderosa-xerohylium) of the ridges and foothills of 

 western Nebraska. When, however, the trees stand 

 sufficiently close that their shadows meet or overlap through- 

 out the day, the increasing diffuseness begins to cause modi- 

 fication and rearrangement of the individuals. By photo- 

 metric methods, the light in a forest is found to be least 

 diffuse just below the facies, while the diffuseness increases 

 markedly in passing to the 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 arrange- 

 ment 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 



