230 PLANT PHYSIOLOGY AND ECOLOGY 



to the reduced light of the habitat. Tall-growing plants which 

 require the most light form the first layer below the facies, while 

 the low forms which will grow in very weak light make up the 

 lowermost layer. The number of layers in a forest depends upon 

 the compactness of the primary layer of facies. When the light 

 beneath the latter is reduced to .002 or .001, layers are impossible, 

 since practically no flowering plants can grow under these con- 

 ditions. Layers are developed to the highest degree when the 

 primary layer permits more or less sunlight to shine through it, 

 i.e., in a light intensity varying from .1 to .OL There is often a 

 more or less incomplete secondary layer of shrubs and small trees. 

 Below this usually occur two or three herbaceous layers. The 

 upper one consists of tall plants about 2 meters high. It is followed 

 by a middle layer about 1 meter high, in which the bushes are 

 usually found, and the latter by a lower layer of small herbs 

 2-5 decimeters high. Beneath these is found a ground layer of 

 mosses, lichens, cup-fungi, toadstools, etc., which is the only one 

 that remains in the densest forests. The herbaceous layers are 

 always more or less interrupted, owing to their close dependence 

 upon light. Either one of them may be absent, or finally all of them 

 may disappear, as has been indicated. 



The explanation of layers, as well as that of consocies and 

 societies, depends upon a knowledge of zonation and alternation 

 and is to be found in the chapter dealing with these principles. 



Experiment 6 1. Layered formations. Make a careful examination of 

 a forest or thicket, noting the number and extent of the layers present. 

 Ascertain the characteristic species of each layer and note the various 

 groups which they form in it. 



244. Classification. Formations are classified with respect to 

 habitat, development, position, dominant species, or their general 

 character. Classification upon the basis of habitat places together 

 formations which are similar in their response to physical factors 

 and in general structure. Developmental classification is based 

 upon the fact that the formations which follow each other in the 

 development of vegetation upon a new or denuded area have a 

 certain organic relation to each other. In many cases it also 

 brings out certain important relations to physiography. Grouping 

 with respect to position is based solely upon geographical factors. 

 The formations brought together in this way have little relation- 



