218 PLANT PHYSIOLOGY AND ECOLOGY 



scattered through forest or meadow, the physical factors are 

 changed, and the species growing on rocks are found to be typical 

 of rock formations elsewhere. Each rock group is a fragment 

 of a formation which is characteristic of a different habitat. The 

 rock may have found its way by accident into the forest or meadow; 

 it may have been uncovered and thus come to serve as a stratum 

 for the lichens, or it may be a relict of a rock formation that has 

 been displaced by other vegetation. In either event the lichen 

 group is foreign to the forest or meadow. 



From the above it is clear that the first task in field work is 

 to distinguish as many different areas as possible in vegetation. 

 The next and more important task is to compare the composition 

 and development of these so carefully that the actual formations 

 may be recognized, and the various pieces referred to the proper 

 one. In any region the number of formations is very small in 

 comparison with the number of parts, fragments, developmental 

 stages, etc. It is a simple matter to recognize a forest, meadow^, 

 bog, or pond as formations in a locality, and very often the forests, 

 meadows, etc., of the neighboring localities are merely different 

 examples of the same formation. 



232. Relation between habitat and formation. Since forma- 

 tions are groups of individual plants and these are dependent upon 

 the habitat, it is evident that the habitat must have the same con- 

 trol over the formation. Strictly speaking, a formation is the mass 

 of plants which cover a habitat. The limits of the two are neces- 

 sarily the same. A habitat with sharply defined limits, e.g., a 

 pond, a rock, a shaded area, is occupied by a formation whose bound- 

 aries are equally definite. One that shades imperceptibly into 

 another shows a formation that grades into the adjoining one. 

 The real characteristic of a habitat is a striking difference in one 

 or more of the direct factors, water content, humidity, or light. 

 A series of habitats thus set off from each other bear characteris- 

 tically distinct formations, as, for example, xerophytic vegetation 

 on a gravel slide, shade plants in a forest, and bog plants in a 

 wet meadow. 



The formation is the product of its habitat. It not only shows 

 a general and typical response to the latter, but it also conforms 

 to the minor differences which occur in it. The conditions of 

 water content and light that produce a spruce forest cause it to 

 be sharply set off from a gravel slide formation produced by very 



