ALTERNATION 289 



ecesis is greater at the edge, where the increased dissimilarity between plant 

 forms diminishes the competition. The actual movement is outward, but it 

 takes place through the normal operation of competition. In this connection, 

 it should be pointed out that the common view that plants require room is 

 inexact, if not erroneous. This is difficult of proof, as it is impossible to dis- 

 tinguish room as such from the factors normally present, light, heat, water, 

 and nutrient salts, but it seems obvious that the available amounts of these 

 will determine the space occupied by a plant, irrespective of the room adja- 

 cent plants may allow it. The explanation of competition upon physical 

 grounds likewise invalidates the view that plants possess spheres of influence 

 other than the areas within which they exert a demonstrable reaction upon 

 the physical factors present. 



Competition plays a very important role in alternation. It produces minor 

 examples of alternation in the physical units of an asymmetrical series. Its 

 greatest influence, however, is exerted in modifying the effects of asym- 

 metry. The reaction of occupants emphasizes or reduces the effect of asym- 

 metry, and has a corresponding action upon alternation. This result of com- 

 petition is typical of succession, in which the sequence of stages arises from 

 the interaction of occupant and invader. 



345. Kinds of alternation. Alternation involves two ideas, viz., the al- 

 ternation of different species or formations with each other, and the alterna- 

 tion of the same species or formation in similar but separate situations. This 

 is the evident result of asymmetry, in response to which contiguous areas 

 are dissimilar and remote ones often similar. Individuals of the same species 

 or examples of the same formation may be said to alternate between two or 

 more similar situations, while dift'erent species or formations are said to al- 

 ternate 7vith each other, occurring usually in situations different in char- 

 acter. From the nature of alternation, the two phenomena are invariably 

 found together. 



It is possible to distinguish three kinds of alternation: (i) of a formation, 

 consocies, layer, facies, or species in similar situations; (2) of similar or 

 corresponding formations, species, etc., in similar situations; (3) of facies 

 and other species with respect to number. The last two are merely varia- 

 tions of the first, arising out of slight dift'erences in the physical factors of 

 the alternating areas, the adjacent flora, or the course of competition. The 

 alternation of different examples of the same formation is a significant fea- 

 ture of greatly diversified areas, such as mountains. It is naturally much 

 less characteristic of lands physiographically more uniform. A xerophytic 

 formation wall alternate from ridge to ridge, a mesophytic formation between 

 the intermediate valleys ; aquatic vegetation will alternate from pond to pond. 



