‘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 occttpants‘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 different species or formations are said to al- 
ternate zeith 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: (1) 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 differences 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 will alternate from ridge to ridge, a mesophytic formation between 
the intermediate valleys ; aquatic vegetation will alternate from pond to pond, 
