ALTERNATION 285 
With réspect to the different portions of an asymmetrical area, migration 
will have one of three effects: (1) it will carry disseminules into both 
favorable and unfavorable areas, (2) into favorable ones only, or (3) into 
unfavorable ones alone. From the radial nature of migration, the first case is 
far the most frequent ; it is typical of sporostrotes, and the highly specialized 
spermatostrotes and carpostrotes. The eftect of migration is uniform here, 
and alternation arises in consequence of the selective power of ecesis. It is 
evident that migration does not have an even indirect effect, when the dis- 
seminules are carried into none but unfavorable situations. Where the move- 
ment is into favorable places alone, alternation is the immediate result. The 
intermittent operation of migration and the presence of barriers are re- 
sponsible for the absence of plants in situations favorable to them, and in 
consequence bring about a certain alternation between corresponding species. 
The selective operation of physical factors upon the disseminules carried 
into the different parts of an asymmetrical area is the usual cause of alterna- 
tion. Asymmetry alone is universal within the more conspicuous structures 
termed zones, down to the smallest areas which a group of plants can occupy. 
The difference between contiguous areas, particularly within the same habitat, 
is often small. It sometimes seems inefficient in the initial stages of a suc- 
cession when a single species is present, but even in extreme cases its effect 
will be recognizable in the size and density of the individuals. Asymmetry 
is clearly evident in vegetation where two symmetrical series cross each other, 
or when a symmetry is interrupted by barrier-like elevations or depressions. 
Within formations, it arises from differences, often very slight, in slope, 
‘exposure, elevation, from irregularities of surface, differences in soil struc- 
ture, or composition, in the amount of cover, and in the reactions of the living 
plants. At the last point, it is in direct connection with plant competition. 
344. Competition. Much uncertainty, as well as diversity of opinion, 
seems still to exist in regard to the precise nature of the competition between 
plants that occupy the same area. It has long been admitted that the 
phrase, “struggle for existence,’ is true of this relation only in the most 
figurative sense, but the feeling still prevails that, since plants live in asso- 
ciations, there must be something mysterious and vitalistic in their ‘relation. 
No one has been able to discover anything of this nature, but nevertheless 
the impression remains. Such a direct relation exists only between 
parasites, epiphytes, and lianes, and the plants which serve to nourish 
or support them. In the case of plants growing on the same stratum, actual 
competition between plant and plant does not occur. One individual can 
affect another only in as much as it changes the physical factors that in- 
fluence the latter. Competition is a question of the reaction of a plant upon 
the physical factors which encompass it, and of the effect of these modified 
