DEVELOPMENT AND STRUCTURE 199 
DEVELOPMENT AND STRUCTURE 
247. Vegetation an organism. The plant formation is an organic unit. 
It exhibits activities or changes which result in development, structure, and 
reproduction. These changes are progressive, or periodic, and, in some 
degree, rhythmic, and there can be no objection to -regarding them as 
functions of vegetation. According to this point of view, the formation is 
a complex organism, which possesses functions and structure, and passes 
through a cycle of development similar to that of the plant. This concept 
may seem strange at first, owing to the fact that the common understanding 
of function and structure is based upon the individual plant alone. Since 
the formation, like the plant, is subject to changes caused by the habitat, 
and since these changes are recorded in its structure, it is evident that 
the terms, function and structure, are as applicable to the one as to the 
other. It is merely necessary to bear in mind that the functions of plants 
and of formations are absolutely different activities, which have no more 
in common than do the two structures, leaf and zone. 
248. Vegetation essentially dynamic. As an organism, the formation 
is undergoing constant change. Constructive or destructive forces are 
necessarily at work; the former, as in the plant, predominate until maturity, 
when the latter prevail. Consequently, it no longer seems fruitful to classify 
the phenomena of vegetation as dynamic or static. The emphasis which 
has been placed upon dynamic aspects of vegetation has served a useful 
purpose by calling attention to the development of the latter. Although it 
is a quarter of a century since Hult, and more than a half century since 
Steenstrup, by far the greater number of ecological studies still ignore the 
problem of development. This condition, however, can be remedied more 
easily by insisting upon an exact understanding of the nature of the forma- 
tion than in any other way. It is entirely superfluous to speak of dynamic 
and static effects in the plant, and the use of these terms with reference 
to the formation becomes equally unnecessary as soon as the latter is 
looked upon as an organism. The proper investigation of a formation can 
no more overlook development than structure, so closely are the two inter- 
‘woven. Future research must rest squarely upon this fact. 
249. Functions and structures. The functions of a formation are as- 
sociation, invasion, and succession: the second may be resolved into migra- 
tion and ecesis, and the third, perhaps, into reaction and competition. 
Formational structures comprise zones, layers, consocies, societies, etc., all 
of which may be referred to zonation, or to alternation. The term associa- 
