GLEASON: THE PLANT ASSOCIATION 465 
It is true that various analogies may easily be drawn between a 
unit of vegetation and an organism, but these analogies are always 
more apparent than real, and never rise to the rank of homologies. 
For example, it is obvious that an association may appear on a 
new area, develop to maturity, and finally disappear, but these 
phenomena are in nowise comparable to the life history of an 
individual. A spore of Rhizopus, for example, given the proper 
environment, will grow to maturity and reproduce without the 
presence of any other living organism. The first pioneer species 
of an association, on the other hand, will merely reproduce them- 
selves, and maturity of the association will never be reached unless 
its other species are also present in a neighboring area. Similar 
exceptions may be taken to all other analogies between the in- 
dividual and the association, designed to demonstrate the organic 
entity of the latter. 
2. Certain common phenomena of the plant individual, namely, 
migration of germules, germination, and growth, when performed 
en masse by numerous individuals, may produce visible effects 
which are worthy of special study and which demand a special 
terminology. Among these effects may be mentioned the structure 
of the plant association and of the subsidiary assemblages of plants 
within it, its space relation to neighboring associations, its de- 
velopment, and its ultimate disappéarance. : 
II. THE ENVIRONMENT 
3. The functions of the individual plant demand a proper 
environment for their operation. Any effective variation in the 
environment causes a variation in the performance of these 
functions, and many environmental changes produce also a vari- 
ation in morphological structure. All these results may be in- 
vestigated by experimental methods, and fall properly within the 
domain of morphology and physiology. 
4. A fixed environment is probably never necessary to the life 
of any individual plant, and certainly never to any spermatophyte. 
Each individual is capable of existing under a variety of environ- 
mental conditions; as a simple example, the annual variation in 
temperature may be mentioned. Also, a fixed environment is not 
necessary to the life of all the individuals of one species. 
