CHAPTER XXIV 
ECOLOGY 
Eco.oey is that department of biology which deals with the 
relations of plants and animals of various habitats to their 
environmental conditions. 
Ecology may be divided into two branches, autecology and 
synecology. AUTECOLOGY deals with the structure and behavior 
of the individual organism and its parts as related to environ- 
ment, whereas synecology deals with plant communities as 
related to soil, light, climate and other environmental factors. 
Every living thing is a creature of circumstance, dominated 
and controlled by heredity and environment. Its fundamental 
structural and functional characteristics are determined by the 
genes in its protoplasm but its characteristics become modified 
by external factors of environment. In order to exist and keep 
healthy it must adapt itself to the various factors of its sur- 
roundings. The environmental factors having to do with the 
existence and health of plants include soils and soil constituents, 
air, moisture, light, range in temperature, gravity, and sur- 
ounding animals and plants of other kinds. Various aspects of 
most of them have been considered in preceding chapters. 
PLANT Communitirs AND ASSOCIATIONS 
An entire group of plants of similar habits occurring in a 
common _ habitat constitutes what is termed a plant community. 
A plant association is | a plant community having a definite com- 
position 7.e., made up of a definite aggregation of plants deveiop- 
ing under similar conditions. Thus, a desert where the cacti 
and euphorbias form the dominant plants with a definite 
assernblage of other plants growing with them would represent 
a cactus-euphorbia desert association. Plant communities may be 
classified either from the point of view of their order of develop- 
ment, as based upon the principle of succession, or upon their 
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