CHAPTER IX 
ECOLOGY 
Ecology is that department of biology which deals with the 
relations of plants and animals of various habitats to their environ¬ 
mental conditions. Every living thing is a creature of circumstance, 
dominated and controlled by heredity and environment. In order 
to exist and keep healthy it must adapt itself to the various factors 
of its surroundings. The environmental factors having to do with 
the existence and health of plants include soil constituents, air, 
moisture, light, range in temperature, gravity, surrounding animals 
and plants of other kinds. 
A group of plants occurring in a common habitat constitutes what 
is termed a plant association or society. Plant associations 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 water 
relation. The latter method, appears to be the one more generally 
adopted, because of its ready application and will now be considered. 
According, therefore, to the relation plant associations have 
assumed in regard to water, they may be grouped as follows: 
1. Hydrophytes or water plants. 
2. Helophytes or marsh plants. 
3. Halophytes or salt plants. 
4. Xerophytes or desert plants. 
5. Mesophytes or intermediate plants. 
6. Tropophytes or alternate plants. 
Hydrophytes. The effect of an aquatic environment on the struc¬ 
ture of water plants is most striking. The root systems are reduced 
both in length and number of branches. The root hairs of those 
immersed in the water are absent. The supportive action of the 
water is such that the fibrovascular elements of the stems, which 
usually function both for support and conduction of crude sap, 
are greatly reduced in size and strength. The leaves, stems and 
roots possess large air-spaces. The mesophyll of the leaves is 
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