CHAPTER IX 

 ECOLOGY 



Ecology is x 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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