IX. ECOLOGY 



ECOLOGICAL FACTORS 



339. Definition. By ecology is meant the relations of 

 plants to their surroundings. These may be classed under 

 three general heads: their relations to inanimate nature, 

 to other plants, and to animals. The subject has been 

 touched upon repeatedly in the foregoing pages, and, in 

 fact, it is impossible to treat of any branch of botany with- 

 out some reference to it. All that was said about the ad- 

 justment of leaves for light and moisture, and their adap- 

 tations for protection and food storage, the devices for 

 fruit and seed dispersal, etc., really belong to ecology, 

 while Sections 330-338, about pollination, may be regarded 

 as a very imperfect review of the ecology of the flower in 

 relation to the insect world. 



340. Symbiosis. Associations for mutual help, like 

 those described in Sections 330-338, between certain plants 

 and their insect visitants, have been included by botanists 

 under the general term, symbiosis, a word which means 

 "living together." In its broadest sense symbiosis refers 

 to any sort of dependence or intimate organic relation 

 between different kinds of individuals, and so may include 

 the climbing and parasitic habits ; but it is more properly 

 restricted to cases where the relation is one of mutual 

 benefit. It may exist either between plants of one kind 

 with another, between animals with animals, or between 

 plants and animals, as in the case of the clover and bumble- 

 bee, and the yucca and pronuba. 



The occurrence of the root tubercles on certain of the 

 leguminosae (Sec. 198) is a clear case of symbiosis, the 

 microscopic organisms in the tubercles getting their food 

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