CHAPTEE XVII 



PLANT ECOLOGY 



173. Introductory.— Ecology has to do with the external 

 relations of plants, and forms the principal subject of the 

 volume entitled Plant Relations, which should be consulted 

 for fuller descriptions and illustrations. It treats of the 

 adjustment of plants and their organs to their physical 

 surroundings, and also their relations with one another 

 and with animals, and has sometimes been called "plant 

 sociology." 



LIFE EELATIOH'S 



174. Foliage leaves. — The life relation essential to foliage 

 leaves is the relation to light. This is shown by their 

 positions and forms, as well as by their behavior when 

 deprived of light. This light relation suggests the answer 

 to very many questions concerning leaves. It is not very 

 important to know the names of different forms and differ- 

 ent arrangements of leaves, but it is important to observe 

 that these forms and arrangements are in response to the 

 light relation. 



In general a leaf adjusts its own position and its relation 

 to its fellows so as to receive the greatest amount of light. 

 Upon erect stems the leaves occur in vertical rows which 

 are uniformly spaced about the circumference. If these 

 rows are numerous the leaves are narrow ; if they are few 

 the leaves are usually broad. If broad leaves were associ- 

 ated with numerous rows there would be excessive shading ; 



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