CHAPTER TWELVE 



THE ENVIRONMENT OF PLANTS 



Even the most casual observation suggests that plants are 

 greatly affected by the conditions under which they grow. 

 The grasslands, the forests, and the deserts of North America 

 are occupied by different plants because of the difference of 

 conditions in these areas. The white oak growing in the open 

 has broad, spreading branches and a large crown, while in the 

 forest it is more columnar and has a crown at the upper level 

 of the forest. The sugar maple becomes a great tree or re- 

 mains a mere shrub according to whether it grows in rich soU 

 or in a crevice of a rock. The internal structure of a plant 

 is also modified by the external conditions under which it 

 grows ; the leaves of many plants which are thin and tender 

 when there is an abundant water supply, become thick and 

 leathery when grown under conditions of drought. Since 

 conditions of growth show so great an effect upon the form, 

 size, structure, abundance, and distribution of plants, we should 

 know the principal factors that make up a plant's environ- 

 ment before going further in the study of the plant itself. 

 It is especially important to understand these factors. Because 

 the purpose of a great part of agricultural practice is to 

 modify the environment of the plants that are being grown. 



Definition of environment. By the environment of a plant 

 is meant the complex of all those influences outside the plant 

 which directly or indirectly affect its physiological processes, 

 its structures, and its development and propagation. These 

 influences are numerous and are usually spoken of as factors. 

 The natural habitat of a plant is a combination of environ- 

 mental factors favorable to the complete development of the 

 plant. The factors include the chemical and physical prop- 



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