CHAPTER XXIV 

 FURTHER DISCUSSION OF PLANT INDUSTRIES 



399. Introductory. Agricultural and horticultural indus- 

 tries are fundamental, since they produce most of the things 

 upon which people live. A scientific study of what plants are 

 and how they live has been the means of raising these indus- 

 tries to their present high efficiency. In preceding and in later 

 chapters there is frequent reference to the practical nature of 

 a knowledge of the principles of botany, but in the present 

 chapter there are presented three topics which relate specifi- 

 cally to agriculture and horticulture. The topics presented 

 are: I. The Soil and the Plant; II. Special Care of Plants; 

 III. Leading Agricultural and Horticultural Plants. 



I. THE SOIL AND THE PLANT 



400. Composition of the soil : rock material. One of the 



most important lines of botanical study has to do with the 

 interrelations of plants and the soil in which they live. Any 

 extended consideration of agricultural and horticultural indus- 

 tries must involve a comprehensive study of the soils, but in 

 the present connection only an outline of the subject is given. 

 In a general way it may be said that the basis of soils 

 consists of more or less finely divided rock. A study of the 

 dumping ground of a stone quarry will show that weathering 

 processes are bringing about the disorganization of some of 

 the stones, and as a result soil is made possible. Sometimes 

 water freezes and expands within the crevices of the stone ; or 

 roots of trees and other plants may grow in these crevices, and 

 by expanding may break the stone. Organic material from 

 plants and animals may help in disorganization, and landslides 



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