THE CONSTITUENTS OF PLANTS. 15 



roots rather than the leaves. The leaves absorb water 

 when there is an insufficient supply in the soil, or when 

 the weather is such as to cause a very rapid exhalation 

 from the leaves. The supply of the mineral constituents 

 depends upon the character and composition of the soil, 

 upon the season, and climate ; these conditions are not uni- 

 form, hence the increased variation in the character of the 

 natural vegetation found in different parts of the world. 



Functions of the Constituents. — The exact work 

 that each constituent performs in plant nutrition has not 

 yet been definitely determined. It has been shown that 

 nitrogen is of vital importance, since it is an essential 

 constituent of the living principle of plants called "pro- 

 toplasm." Phosphorus and sulphur have been found 

 essential constituents in the formation of albuminoids, a 

 very important compound of all plants. Potash is neces- 

 sary to the formation and distribution of starch, while the 

 green color of plants, or chlorophyll, cannot be formed 

 without iron. All the constituents mentioned are neces- 

 sary, and are concerned in the various processes which 

 result in the perfect plant. 



The Natural Tendency of Plant Growth. — The ulti- 

 mate object of all plants in their natural state is to form 

 seed, or the beginning of new plants. The seed of every 

 plant contains within itself a sufficiency of food to nourish 

 the germ till the root and leaves of the new plant are 

 sufficiently developed to acquire food from the sources 

 mentioned. 



Germination : the Conditions Necessary. — The 

 first evidences of growth are shown in the germination 

 or sprouting of the seed. This beginning depends chiefly 



