234 PLANT AND ORGANIC CHEMISTRY 



Most of the ash- constituents essential or injurious to the 

 growth and development of plants are known, and also the va- 

 riations in growth caused by the presence or absence of certain 

 inorganic compounds. 



The chemical changes through which many plants pass from 

 the germination of their seed to maturity and decay are also 

 known, each separate stage of growth showing a distinct chem- 

 ical composition or a predominance of some one chemical com- 

 pound. 



It should be especially noted that some chemical compounds 

 occur in certain species of plants and do not occur in others. 

 Certain classes of compounds are found widely distributed 

 through the plant kingdom, accompanied by correlated mor- 

 phological characters. Some one compound, as saponin, will 

 be found with similar botanical characters in plants of distinct 

 genera and families, on the same plane of evolution or devel- 

 opment. 



It cannot be the result of accident that cinchona and related 

 plants contain, quinine; and other plants, distributed through 

 the vegetable kingdom, their own typical compounds. Nor can 

 it be the result of accident, or changes produced by climate 

 or other causes, that an absence of some one or more com- 

 pounds is accompanied by a modification of the exterior of 

 the plant. 



Before taking up the consideration of the above statements 

 in detail, it may be well to study briefly two properties 1 of liv- 

 ing protoplasm, namely, absorption and metabolism. 



The seeds of plants are the storehouses of a certain amount 

 of latent energy or life, elaborated by the parent plant and 

 stored up in the form of complex chemical compounds. Under 

 suitable conditions of warmth and moisture, certain chemical 

 changes take place within the seed. The latent energy becomes 

 active, and the seedling grows, feeding upon its food supply 

 until it has exhausted its store. 



At this stage the little plant must seek its food from without, 

 from the atmosphere and the soil. The soil is of varying and 

 complex composition, containing, between its particles, gases 



1 Lectures on the Physiology of Plants, by S. H. Vines. Cambridge, 1886. 



