OBJECTS.] INTRODUCTORY. 89 



green plants. These grow taller and taller until they 

 attain many times the size which they had when they 

 first appeared ; and they produce the heads of flowers 

 which eventually change into ears of corn. 



In so far as this is a process of growth, accompanied 

 by the assumption of a definite form, it might be 

 compared with the growth of a crystal of salt in 

 brine : but, on closer examination, it turns out to be 

 something very different. For the crystal of salt grows 

 by taking to itself the salt contained in the brine, 

 which is added to its exterior ; whereas the plant 

 grows by addition to .its interior : and there is not a 

 trace of the characteristic compounds of the plant's 

 body, albumin, gluten, starch, or cellulose, or fat, in 

 the soil, or in the water, or in the air. 



Yet the plant creates nothing ( 50) and, therefore, 

 the matter of the proteids and amyloids and fats which 

 it contains must be supplied to it, and simply manu- 

 factured, or combined in new fashions, in the body of 

 the plant. 



It is easy to see, in a general way, what the raw 

 materials are which the plant works up, for the 

 plant gets nothing but the materials supplied to it by 

 the atmosphere and by the soil. The atmosphere 

 contains oxygen and nitrogen, a little carbonic acid 

 gas, a minute quantity of ammoniacal salts, and a 

 variable proportion of water. The soil contains clay 

 and sand (silica), lime, iron, potash, phosphorus, 

 sulphur, ammoniacal salts, and other matters which 

 are of no importance. Thus, between them, the soil 

 and the atmosphere contain all the elementary bodies 

 which we find in the plant : but the plant has to 

 separate them and join them together afresh, 



