172 PRACTICAL COURSE IN BOTANY 



The water not used up in the process is given off as a waste 

 product in transpiration, while the oxygen is returned to the 

 air, as shown by Exp. 66. This equation is not to be under- 

 stood as representing the chemical changes that actually take 

 place in the leaf. These are too complicated, and at present 

 too imperfectly known, to be considered here. It will serve, 

 however, to give a fair idea of the final result from the process 

 of photosynthesis, however brought about. 



Simple as the operation appears, the chemist has not, as 

 yet, been able to imitate it. He can analyze starch into its 

 original constituents, but while he has the ingredients at 

 hand in abundance, and knows the exact proportions of their 

 combination, it is beyond his power, in the present state of 

 our knowledge, to put them together. Hence, both man 

 and the lower animals are dependent on plants for this most 

 important food element. The so-called factories that supply 

 the starch of commerce do not make starch any more than 

 the miller makes wheat, but merely separate and render 

 available for use that already elaborated by plants. 



1 88. Proteins. Foods of this class are mainly instru- 

 mental in furnishing material for the growth and repair of 

 the tissues out of which the bodies of both plants and animals 

 are built up. They embrace a great variety of substances, 

 but their chemical nature is very complex and very imper- 

 fectly understood. Nitrogen is an important element in 

 their composition, whence they are commonly distinguished 

 as " nitrogenous foods." Besides nitrogen, there are present 

 carbon, hydrogen, oxygen, and sulphur, and traces of the 

 mineral salts absorbed from the soil are found in varying 

 quantities in the ash of different proteins. The percentages 

 in which these ingredients are combined and the processes 

 concerned in their formation are at present a matter of pure 

 hypothesis. Botanists are not agreed even as to whether 

 they are made in the leaf or in some other part or parts of 

 the plant, though the weight of opinion inclines to the view 

 that their construction takes place in the leaf. 



