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. 
188. 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. 
