76 CHANGES OF COMPOSITION [chap. 



grow and multiply, and in doing so they split up by the 

 aid of an enzyme the sugar into alcohol and carbon 

 dioxide. The yeast cells do not feed upon the sugar, 

 but they derive energy from its splitting up, just as the 

 ordinary plant cell derives energy from the oxidation of 

 the sugar to carbon dioxide and water which takes place 

 during respiration in the presence of oxygen. 



In discussing the changes which take place in the 

 composition of parts of the plant, we have already had 

 occasion to point out that the changes due to variations 

 of environment are not very great; the variations 

 induced by season are much greater than those due to 

 soil or manuring, as may be illustrated by the figures 

 for the nitrogen-content of the grain of wheat in 

 Table VII. In the wet season of 1852 there was about 

 2 per cent, of nitrogen in the wheat grain, whereas in the 

 dry year of 1863 there was little more than 1-5 per 

 cent. ; this season alone has caused a difference of 0-5 

 per cent, or 25 per cent, of the total, whereas the greatest 

 difference in nitrogen-content between the grain grown 

 on the wholly unmanured plot and on that which 

 receives an annual heavy dressing of farmyard manure 

 only amounted to 0-2 per cent. As a matter of fact, the 

 difference in the composition of the crop brought about 

 by different soils or different manuring may be regarded 

 as due to the change of climate which the soil and manur- 

 ing sets up for the particular plant. For example, a heavy 

 soil will maintain not only moisture about the root but 

 a moister atmosphere about the leaves, and so become 

 the equivalent of a wetter season ; on the other hand, a 

 nitrogenous manure will prolong the growth of a plant 

 and so cause the ripening process to take place under 

 other climatic conditions than those prevailing for an 

 earlier ripening crop. If, however, the differences of 

 composition induced by environment — soil, manuring, 



