52 GENERAL BIOLOGY 



the chemist supplies. Furthermore, if a compound 

 sugar, like the cane-sugar just described, should 

 be broken up, it would not resolve itself into its 

 ultimate components (atoms) at once, but the line 

 of cleavage would occur first at the point where the 

 two larger groups had joined. In other words, the 

 affinity of the two simple sugars for one another is 

 much weaker than the affinities of their constituent 

 atoms for each other. In general, the simplest 

 compounds, such as CO2, H^O, NH 3 , etc., are bound 

 together by very strong chemical affinity and require 

 much force to disrupt them, whereas the chemical 

 affinity binding together very complex organic 

 substances is usually so weak that these molecules 

 often appear to disintegrate spontaneously, for 

 which reason they are spoken of as unstable. These 

 facts, as we shall see, have an important bearing on 

 the utilization of food substances by plants and 

 animals. 



Chemical Synthesis in the Organism. Green 

 plants not only require the normal conditions of 

 heat and moisture demanded by all living things, 

 but they require sunlight as well, else they grow 

 pale and sickly. This is true even of those plants, 

 like ferns, that thrive best in the shade. The 

 position and attitude of every leaf on a tree is ad- 

 justed to receive the maximum amount of sunshine. 

 If we inclose a leaf in a glass tube filled with CO 2 , 

 and then expose it to the sunshine, after several 

 hours we will find by tests that a large part or all 



