76 AGRONOMY 



through the intercellular spaces, mixes with the moisture in 

 the cell walls and thus enters the cells. Here it is combined 

 into food and the excess oxygen given off. The whole process 

 is known as photosynthesis. It is popularly supposed that photo- 

 synthesis in plants is the equivalent of respiration in animals, 

 but this is an error. Plants also respire, exactly as animals do, 

 taking in oxygen and giving off carbon dioxide, but, unlike 

 animals, they have the additional process of photosynthesis, 

 in which carbon dioxide is taken in and oxygen released. 



The two processes 

 differ also in other 

 W respects. Respiration 



occurs in every living 

 cell, in roots as well as 

 in stems and leaves, 

 ^ ^ ^ and goes on continu- 



.> ^% ally? while photosyn- 



fs i^ggO^ thesis goes on only 



in the green cells in 

 sunlight. 



The grape sugar 

 FIG. 64. Cells of the carrot with crystals of ~ 1-0.11 



, . , . ., . -.. formed in the leaves 



carotin, which give the root its orange color 



is, as we have noted, 



almost immediately turned to starch. Later, especially at 

 night, this food is distributed through the plant, by way of the 

 sieve tubes, to be used in the formation of new tissues, or it is 

 stored in stems, roots, and other organs until needed. Starch, 

 however, cannot pass through the cell walls, and before it can 

 be moved it must be turned back to grape sugar again. This 

 is accomplished by means of vegetable ferments called enzymes, 

 and the process is called digestion. A green and starchy 

 banana or pear laid aside for a time will become sweet by the 

 same process. The underground parts of the plant are favor- 

 ite places for the storage of food. Here the grape sugar is 



