328 NATURE STUDY AND AGRICULTURE 



essential to life, for unless the protoplasm can work none 

 of the phenomena of life appear. The germinating seed 

 shows that respiration is going on by taking in oxygen from 

 the air and giving out carbon dioxide. 



The conspicuous work done by the protoplasts in the 

 seed is the formation of new cells, thus causing the little 

 plant to grow. In this work of forming new cells, each 

 protoplast divides, beginning with the nucleus, and a wall is 

 laid down between the two halves. Then each half in- 

 creases in size until it becomes as large as the original cell. 

 If every protoplast in the embryo should divide in this way, 

 the result would be an embryo twice as large as before. In 

 this way the young plant increases in size, and finally 

 bursts through the seed coat. 



Thus the protoplasts, by digesting food, by assimilating 

 it, and by constantly respiring cause the plant to grow from 

 the embryo to the full-grown tree. Long before this growth 

 is completed, in fact soon after the plant escapes from the 

 seed, another process is necessary. The store of food laid 

 up in the seed is soon exhausted, and new food must be 

 provided. 



The tree, as all green plants, has the power of manu- 

 facturing food, and this process is called photosynthesis. 

 This work can be done only by the green parts of plants, 

 and since the leaves constitute the larger part of the green 

 tissue, they are properly spoken of as the organs of photo- 

 synthesis or the organs of food manufacture. This process 

 is of supreme importance, for all life is dependent upon it. 

 This means that green plants make more food than they 

 use, and upon this surplus all other plants and all animals 

 live. 



