AN INTRODUCTION TO PLANT BIOLOGY 109 



is like a moving machine in that activity causes wear or 

 waste, and hence some food must be used for repair or the 

 entire plant will soon die. In short, some food must be 

 used continually in making new protoplasm. If more 

 protoplasm is made than is needed for repair of waste, the 

 result will be growth ; and usually growth means the forma- 

 tion of new cells. This is especially true at the growing 

 tips of plant stems. The making of new protoplasm is known 

 as assimilation, or constructive metabolism. 



(2) Stored in Cells. If the foods received from the leaves 

 are not needed for immediate repair, they are stored in various 

 parts of the plant (stem, roots, leaves, or seeds). In later 

 lessons (Chapter VIII) we shall study various modifications of 

 plants adapted to storing foods ; but for our present purposes 

 it will be sufficient to mention that carrots, turnips, and 

 sugar-beets are examples of plants that store large quantities 

 of plant food in their roots ; sugar-cane and sago-palm store 

 food in their stems; the head of a cabbage is a bundle of 

 leaves stored with food ; and beans, corn, and nuts are seeds 

 stored with food. These are examples of various plant or- 

 gans in which food is stored for the future use of the plants. 

 It has happened that man and the herbivorous (plant-eating) 

 animals have found it convenient to appropriate many of 

 these reserves of plant food for use in their own food-supply. 



These reserved foods of the plant are commonly stored as 

 oil and starch, which are easily seen (with low power of micro- 

 scope) in thin sections of various stems and roots, especially 

 in the late autumn or winter after storage has been going 

 on during the entire growing season. In cases of such 

 starch storage, the sugar solutions derived from starch in 

 the leaves osmose into the cells of the root, stem, or fruit, and 

 is then changed and stored as starch-grains. When the plant 

 needs this stored starch, the starch-grains are changed back 

 again into sugar, which is able to osmose out of the cell and 

 into other cells. 



