86 Science of Plant Life 



summer, or it may be hundreds of years, as in some trees. In 

 the process of growth vast quantities of food are consumed. 

 During the early stages of a plant's development most of the 

 food it manufactures is used in this way. In order to grow, 

 a plant must make new protoplasm, develop new cell walls, 

 and thicken and strengthen old cell walls. Growth requires 

 not only food, but energy as well. Indeed, a considerable 

 part of the energy derived from respiration is used in growth. 

 We might expect assimilation and food consumption to be 

 most active in young growing parts, and that this is the case 

 has many times been verified by experiment. Growth takes 

 place through the enlargement of cells already present in the 

 plant, through cell division, and tlu'ough modification of cells 

 without enlargement. 



The making of the cell walls. The wall that surrounds each 

 cell of the plant is composed largely of a substance called 

 cellulose, which is secreted by the living protoplasm. When 

 the cell is growing, its wall is exceedingly thin and it stretches 

 as the cell enlarges. When a cell divides, a new wall is formed 

 between the two parts. As the cell grows older, new layers 

 of cellulose and allied substances are added. In some tis- 

 sues, as in the shells of nuts, the walls become so thick as to 

 occupy most of the volume of the cell. In other tissues, hke 

 the mesophyll of leaves, the cell walls always remain thin. 



Chemically, cellulose is a carbohydrate, closely related to 

 sugar and starch. Sugar and starch are the plant foods that 

 are mainly used in its manufacture, just as the proteins are 

 mainly used in the building of new protoplasm. Cotton 

 fiber is pure cellulose, and exemplifies the strength, lack 

 of color, and insolubility in water that are characteristic of 

 cellulose. 



