i6 



Science of Plant Life 



the cell dies when it is removed, and it is thought to control 

 many of the activities that go on within the cell. The cells 



Fig. io. 



Leaves with prominent stipules : pea, black willow, red clover, 

 Japanese quince, rose. 



are the structural units of plants. Figure 1 2 shows how a leaf 

 is built of cells of different sizes and shapes. 



The cytoplasm makes up the bulk of the living matter of 

 a cell, but in mature plant cells most of the space inclosed by 

 the cell wall is occupied by one or more vacuoles or cavities 

 containing the cell sap. This is water with sugars, mineral 

 salts, acids, and other substances dissolved in it. Lying 

 within the cytoplasm are structures called plastids, small 

 bodies that contain food substances and coloring matters. 



The cell wall. The wall which surrounds the cell is com- 

 posed of a transparent material called cellulose. Its impor- 

 tance lies in the fact that it gives firmness to the cell. It 



