Leaves and Their Structures 



17 



supports the soft cytoplasm as the wax of the honeycomb sup- 

 ports the honey within, and it helps to give stiffness to all 

 parts of the plant. You have 

 seen pure cellulose in the form 

 of cotton. Filter paper and 

 most book papers are made of 

 cellulose fibers derived from 

 wood. Water passes freely 

 through the cellulose walls of 

 plant cells, as do most substances 

 that are dissolved in the water. 



Animals, as well as plants, are 

 composed of cells ; but the 

 animal cell, instead of having a 

 stiff cellulose wall like a plant 

 cell, has a soft wall, or, as in the 

 case of nerve cells and white 

 blood corpuscles, it may lack a 

 wall entirely. Consequently, the 

 tissues of animals (except the 



skeletal tissues) are usually softer and more pliable than 

 plant tissues. This makes it easy for an animal to bend and 

 to move about. The difference in cell walls and in the 

 phabihty of tissues is so general throughout the plant and 

 animal kingdoms, that it is one of the important distinc- 

 tions between plants and animals. 



The epidermis and the stomata. The cells of the epidermis 

 are flat, irregularly shaped, closely united, and, for the most 

 part, colorless. The cell walls on the side of the epidermis 

 which is exposed to the air become thickened with a waxlike 

 material called cutiuj which forms a layer over the surface of 



Fig. II. Plant cells: A is from a 

 moss leaf; B is from a squash-vine 

 hair; C is a starch-filled cell from a 

 potato tuber; and Z? is a cell from 

 the palisade layer of a leaf. E shows 

 a cell in cross section. 



