i8 



Science of Plant Life 



the leaf. This layer is called the cuticle. It is useful to the 

 plant because water does not pass through it readily, and it 



" ^<^ -^ j-^"^ ^Uppcr epidermis 



\ Palisade layers 



I Intercellukr 

 y^ 3pace 



V-. \ -^ <\ \y^^ layers^ 



^\ 





Lower 

 epidermis 



Fig. 12. Model of a small piece of vinca leaf, showing cells and tissues. 



protects the plant from water loss. It may be compared to 

 the enamel covering of oilcloth and acts in much the same 

 way. The cuticle is useful to the plant also because it serves 

 as a first Hne of defense against disease germs. The impor- 

 tance of the epidermis as a protective covering for the delicate 

 inner tissues of the plant may be judged from the drying and 

 decay that follow the breaking of the thin epidermal coat of 

 an apple or a pear. 



Scattered among the colorless cells of the epidermis are 

 pairs of small, crescent-shaped green cells, the guard cells. 

 Each pair of these surrounds a small opening or pore, the 



