Leaves and Their Structures 19 



stoma (Greek: stoma, mouth; plural, slomata),^ which is 

 opened or closed by the expansion or contraction of the guard 



Figs. 13 and 14. Upper epidermis of "Wandering Jew" (Zebrina) leaf, on the left, and 

 lower epidermis, on the right. Si is a stoma, G a guard cell, and Sc a subsidiary cell. 

 The stomata are found only on the lower surface of the leaf. 



cells. The stomata are very important, for they connect the 

 air spaces among the cells inside the leaf with the external 

 atmosphere. When open, they allow the exchange of water 

 vapor and other gases through the epidermis; and when 

 closed, they complete the barrier to gas movements in either 

 direction. In many plants stomata occur only on the lower 

 surfaces of the leaves ; but in some plants they are found on 

 both the upper and lower leaf surfaces. 



The mesophyll. The mesophyU tissue is composed of the 

 soft, thin-walled cells that Ue among the veins in the in- 

 terior of the leaf. In most leaves there is beneath the upper 

 epidermis one or more palisade layers, which are composed of 



' Stomata are so small that 2500 of them have an area about equivalent to 

 that of an ordinary pin hole. They are so numerous, however, that they 

 occupy about ^^ of the area of the average leaf. On a square centimeter 

 of the lower surface of a sunflower leaf there are about 150,000 of them. 



