HOW WATER MOVES THROUGH THE PLANT. 59 



spaces are all connected, and that the stomata, \vhere they 

 occur, open also into intercellular spaces. There is here an 

 opportunity for the water vapor in the intercellular spaces to 

 escape when the stomata are open. 



104. Action of the stomata. Besides permitting the escape 

 of the water vapor when the stomata are open they serve a very 

 important office in regulating the amount of transpiration. 

 During normal transpiration the stomata remain open, that is, 

 when the amount of transpiration from the leaf is not in excess 

 of the supply of water to the leaves. But when the transpiration 

 from the leaves is in excess, as often happens, and the air 

 becomes very dry, the stomata close, and thus the rapid trans- 

 piration is checked. 



For further discussion of transpiration and root pressure see 

 the author's larger " Elementary Botany." 



Synopsis. 



Structure of a leaf 

 (cross-section). 



Epidermis. The epidermal cells usually lack chloro- 

 phyll. 



Upper epidermis, a layer of cells over the upper 



surface of the leaf. 

 Lower epidermis, a layer of cells over the 



lower surface of the leaf. 

 Guard cells of the stomates (openings in the 



epidermis) contain chlorophyll. 

 (Hairs of various kinds on different leaves are 

 often present: see synopsis of tissues at close 

 of Chapter XI.) 



Mesophyll (the cells of the leaf between the upper 

 and lower epidermis). 



1. Palisade layer of cells, usually next the 



upper epidermis. Contains chlorophyll. 



2. Loose parenchyma cells, with large inter- 



cellular spaces where the air and water 

 vapor can circulate. Cells contain chloro- 

 phyll. 



(Vascular bundles are present in the "veins" oi 

 the leaf : see Chapter XI.) 



