118 THE ELEMENTS OF VEGETABLE HISTOLOGY 



products resulting from the activity of the cell and 

 is excreted through the stomata. In the consideration 

 of assimilation processes (Chapter XI), it will be noted 

 that carbon dioxide is consumed and oxygen produced, 

 the parts played by the two gases being exactly the 

 reverse of those observed in plant respiration. 



Transpiration. — The mineral salts absorbed by 

 the root hairs are in extremely dilute solution, and, 

 in order to secure appreciable amounts of these inor- 

 ganics, the plant must absorb an enormous amount of 

 water. Water is a necessary constituent of cell pro- 

 toplasm but the quantity absorbed is far in excess 

 of the needs of the plant. The excess water must 

 be eliminated, or excreted, by the stomata. Mention 

 of the mechanism by which the. size of the breathing 

 pore is regulated has previously been made. This 

 regulation serves to control the rate of transpiration 

 or elimination of excess water. When large amounts 

 of water are present in the guard cells and surrounding 

 tissues, the stoma is wide open so that water vapor 

 may readily pass from the intercellular spaces to the 

 atmosphere. When the amount of water in the 

 guard cells is small, they flatten out and close the 

 stoma, thus preventing evaporation from the inner 

 tissues. 



In a few genera, elimination of excess water is 

 accomplished by openings termed water pores, which 

 are located around the margin of the leaf, and which 

 communicate directly with the tubular structures 

 found in the veins of these parts. Water pores 

 are similar to stomata in structure, but the pore or 

 opening is not regulated by variations in the size of 

 the surrounding cells and is always open. 



