88 ^ THE NUTRITIVE FUNCTIONS. 



eeeds the supply at the roots. A copious supply of water, 

 however, usually accumulates on the leaves during the 

 cloudless nights of summer, which is absorbed through the 

 pores on their surface into their organism, and in the mor- 

 ning, the plants refreshed by the night dews, have assumed 

 their wonted rigidity and freshness. In this case the only 

 absorbents are the stomata on the epidermis. How beauti- 

 ful this provision of nature, which, at the time the soil is 

 dried to dust, causes the moisture to distil from the atmos- 

 phere on every forest leaf and blade of grass in gently 

 descending dews, to remedy as it were in some measure the 

 evils arising from an insufficiency .of water in the soil. 



The stomata or pores of plants vary in their figure and 

 size. They are, however,' usually of an oval shape with a 

 slit in the middle, and are so situated as to open directly 

 into the intercellular spaces of the parenchyma. These 

 pores may be called the vegetable governor. They regulate 

 and control the evaporation from the surface of plants, thus 

 promoting a healthy passage of the fluids through the 

 system. 



This is done on the following principle. The slit or per- 

 foration in the epidermal surface lies between two cells, 

 which differ from the other in being very hygometrical, or 

 easily affected by the moisture of the atmosphere. When 

 the air is damp, and there is an abundance of moisture in 

 the ground, these two cells become swollen and turgid, and 

 by their outward curvature open the pore and allow the 

 escape of the superfluous water ; but when the atmosphere 

 is dry they straighten and lie parallel, their sides being 

 brought into close contact. The pore is thus closed, and 

 the evaporation stopped the moment it becomes injurious 

 to the plant. 



The structure of the stomata or pores of plants may be 



