HOW THE WILD FLOWERS GROW. 21 



structure of the plant, to be aerated and spread out by 

 the leaves, which act as the lungs of the plant, throw- 

 ing out refuse or absorbing new matter from the atmo- 

 sphere. The process of evaporation and absorption is 

 aided by the peculiar organization of the epidermis 

 or skin of the leaves. The porous openings in the 

 skin are called stomata, and are really beautiful self- 

 acting valves. If we take the leaf of a lily, where 

 these stomata are large, and carefully remove the skin 

 and the chlorophyll or leaf-green, and call in the aid 

 of a microscope, we shall see that these valves consist 

 of two oval cells, as a rule, with a slit in the middle, 

 and these open directly into the hollow chambers or 

 air-cavities in the interior of the leaf. By these the 

 necessary gases are absorbed from the atmosphere, and 

 the surplus moisture of the sap evaporated. The 

 growth of the plant is now rapid. New leaves and 

 new rootlets are produced in rapid succession. At 

 first the growth of the plant is accelerated, and then 

 retarded. The " wave of growth," if I may use the 

 term, begins to recede about the middle of the plant. 

 Afterwards the force and energy of the plant is de- 

 voted to a new function that of reproduction. The 

 leaves are crowded into rosettes, or clusters, of dif- 



