142 PLANT PHYSIOLOGY 



STOMATA AND THE AERATING SYSTEM 



The preceding paragraph leads us to a consideration of 

 those special epidermal structures, the stomata, which are 

 the chief means by which the plant controls the transpira- 

 tion of water-vapor and the exchange of oxygen and carbon- 

 dioxide. The stomata are the guarded openings, on the 

 surface of the plant, of those intercellular spaces which form 

 throughout its body a system of continuous passages 

 through which gases, passing diosmotically into and out 

 of the adjacent living cells, make their way from and to 

 the outside air. 



Through open stomata and intercellular passages gases 

 can diffuse more rapidly than they can pass by osmosis 

 through cell-walls soaked with water. Besides the uninter- 

 rupted diffusion which these passages and openings make 

 possible, still more rapid movement of enclosed gases and 

 vapors must take place whenever the plant is agitated, 

 swayed by the wind, or by passing animals. Even changes 

 in the positions of the organs of the plant, resulting in 

 changes in the diameter of the intercellular spaces in one 

 region and a consequent change in the equilibrium of the 

 enclosed gases and vapors, will facilitate the movement of 

 gases throughout the whole aerating system. Thus the 

 constant trembling of the leaves of the aspen (Populus 

 tremula, and P. tremuloides), and often of other plants 

 also, in the slightest breeze, and the autonomic movements 

 of the leaflets of Desmodium gyrans are claimed by Stahl* 

 to increase transpiration. If they do this, they must also 

 accelerate the interchange of gases. 



The intercellular spaces which, uniting together, form with 

 the stomata the aerating system of larger massive plants, 

 .are of very different sizes according to their position and 

 according to the needs of the cells enclosing them. Between 

 the chlorophyll-containing, food-making cells of the leaf the 

 intercellular spaces are comparatively large. In the leaves 

 of plants growing in damp places they are even larger than 



* Stahl, E. f T ber den Pflanzenschlaf und verwandte Erseheinungen. Bot. { 

 Zeitung, 1897. 



