RELATION TO ENVIRONMENT 499 



ABSORPTION OF FOOD 



Where a plant is completely submersed, the outer cells of all parts 

 are capable of absorbing water with the various food elements in 

 solution. In terrestrial plants this is of course impossible, and these 

 plants must have special organs for the absorption of food. For the 

 absorption of C0 2 from the atmosphere, the green parts are provided 

 with stomata, which alone permit the entrance of gases from the 

 atmosphere. For the absorption of water and dissolved mineral 

 salts, the roots are the chief agents, serving not merely as organs of 

 attachment, as in aquatic plants, but also as absorbents of water, both 

 to supply the loss due to transpiration, and as a vehicle for the 

 transport of the food constituents from the earth. 



In most land plants there are well-developed special organs for 

 the assimilation of C0 2 , leaves which are far better developed than 



FIG. 467. Acer saccharinum, showing the arrangement of the leaves to avoid 

 shading. (After BAILEY.) 



in even the highest of the Seaweeds. The leaf, in order to insure 

 firmness, is in the most perfect forms provided with a complicated 

 skeleton of woody fibres, the veins, between which are placed the 

 spongy green cells which are concerned with photosynthesis. Cover- 

 ing it is the epidermis, checking loss of water except through the sto- 

 mata, which communicate with the intercellular spaces of the green 

 mesophyll. Stomata are never found except upon aerial organs. 



The development of these special organs, and the segregation of 

 special functions, necessitates a much more perfect system of con- 

 ductive tissues than is found in aquatic organisms, and these tissues 

 are best developed among the higher terrestrial plants. 



The conditions which determine plant growth i.e. light, heat, 

 moisture, and food are of course variable in quantity, and we find, 

 as might be expected, that the plant-organism varies in response to 

 changes in these life-conditions. 



