How Plants Draw in Various Materials 191 



accompanying figure 66, .4. Here, for simplicity, the passages, 

 represented in black, are imagined to fall into one plane; and here 

 also, by the way, a partial interruption in the system due to 

 the presence of the longitudinally-running woody bundles is 

 shown by the blank spaces. In young green tissues, as shown by 

 the detailed diagram (figure 66, B), in which, as in C and D, the air 

 passages are partially reduced to one plane, the passages open 

 through the epidermis by the stomata, while on older stems, 

 where a corky bark has formed, they open through the lenticels 

 (figure 66, C), those corky wart-like excrescences prominent on all 

 young stems, and consisting simply of open gashes in the bark, 

 partially sealed in the winter by corky cortical cells. In young 

 roots, however (figure 60, D), neither stomata nor lenticels are 

 present, but the continuous epidermis and hairs are commonly 

 and normally covered with films of water, through which the 

 gases diffuse in solution from the air spaces in the soil to those in 

 the root, and vice versa. 



Thus much for the aeration system, whereby every living cell 

 of the plant is brought into communication with the external 

 reservoir the air. But what is the power impelling the gases 

 along these passages, — which are often of great length, small size, 

 and extreme irregularity? Plants possess no mechanism for the 

 forcible indrawing and expulsion of the air en masse, such as 

 animals have developed in their muscular chest-and-lung breath- 

 ing arrangements. In some degree a movement of air through 

 the inter-cellular system is promoted by the swaying of parts in 

 the wind, and by the expansions and contractions of the air under 

 varying temperature and barometric pressure; but such effects 

 are insignificant. The primary cause of the gas movement is 

 found in diffusion, that process, already described, whereby the 

 molecules, driven by the energy of heat absorbed from the sur- 

 roundings, tend ever to move forward from places of greater to 

 places of lesser concentration, and therefore from places where 

 they are being formed or released to places where they are not. 



