222 • BOTANY 



oxygen into the plant body is not accompanied by any respiratory 

 movements, as in the case of animals ; but takes place solely through 

 diffusion. Those cells which are in direct contact with the air or 

 water can absorb their requisite oxygen directly ; while cells in the 

 midst of tissues are dependent upon ■ the oxygen which can diffuse 

 through the surrounding cells. Such a diffusion from cell to cell 

 would not, however, be adequate, in the case of the vast cellular 

 bodies of the higher plants, to provide the living cells of the interior 

 with a sufficient supply of oxygen. This is accomplished by means of 

 the air-spaces, which, as intercellular PASSAGES, penetrate the tissues 

 in all directions and so bring to the protoplasm of the inner cells the 

 air entering through the stomata and LENTICELS (p. 143). That the 

 intercellular spaces were in direct communication with each other 

 and also with the outer atmosphere, was rendered highly probable 

 from anatomical investigation, and has been positively demonstrated 

 by physiological experiment. It is, in fact, possible to show that air 

 forced by moderate pressure into the intercellular passages makes 

 its escape through the stomata and lenticels ; and conversely, air 

 which could enter only through the stomata and lenticels can be 

 drawn out of the intercellular passages. The method of conducting 

 this experiment can be seen from the adjoining figure (Fig. 192). 

 Through the cork of the bottle (G), which is partially filled with water, 

 a glass tube (R) and a leaf (P) are inserted ; when the air in the bottle 

 (Q) is drawn out by suction through the glass tube (R), a stream of air- 

 bubbles passes out through the intercellular spaces of the severed leaf- 

 stalk, and is maintained by the air entering through the stomata of 

 the leaf-lamina. By a similar experiment it can also be shown that 

 in corky stems the communication between the intercellular spaces 

 in the medullary rays, cortex, and wood and the external atmosphere 

 is maintained through the lenticels. The movement of the gases 

 within the intercellular spaces is due partly to the diffusion, induced 

 by the constant interchange of gases caused by respiration, assimila- 

 tion, and transpiration, and partly to their own instability, arising 

 chiefly from modifications of the temperature, pressure and moisture of 

 the surrounding atmosphere, but which is also increased by the move- 

 ment of the plants themselves, through the action of the wind. 



Intercellular air-spaces are extensively developed in water and marsh plants, 

 and occupy the greater part of the plant body. The submerged portions of water- 

 plants unprovided with stomata secure a special internal atmosphere of their 

 own, with which their cells maintain an active exchange of gases. This internal 

 atmosphere is in turn replenished by the diffusion taking place with the surround- 

 ing atmosphere. In marsh-plants, which stand partly in the air, the large inter- 

 cellular spaces form connecting canals through which the atmospheric oxygen, 

 without being completely exhausted, can reach the organs growing deep in the 

 swampy soil, surrounded by marsh-gas and otherwise cut off from any communica- 

 tion with the atmosphere. 



