104 = _ The American Naturalist. 
coefficient of absorption and the density of the gas. Cuticular — 
and corky formations also permit the passage of gases when. 
dry. Thus we see, that gases may be forced through the 
stomata, or breathing pores, by varying pressure, but can only 
pass through the epidermis and bark of plants by diffusion. 
We therefore arrive at the conclusion that the gases inside and 
outside of the plant are brought to an equilibrium by direct 
interchange through the stomata and intercellular spaces, 
aided by the comparatively slow process of diffusion through — : 
the whole surface of the plant, both above and below ground. 
In CONCLUSION. 
After so long a discussion of the subject of the origin, kind 
and movement of gases in plants, I trust I have not only 
brought out the main facts regarding our present knowledge, 
but also made evident some of the numerous directions in — 
which further experiment and research is needed. It would 
be most appropriate to transfer the activity in this subject to 
the laboratories of this country, and let the Americans take up 
the line of discoveries carried on so far by the English, French, 
and lastly, the German scientists: 
