XL'TKIEXT GASES. 63 



latiou of uiitritiou, there is an adaptation of structure of the following nature. 

 Amonw the firmly connected epidermal cells with their thickened outer walls al- 

 most impervious to air, other cells ai-e interspersed at intervals. They are always 

 in pairs, are generally rather smaller than the rest, and have a little cleft open 

 between them. Inasmuch as these apertures (stomata) always exist where passages 

 and canals, the so-called intercellular spaces, have arisen from the separation of 

 individual cells of the sub-epidermal tissues, each stoma constitutes the mouth of a 

 system of channels ramifying between the thin-walled cells of the interior. The 

 components of the atmosphere, especially carbon-dioxide, are able to reach these 

 internal passages through the stomata, and in them they travel to the chlorophyll- 

 containing cells. Through the thin, saturated walls of these cells they are able to 

 penetrate with ease, and so they reach the living protoplasts, with their equipment 

 of chlorophyll, whose daily work it is, as already mentioned, to decompose — under 

 the transforming power of light — the carbonic acid as it reaches the chlorophyll- 

 bodies, to work up the carbon and expel by the same path as they entered not only 

 the oxygen but also all other aerial constituents which may have penetrated and for 

 the moment find no employment. 



These ventilation-canals, with stomata as orifices at the epidermis, have other uses 

 besides the importation of carbon-dioxide (and therefore of carbonic acid) and the 

 exportation of oxygen. For the same pores, passages, and lacunae, as serve for the 

 influx and exit of carbon-dioxide and oxygen respectively, are the channels of a 

 plant's respiration. Moreover, they play a very important part also in the escape 

 of aqueous vapour, the process known as "transpiration;" and as the variety in 

 their structure is to be interpreted chiefly as an adaptation to the diflerent condi- 

 tions under which transpiration occurs, it cannot be profitably discussed until we 

 treat of that jjrocess. 



Those saprophytes and parasites which contain no chlorophyll or practically 

 none, do not absorb any fi-ee carbon-dioxide from the atmosphere, but supply them- 

 selves with carbon from the oi-ganic compounds in the nutrient substratum on 

 which they grow. But saprophytes and parasites, abundantly furnished with 

 chlorophyll, douljtless do attract free carbon-dioxide in addition. They may do so 

 either after the manner of water-plants and lithophytes, as is the case with Euglenje, 

 and with mosses growing on the dung of mammalia; or else after the manner of 

 land-plants, as instances of which the cow-wheat, yellow-rattle, and eye-bright may 

 be (juoted. 



It is a very remarkable fact that no plant is known which takes up carbon- 

 dioxide or carbonic acid from the earth. One might expect that the roots of land- 

 plants at any rate, ramifying as they do in a stratum of earth saturated with water 

 containing carbonic acid in solution, would suck up to some extent so important a 

 food, and that it would be from them conducted to the green -foliage leaves. But 

 so far as experiments have gone, they indicate that this is not the ease. 



Ecjually curious is the circumstance that nitrogen, which is an indispensable 

 constituent of protoplasm, and therefore a very important means of subsistence, is 



