NUTRIENT GASES. 63 
lation of nutrition, there is an adaptation of structure of the following nature. 
Among the firmly connected epidermal cells with their thickened outer walls al- 
most impervious to air, other cells are 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 lacunz, 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 different condi- 
tions under which transpiration occurs, it cannot be profitably discussed until we 
treat of that process. 
Those saprophytes and parasites which contain no chlorophyll or practically 
none, do not absorb any free carbon-dioxide from the atmosphere, but supply them- 
selves with carbon from the organic compounds in the nutrient substratum on 
which they grow. But saprophytes and parasites, abundantly furnished with 
chlorophyll, doubtless 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 Euglene, 
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 quoted. 
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 case. 
Equally curious is the circumstance that nitrogen, which is an indispensable 
constituent of protoplasm, and therefore a very important means of subsistence, is 
