60 NUTRIENT GASES. 
those of the cell-wall, having different chemical affinities, and these molecules 
again are differently grouped; nor are the passages for aqueous solutions the same. 
All this cannot but have an important bearing on the permeating capacity of 
the substances that are being interchanged. 
Although all these ideas concerning the molecular structure of cell-membranes 
and of protoplasm, concerning the intermixture and exchange of materials and 
the absorption on the part of cells and their swelling up, have only the value 
of theories, still we have good ground for assuming that they are fairly near 
the truth. They give us, at all events, an intelligible representation of the inter- 
action which takes place between living protoplasts, with their need for food, and 
the environment, which supplies the nutriment. 
2. ABSORPTION OF INORGANIC SUBSTANCES. 
Nutrient Gases.—Nutrient Salts.—Absorption of Nutrient Salts by Water-plants, Stone-plants, 
and Land-plants.—Relations between the position of Foliage-leaves and Absorption-roots. 
NUTRIENT GASES. 
One of the most important sources of the nourishment of plants is carbonic 
acid. The living protoplasts appropriate it from water and from air, in the latter 
case chiefly by attracting the carbon-dioxide This gas penetrates a cell-wall satur- 
ated with water more readily than the other constituent gases of the atmosphere 
(nitrogen and oxygen). In the wall it is converted into carbonic acid, and it then 
passes on into the cell-sap contained in the cavities of the protoplast. Apart from 
the effects of temperature and atmospheric pressure, the quantity of carbonic 
acid absorbed is chiefly determined by the requirements of the cells whose nourish- 
ment is in question. These requirements, however, vary considerably according 
to the specific constitution of the protoplasm and with the time of day. During 
daylight the need of carbon is very great in all green plants. As soon as the 
carbonic acid reaches the cell-sap it is decomposed and reduced by the action of 
sunlight, and from it are formed compounds known as carbo-hydrates. The 
oxygen thus set free is, however, removed from the cell precincts, and expelled into 
the surrounding air or water. In this way the gas when barely absorbed is 
withdrawn, as such, from the cell-sap, the carbon alone being retained and the 
oxygen eliminated, and a renewed attraction of carbon-dioxide from the sur- 
rounding medium ensues. The fresh supply again is immediately worked up in the 
green chlorophyll-bodies, so that there is a constant influx of earbon-dioxide, and 
therefore indirectly of carbonic acid, from the environment into the interior of 
green cells to the part where its consumption takes place. Were it possible to see 
1 The atmosphere contains free carbon-dioxide and not carbonic acid. But carbonic acid is formed when the 
dioxide is absorbed into water. 
