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. 1 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 withdrawn, as such, from the cell-sap, the carbon alone being retained and the 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 carbon- 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. 



