368 TRANSMISSION OF THE FOOD-GASES. 



withered turf and dry scales, as if dead. But as soon as they are moistened, or can 

 condense moisture from the air, they are aroused to renewed vitality, and then suck 

 up with great eagerness atmospheric water, which always contains small quantities 

 of carbonic acid gas, and also traces of nitric acid. In the rock-inhabiting mosses 

 the cells, which absorb water from the atmosphere containing carbonic acid, are also 

 those in which the decomposition of carbonic acid takes place. In this respect 

 these mosses behave exactly like aquatic plants; nor is it perhaps superfluous 

 here again to point out the interesting fact already mentioned, that there are mosses 

 which permanently live under water, and there behave like true water plants, 

 though they are able equally to live on rocks, where they remain dried up for 

 weeks together, and only resume their activity when wetted by rain. It is to be 

 taken for granted that such damp, water-saturated mosses have the capacity of 

 absorbing carbon dioxide from the surrounding atmosphere. The carbon dioxide is 

 changed into carbonic acid by its passage through the cell-wall saturated with 

 water. Probably it is only when carbonic acid is dissolved in water that it reaches 

 the active protoplasm in the cells in question. In lichens the carbonic acid which 

 reaches the protoplasm provided with chlorophyll is also dissolved in water; 

 however, in most lichens the green cells do not come in contact with the atmosphere, 

 but are separated from it by a layer of hyphal threads. Thus the conduction to 

 the green cells takes place by means of the hyphal layer destitute of chlorophyll. 



In land plants also the cells which are filled with chlorophyll-bearing protoplasm 

 seldom come directly into contact with the atmosphere; usually the green tissue is 

 surrounded with an actual mantle of water. That is to say, the cavity of each 

 epidermal cell contains very watery fluid, or, in other words, in the fully-formed 

 epidermal cells the protoplasm constitutes merely the parietal layers without 

 chlorophyll, their large cavities being filled with water. These epidermal cells fit 

 closely to each other, and on the upper side of the leaf are only rarely interrupted 

 by stomata. Usually the epidermis on the upper side of the leaf gives rise to a 

 layer of cells with clear watery contents, directly bordering on the green palisade 

 tissue; and as the carbon dioxide of the atmosphere has to pass from the upper side 

 to this green tissue, it must first of all pass through this watery cell-layer of the 

 epidermis. There it becomes changed into carbonic acid, and passes from this 

 epidermal sphere of activity, not in the form of gas, but dissolved in water, to the 

 cells of the palisade tissue below. Since the green palisade tissue under the 

 influence of sunlight uses up the carbonic acid in the manufacture of organic 

 material, it becomes a centre of attraction for this acid as long as the illumination 

 continues. At first the carbonic-acid-bearing contents of the contiguous cells are 

 eagerly absorbed, and indirectly carbon dioxide also is drawn from the surrounding 

 air and made to force its way into the epidermal cells. The cell-wall offers no 

 great resistance to this entrance. It has been proved that carbonic acid, or rather 

 carbon dioxide, passes very easily through the cell- wall. According to all this, it is 

 evident that the small quantity of carbon dioxide is drawn from the air by the 

 green illumimated tissue of the leaves and stem, that carbon dioxide streams 



