NUTRIENT GASES. 61 
the molecules of carbon-dioxide in the air, we should observe how much faster they 
are impelled towards the leaves and other green parts of plants, where the intense 
craving for carbon is localized, than are the other constituent particles of the air. 
This impulsion and influx lasts so long as the green cells are under the influence of 
daylight. The first thing in the morning when the first ray of sunshine falls upon 
a plant the protoplasts begin work in their little laboratories decomposing carbonic 
acid, and producing from it sugar, starch, and other similar organic compounds. 
And it is not till the sun sets that this work is suspended, and the influx of carbon- 
dioxide stopped till the following morning. 
The green plants that spend all their lives under water are supplied with car- 
bonic acid by the water surrounding their cells, which always contains some of that 
material. In the case of unicellular plants of this class, absorption of carbonic acid 
takes place through the whole surface of the cell-membrane. Multicellular plants, 
with their cells arranged in filaments or plates, only take in carbonie acid through 
those parts of the walls of their cells which are in immediate contact with the 
water. This applies also to submerged plants composed of several layers of cells 
and of considerable dimensions. Thus, in plants of this kind, the cells in contact 
with the water constitute the skin. They are always pressed closely together 
and squeezed flat, are not thickened on the side exposed to the water, and are 
united everywhere edge to edge leaving no gaps. But in the interior of these 
water-plants large lacun® and cavities are formed from earliest youth, owing to 
the detachment of single rows of cells; and the spaces so formed are filled with 
a quantity of nitrogen, oxygen, and carbon-dioxide, that is to say, with a gaseous 
mixture not essentially different from atmospheric air. Although this organiza- 
tion may have as its primary object the reduction of the plant’s weight as a 
whole, it cannot be without a further importance inasmuch as carbonic acid can 
be taken up from the air-spaces into adjacent cells. But there is no doubt that, 
even in this case (of water-plants provided with large internal air-cavities), 
the chief absorption of carbonic acid is through the epidermis, or more precisely 
through those walls of the epidermal cells which are in immediate contact with 
the water. 
The carbonic acid taken up by cells, wholly or partially immersed in water, 
is either contained as such dissolved in the watery medium, or occurs in com- 
bination with calcium as bicarbonate of lime. Part of the carbonic acid in this 
bicarbonate in aqueous solution is susceptible of being withdrawn by water-plants, 
mono-carbonate of lime, which is insoluble in water, being then precipitated on 
the cell-wall through which the rest of the carbonic acid has passed into the 
cell-interior. Accordingly, a large number of water-plants are found inerusted 
with lime in both fresh and salt water. We shall return to this important pheno- 
menon when we treat of the influence of living plants on that part of the environ- 
ment which comes within their sphere of action for purposes of nutrition. 
Lithophytes obtain carbonic acid from the moisture deposited upon them from 
the aqueous vapour in the atmosphere, and attract carbon-dioxide direct from the 
