62 NUTRIENT GASES. 
air around them. The chief members of this class are those mosses, liverworts, and 
lichens which, though clinging to dry rocks, behave just like water-plants as regards 
the absorption of carbonic acid. There is no reason to think that these plants 
absorb carbonic acid in dry weather; for under the influence of dry air they lose 
water fast, and meanwhile receive no compensation from the rock to which they 
are attached, and in a short time they become so dry that they crumble into 
powder when rubbed between the fingers. Vitality is suspended for a time, and 
it is out of the question that there should be any absorption of carbon-dioxide 
from the atmosphere under such circumstances. But the moment the plant is 
moistened by rain or dew, the cell-walls directly exposed to the air become 
saturated, and are enabled to admit water into the interior. Then the lithophytes 
suck up water very fast; the dry, apparently dead, inerustations swell up again, 
and, together with the rain and dew, carbonic acid is absorbed, it being contained 
in all depositions of atmospheric moisture. A tumescent moss tuft can, in addi- 
tion, absorb carbon-dioxide direct from the atmosphere through its saturated 
superficial cells; but the quantity of carbonic acid thus acquired by a plant is in 
any case only secondary. Many mosses, as for example the widely-distributed Grim- 
mia apocarpa, are also able to live just as well under water as in air; nor is any 
alteration of their leaves necessary in either condition, nor any special contrivance 
for the absorption of carbonic acid and water. These substances reach the interior 
by similar passage through cell-walls of identical construction, whether the 
Grimmia spends its life attached to submerged rocks or in the open air at the 
top of a mountain; whence we may infer that there is a greater resemblance 
between lithophytes and water-plants as regards nutrition than between litho- 
phytes and land-plants. 
Land-plants satisfy their need of carbon almost exclusively by withdrawing 
the dioxide from atmospheric air. For the purpose of this direct appropriation, 
specially adapted structures are found in them. Seeing that these plants are 
not able to endure periodic desiccation in times of drought, as lithophytes are, 
it is necessary for them to be secured against excessive loss of water. Accord- 
ingly, the cell-walls in immediate contact with the air, that is to say, the outer 
walls of the epidermis, are thickened by a layer (cuticle) which is impermeable 
by air or water, and, in general, they are so organized that water cannot readily 
escape from the interior of the cells. Obviously, however, a cell-wall which opposes 
a strong resistance to the extravasation of water will not give easy admittance to an 
influx either, and the conditions for the passage of gases through a cell-membrane, 
thickened and cuticularized in this way, would be far from favourable. As a 
matter of fact many of the constituent gases of the atmosphere permeate these 
thickened walls of the epidermal cells only with great difficulty, and others not at 
all. Carbon-dioxide alone has the power of penetrating, but even in the case of 
this gas the quantity is not always sufficient to satisfy the demand. To ensure 
that so important a form of plant-food should reach in proper amount those cells 
lying under the epidermis, which are occupied by protoplasts engaged in the regu- 
