G2 XUTBIEXT 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, incrustations 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 

 1 »'t ween lithoplrytes 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 nut 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 Ear 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 

 thai 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- 



