3 i8 PHYSIOLOGY 
Waterproofing vs. gases. Though water and salts might still bt 
admitted, a complete waterproofing of aerial surfaces would exclude the 
gases of the air, because all substances must enter in solution. So, as a 
matter of fact, plants possess aerial surfaces of large extent, freely per- 
meable, but shielded by covers which, while more or less waterproof, 
are perforate, so that gases have access to the moist cells underneath. 
There is one gas, oxygen, needed by almost every plant for respiration, 
which the terrestrial plants can get satisfactorily only from the atmos- 
phere. There is another gas, carbon dioxid, which is absolutely essen- 
tial for the food making of green plants, and this likewise can enter land 
plants only from the air. As the food made by green plants is the sole 
supply for them and for most other living things, even for man, and 
further is the chief source of energy for doing the world's work, it is 
evidently of some importance that the aerial parts of green plants should 
expose wet surfaces to the air and so make possible the solution and 
admission of oxygen and carbon dioxid. 
Protective tissues. The admission of oxygen and carbon dioxid 
by the smaller plants, mosses, liverworts, and the like, is made possible 
by the fact that the whole surface of the body is moist and therefore 
permeable. But the larger plants expose wet cell walls only as the bound- 
ing surfaces of internal chambers that constitute an aerating system, 
shielded by a nearly waterproof epidermis or a layer of cork tissue. 
The outer wall of the epidermis has its outermost layer so completely 
cutinized as to constitute a continuous sheet, the cuticle; and the sub- 
jacent layers are often infiltrated with cutin to a greater or less extent. 
Besides this, the epidermal cells not infrequently form wax, resin, 
and similar substances which are secreted in granules or continuous 
sheets on the outer wall. These substances all repel water, so that only 
minute amounts occupy these parts of the wall; consequently very little 
can escape into the air as vapor. On the older parts of the stem, the 
epidermis is at first underlaid, and later, sloughing off, is replaced by 
layers of cells, which, before losing their living contents, impregnate the 
walls with suberin, so that they become nearly impermeable to water 
(cork). Both these superficial waterproof tissues, epidermis and cork, 
are perforate at numerous points (stomata and lenticels), which com- 
municate with and indeed form a part of the aerating system. (See Part 
III on cutin and cork.) 
Aerating system. This is a network of canals and spaces, of the 
utmost irregularity in land plants, and connected throughout. The 
