§ 274. From this most general contrast between aërial surface-tissues--those of axes and those of folia--we pass now to the more special contrasts of like kind existing in folia themselves. Leaves present us with superficial differentiations of structure and function; and we have to consider the relations between these and the environing forces. Over the whole surface of every phænogamic leaf, as over the fronds of the _Pteridophyta_, there extends a simple or compound epidermal layer, formed of cells that are closely united at their edges and devoid (in the Flowering Plants) of that granular colouring matter (chlorophyll) contained in the layers of cells they inclose: the result being that the membrane formed of them is comparatively transparent. On the submerged leaves of aquatic Phænogams, this outer layer is thin, delicate, and permeable by water; but on leaves exposed to the air, and especially on their upper surfaces, is comparatively strong, dense, often smooth and impermeable by water: being thus fitted to prevent the rapid escape of the contained juices by evaporation. Similarly, while the leaves of terrestrial plants which live in temperate climates, usually have comparatively thin coats thus composed, in climates that are both hot and dry, leaves are commonly clothed with a very thick cuticle. Nor is this all. The outside of an aërial leaf differs from that of a submerged leaf by containing a deposit of waxy substance. Whether this be exuded by the exposed surfaces of the cells, as some contend, or whether it is deposited within the cells, as thought by others, matters not in so far as the general result is concerned. In either case a waterproof coating is formed at the outermost sides of these outermost cells; and in many cases produces that polish by which the upper surface of the leaf is more or less distinguished from the under surface. This external pellicle presents us with another contrast of allied meaning. On the upper surfaces of leaves subject to the direct action of the sun’s rays, there are either few or none of those minute openings, or stomata, through which gases can enter or escape; but on the under surfaces these stomata are abundant: a distribution which, while permitting free absorption of the needful carbonic acid, puts a check on the exit of watery vapour. Two general exceptions to this arrangement may be noted. Leaves that float on the water have all their stomata on their upper sides, and leaves that are submerged have no stomata--modifications obviously appropriate to the conditions. What is to be said respecting the genesis of these differentiations? For the last there seems no direct cause: its cause must be indirect. The unlike actions to which the upper and under surfaces of leaves are subject, have no apparent tendency to produce unlikeness in the number of their breathing holes. Here the natural selection of spontaneous variations furnishes the only feasible explanation. For the first,