THE OUTER TISSUES OF PLANTS. 243 



tho fronds of the higher Acrogens, there- extends a simple or 

 compound cuticular layer, formed of cells that are closely 

 united at their edges and devoid of that granular colouring 

 matter contained in the layers of cells they inclose : the 

 result being that the membrane formed of them is compara 

 tively transparent. On the submerged leaves of aquatic 

 Phaenogams, this outer layer is thin, delicate, and permeable 

 by water ; but on leaves exposed to the air, and especially 

 on their upper surfaces, it is comparatively strong, dense, 

 often smooth, and impermeable by water : being thus fitted 

 to prevent the rapid escape of the contained juices by evapo 

 ration. Similarly, while the leaves of terrestrial plants that live 

 in temperate climates, usually have comparatively thin coats 

 thus composed, in climates that are both hot and dry, leaves 

 are commonly clothed with two, three, or more layers of such 

 cells. Nor is this all, The outside of an aerial leaf differs from 

 that of a submerged leaf by containing a deposit of waxy sub 

 stance. 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. m 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 pre 



sents 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 abun - 

 dant a distribution which, while permitting free absorp 

 tion of the needful carbonic acid, puts a check on the 

 exit of watery vapour. Two general exceptions to this ar 

 rangement 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 ap- 



