260 PHYSIOLOGICAL DEVELOPMENT. 



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 differen- 

 tiations of structure and function; and we have to consider 

 the relations between these and the environing forces. 



Over the whole surface of every phaanogamic 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 Phaenogams, this outer 

 layer is thin, delicate, and permeable by water; but on 

 leaves exposed to the air, and especially on their upper sur- 

 faces, is comparatively strong, dense, often smooth and 

 impermeable by water: being thus fitted to prevent the 

 rapid escape of the contained juices by evaporation. Simi- 

 larly, while the leaves of terrestrial plants which live in tem- 

 perate 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 aerial 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 con- 

 cerned. 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 



