THE OUTER TISSUES OF PLANTS. 261 



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 ar- 

 rangement may be noted. Leaves that float on the water 

 have all their stomata on their upper sides, and leaves that 

 are Bubmerged have no stomata — modifications obviously ap- 

 propriate 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, however, there is a 

 possible cause in the immediate actions of incident forces, 

 which survival of the fittest continually furthers. 



The fluid exhaling through the walls of the cells next the 

 air, will be likely to leave behind suspended substances on 

 their outer surfaces. On remembering the pellicle which is apt 

 to form on thick solutions or emulsions as they dry, and how 

 this pellicle as it grows retards the further drying, it will be 

 perceived that the deposit of waxy matter next to the outer 

 surfaces of the cuticular cells in leaves, is not improbably 

 initiated by the evaporation which it eventually checks. 

 Should it be so, there results a very simple case of equilibra- 

 tion. Where the loss of water is too great, this waxy pellicle 

 left behind by the escaping water will protect most those in- 

 dividuals of the species in which it is thickest or densest ; and 

 by inheritance and continual survival of the fittest, there will 

 be established in the species that thickness of the layer which 

 brings the evaporation to a balance with the supply of water. 



Another superficial differentiation, still more familiar, has 

 to be noted. Every child soon learns to distinguish by its 

 colour the upper side of a leaf from its under side, if the leaf 

 is one that has grown in such way as to establish the rela- 

 tions of upper and under. The upper surfaces of leaves are 



