THE OUTEIl TISSUES OF PLANTS. 245 



outer cells contain, must be deposited as fast as the fluid 

 escapes. Where will it be deposited ? The fluid exhaling 

 through the walls of the cells next the air, will be likely to 

 leave behind the suspended substance attached to these walls. 

 On remembering the pellicle that 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 n'axy su"bstance next to the outer surfaces of 

 the cuticular cells in leaves, is probably initiated by the 

 evaporation which it eventually checks. We have here, 

 ir.deed, a ver}' simple case of equilibration. Where the loss 

 of water is too great, this waxy pellicle left behind by the 

 escaping water will protect most those individuals of the 

 species in which it is thickest or densest ; and by inheritence 

 and continual survival of the fittest, there will be established 

 in the species that thickness of the laj'er 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 

 habitually of a deeper green than the under. Microscopic 

 examination shows that this deeper green results from the 

 closer clustering of those parenchyma-cells full of chlorophyll 

 that are in some way concerned with the assimilative actions; 

 while beneath them are more numerous intercellular passages 

 communicating with those openings or stomata through which 

 is absorbed the needful air. Now when it is remembered that 

 the formation of chlorophyll is clearly traceable to the action 

 of light — when it is remembered that leaves are pale where 

 tliey are much shaded and colourless when developed in the 

 dark, as in the heart of a Cabbage — when it is remembered 

 that succulent axes and petioles, like those of Sea-kale and 

 Celery, remain white while the light is kept from them and 

 become green when exposed ; it cannot be questioned that 



