244 PHYSIOLOGICAL DEVELOPMENT. 



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 in- 

 direct. 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 

 substance contained in the ceils of leaves consists partly 

 of wax and partly of chlorophyll. According to Mulder, 

 " there is a genetic connexion between the production of wax 

 and that of the green colouring matter in the leaves ;" and 

 he alleges, as the result of his own experiments and those of 

 Berzelius, that chlorophyll " may be decomposed, both by 

 oxidizing and de-oxidizing substances, so as to become colour- 

 less at last ; and that wax seems to be producible from it by 

 de-oxidizing actions." Now the superficial cells of leaves 

 are more exposed to the de-oxidizing influence of light than 

 the inner cells ; those forming the upper surface are more 

 exposed to it than those forming the under surface ; and 

 those which coat leaves in hot dry climates are more exposed 

 to it than those by which leaves in temperate climates are 

 coated. May it not be that the action of light, whence 

 chlorophyll results as a transitional compound which after- 

 wards passes into a colourless compound, is an action directly 

 tending to form these bleached and transparent outer layers ; 

 and directly tending to produce a greater thickness of such 

 layers in proportion as it is intense ? There are difficulties 

 in the way of this supposition ; for I learn from Dr. Hooker 

 that some of the Balanojjhorcz, which grow in the shade, are 

 very full of wax. As these are parasites, however, and absorb 

 the prepared juices of other plants, the comparison is interfered 

 with. But whatever be its origin, we have to note that this 

 waxy substance suspended in the fluid which these bleached 



