Comparative Physiology. 575 



toiy motion, it would tend to confirm this oj^inion. Old wood, 

 being comparatively inert, could have no share in such a process. 



A great deal of action undoubtedly takes place in the leaf; 

 plants requiring to j)repare organic compounds from their ele- 

 ments principally, or wholly, require greater chemical power to 

 assist organic action. From the similarity between organic action 

 and chemical, and from chemistry being able to imitate some 

 organic actions, it has been assumed by some that the whole 

 action taking place there is chemical ; and it has been customary 

 to talk of the peculiar secretions as prepared there, and sent 

 down the channels of conveyance in a prepared state. It is 

 more likely, however, that, as in animals, the elaborated sap of 

 the plant, like the blood of the animal, contains only matter 

 capable of enabling the secreting and assimilating organs to 

 prepare their several products, each in its own way. The 

 sap has not yet been analysed at its outset from the leaf, so as 

 to show whether it contains the peculiar secretions in a prepared 

 state, or only the proximate principles necessary to enable the 

 organs to perform their functions, which is more likely. The 

 action is also probably as much organical as chemical, though 

 it will be difficult to define between these; yet the chemical 

 power of light cannot separate carbonic acid, water, and ammonia 

 into their elements, without the organic action of the leaf, in the 

 same manner as it does by its assistance ; and, though the leaf 

 cannot perform its action without the assistance of the light, 

 the latter may perhaps act greatly by stimulating the organic 

 action of the leaf, as well as by its chemical powers. The full 

 intense white light of the sun is necessary to produce perfect 

 action ; the blue rays are the most indispensable, showing the 

 chemical power resident in them to be the most necessary ; but 

 they must have organic assistance, and are most efficient when 

 combined with all the rays into the perfect sunbeam. 



The laticiferous vessels cannot be the main channel of the 

 returning sap, as they do not contain fluids capable of supplying 

 the wants of the nutritive or assimilating organs. According 

 to the analysis given by Professor Thomson of the milky juices 

 (^Vegetable Chemistry, p. 792, 793.), they abound mostly, be- 

 sides water 73 to 77 per cent, in fixed oils, resins, caoutchouc, 

 and cerin and myricin, the constituents of bees' wax ; all sub- 

 stances containing a superabundance of hydrogen, unfitting 

 them for assimilation, independently of their being in a state 

 requiring decomposition before becoming nutritive, for which 

 there is no apparent reason : of nitrogen, an indispensable 

 requisite, they contain only a trace. It is clear, therefore, that 

 the fluids circulating in the laticiferous vessels are unfit for the 

 general nutrition of the plant ; and, as these have been deno- 

 minated latex and peculiar juices, that either of these terms 



p I' 3 



