NERVOUS AGENT OF PLANTS. 99 



of animals, no matter whether the mechanism be the lungs of a mammalian or the 

 branchial organs of fishes. Every variety of breathing apparatus has for its object the 

 evolution of heat by the oxidation of carbon, or of carbon and hydrogen conjointly ; 

 but the object of the agency of leaves upon the air is to obtain from it carbon, or car- 

 bon and hydrogen. At certain periods of their history, plants themselves become ma- 

 chines of combustion, when the process of fertilization requires that for a time, in a 

 given place, and for a specific object, there should be an elevation of temperature. Re- 

 sort is then had to those same processes which obtain in animal systems ; and sugar, 

 or what comes to the same thing, honey, is burned. 



CHAPTER XII. 



ON THE NERVOUS AGENT OF PLANTS. 



CONTEXTS : Subdivisions of Nervous Mechanism in Animals. Excessive Sapidity of 

 Motion arising in these Nervous Actions. Plants constructed on a Surface-type. Ox- 

 idating Processes replaced in them by the Application of Radiant Heat. Difference of 

 Action on the Upper and Under Face of the Leaf. Light applied to one, and Heat to 

 the other Face. Specific Effects produced by the different-coloured Rays. Effects of 

 these Radiant Principles on the Lower Tribes of Animals. Centralization of Appa- 

 ratus for different Functions. Analogies between Nervous Action in Animals and 

 Imponderable Agency in Plants. Vegetables are the Representatives of the Resultant 

 Action of the Ethereal Agents on Ponderable Matter. Conclusion. 



388. IT was the beautiful discoveries of Sir C. BELL which first forcibly drew the 

 attention of physiologists to the fact that different portions of the nervous system are 

 devoted to different functions ; that in the spinal axb there is one column devoted to 

 sensation, and another to motion. This division of offices is doubtless carried to a far 

 greater extent than we have at present any means of proving. Analogy would lead us 

 to suppose that every function is represented by its own appropriate mechanism. It 

 is not alone in the great and more striking characteristic divisions of action that these 

 divisions of machinery are observed ; the cerebrum, the cerebellum, or the sympathetic 

 system, being each devoted to its specific end, they doubtless, also, exist on a far more 

 minute scale, in connexion with more trivial purposes. 



389. If, thus, the intellectual processes and processes of movement, which are things 

 appertaining to the interior constitution of the animal system, are under the control of 

 a divided agency, a similar plan is resorted to in the case of those functions which put 

 the system in relation and communication with the exterior world. On these, the out- 

 ward physical agents have to expend their operation. The optic nerve, which gath- 

 ers on its retinal expansion the images of outward forms, transmits them to the brain. 

 To that cerebral tract to which it goes, the power is given to be affected by luminous 



