20 HANDBOOK OF PHYSIOLOGY. 



table cells are peculiarly associated with the possession of chlorophyll 

 and that these operations are dependent upon the light of the sun. It 

 has been further shown that a solution of chlorophyll has a definite 

 absorption spectrum when examined with the spectroscope,, and that it 

 is particularly those parts of the solar spectrum corresponding to these 

 absorption bands which are chiefly active in the decomposition of car- 

 bonic anhydride, and that, moreover,, the position of the maximum absorp- 

 tion corresponds with the maximum of energy of light. In the synthet- 

 ical processes of the plant then, by aid of its chlorophyll, the radiant 

 energy of the sun's rays becomes stored up or rendered potential in the 

 products formed. The potential energy is set free, or is again made 

 kinetic, when these products simply by combustion produce heat, or 

 when they are taken into the animal organism and used as food and to 

 produce heat and motion. 



The influence of light is not an absolute essential to animal life; in- 

 deed, it is said not to increase the metabolism of animal tissue to any 

 extent, and the animal cell does not receive its energy directly from the 

 sun's light, nor yet to any extent from the sun's heat, but from the 

 products formed by vegetable metabolism supplied as food, either di- 

 rectly, as in the case of herbivora, or indirectly in the case of carniyora. 

 The potential energy of these food stuffs is set free in the destructive 

 metabolism of the animal cell already alluded to. But it must be always 

 recollected that anabolism is not peculiar to vegetable, or katabolism to 

 animal cells; both processes go on in each, but the chief function, as 

 far as we know at present of the former, is to transform kinetic into po- 

 tential energy, and of the latter to render potential energy kinetic, as 

 in heat, motion, and electricity. 



With reference to the food of plants, it should not be forgotten that 

 some of the lowest forms of vegetable life, e.g., the bacteria, will live 

 only in a highly albuminous medium, and in fact seem to require for 

 their growth elements of food stuffs which we shall see later on are es- 

 sential to animal life. In their metabolism, too, they very closely ap- 

 proximate to animal cells, not only requiring an atmosphere of oxygen, 

 but giving out carbonic anhydride freely, and secreting and excreting 

 many very complicated nitrogenous bodies, as well as forming proteid, 

 carbohydrates, and fat, requiring heat but not light for the due perform- 

 ance of their functions. It must be added, however, that certain bac- 

 teria grow only in the absence of oxygen. 



(3.) There is, commonly, a difference in general chemical composition 

 between vegetables and animals, even in their lowest forms; for associated 

 with the protoplasm of the former is a considerable amount of cellulose, 

 a substance closely allied to starch and containing carbon, hydrogen, and 

 oxygen only. The presence of cellulose in animals is much more rare 



