DIFFERENCES BETWEEN ANIMALS AND PLANTS 13 



those which do not possess it. It is probably present only in slight degree 

 as one of the properties of animal protoplasm. 



It must be recollected, however, that chlorophyll without the aid of the 

 light of the sun can do nothing in the way of building up substances, and a 

 plant containing chlorophyll when placed in the dark, while it continues to 

 live, and that is not as a rule long, acts as though it did not contain any of 

 that substance. It is an interesting fact that certain of the bacteria have the 

 chlorophyll replaced by a similar pigment which is able to decompose carbon 

 dioxide gas. 



Animal cells do not possess the power of building up or synthesizing from 

 simple materials; their activity is chiefly exercised in the opposite direction, 

 viz., they have brought to them as food the complicated compounds pro- 

 duced by the vegetable kingdom. With these foods they are able to perform 

 their complex functions, setting free energy in the direction of heat, motion, 

 and electricity, and at the same time eliminating such bodies as carbon di- 

 oxide and water, and producing other bodies, many of which contain nitrogen 

 but are derived from decomposition. 



With reference to the substance chlorophyll it is necessary to say a few 

 words. It has been noted that the synthetical operations of vegetable 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 carbon dioxide. In the synthetical 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 chemical products 

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

 these products by simple 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; indeed, 

 it is said not to increase the metabolism of animal tissue to any great 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 potential energy of the 

 food stuffs. But it must be always kept in mind that anabolism is not pecu- 

 liar to vegetable, or katabolism to animal cells; both processes go on in each. 

 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 are essential to animal life. In their metabo- 

 lism, too, they very closely approximate animal cells, not only requiring an 

 atmosphere of oxygen, but giving out carbon dioxide freely, and secreting 

 and excreting many very complicated nitrogenous bodies, as well as forming 

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



