DIFFERENCES BETWEEN ANIMALS AND PLANTS 13 



to do in consequence of their containing a certain coloring matter called 

 chlorophyll, the presence of which is the cause of the green hue of plants. 

 In all plants which contain chlorophyll two processes are constantly going 

 on when they are exposed to light: one, which is called true respiration and 

 is a process common to animal and vegetable cells alike, consists in the 

 taking of the oxygen from the atmosphere and the giving out of carbon 

 dioxide; the other, which is peculiar apparently to bodies containing chloro- 

 phyll, consists in the taking in of carbon dioxide and the giving out of oxygen. 

 It seems that the chlorophyll is capable of decomposing the carbon dioxide 

 gas and of fixing the carbon in the structures in the form of new compounds, 

 one of the most rapidly formed of which is starch. 



Vegetable protoplasm by the aid of its chlorophyll is able to build up a 

 large number of bodies besides starch, the most interesting and important 

 being protein or albumin. It appears to be a fact that the power which 

 plants possess of elaborate chemical synthesis is to a large extent dependent 

 upon the chlorophyll they contain. Thus the power is present to a marked 

 extent only in the plants in which chlorophyll is found, and is absent in 

 those saphrophytic plants which do not possess chlorophyll. 



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, though 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, though higher organic synthesis can no longer be ques- 

 tioned. Their activity is chiefly exercised in the opposite direction, viz., the 

 oxidations of the complicated compounds produced by the vegetable kingdom 

 which they have brought to them as foods. With these foods animals 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 dioxide and water, and producing other bodies, many of which contain 

 nitrogen but are derived from decomposition. 



With reference to the substance chlorophyll 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 

 when examined with the spectroscope reveals a definite absorption spectrum, 

 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 ren- 



