ELEMENTARY VITAL PHENOMENA 217 



because their atoms are coupled together by very strong affinities. 

 Hence, in order to make them free and serviceable for new labours, 

 they must first be split up, and for this an introduction of energy 

 is necessary. The energy that performs this cleavage is light in 

 combination with the chemical energy of the living plant-sub- 

 stance. Without light no plant-life is possible, and since without 

 plant-life no animal-life can exist, it may be said that without 

 light no life whatever would exist. Hence, although light plays 

 an essential role as a direct source of energy only in the plant, it 

 is as indispensable for the maintenance of life upon the earth's 

 surface as the chemical energy of food. 



The places in the plant where light effects the cleavage of 

 carbonic acid are the green parts of the plant-body, and hence 

 especially the leaves. This can best be demonstrated by the 

 experiment on assimilation already described. 1 This experiment 

 shows that in the part played by the rays of light in the cleavage 

 of carbonic acid in the green plant-cell, two factors are present, 

 the intensity and the wave-length of the rays. The efficiency of 

 the light increases with the intensity, so that in a brighter light, 

 more carbonic acid is split up than in a feebler one. Moreover, 

 with the same intensity the rays of red light (not those of yellow, 

 as botanists formerly supposed) are the most effective. Engelmann 

 ('81, 1 ; '94) in a series of researches placed this beyond all doubt 

 by a microscopic method that depends upon the action on bacteria 

 of the oxygen set free in the cleavage of carbonic acid. At the 

 same time these researches confirmed the view that the cleavage 

 of carbonic acid in the green plant-cell takes place in the chloro- 

 phyll-bodies only, and established the fact that the cleavage 

 begins at once upon the admission of light and ceases immediately 

 upon darkening. Hence the dependence of this property of the 

 chlorophyll-body upon light is extremely close. 



The heat that comes into the living organism from the outside, 

 partly by radiation and partly by conduction, plays, like light, a 

 role in the chemical transformations within living substance. 

 Since with increasing temperature the power of decomposition 

 increases in all chemical compounds, the heat that is introduced 

 takes part especially in the processes of decomposition in the 

 living substance. The rdle of heat as a source of energy may be 

 recognised especially clearly in the so-called cold-blooded animals. 

 These are better termed animals possessing a changeable tempera- 

 ture (poikilothermal), since in contrast to the so-called warm- 

 blooded animals, or animals possessing a uniform temperature 

 (homothermal), the temperature of their bodies changes continually 

 with that of their environment : with a high external temperature 

 they may have a body-temperature equal to that of the warm- 

 blooded animals. When the temperature of the medium in which 



1 Cf. p. 158. 



