FOOD DIGESTION AND RESPIRATION 51 



We have, first of all, to convince ourselves that a green plant 

 is able to make use, in some way, of the carbon dioxide in the 

 air as a source of carbon to build up the complex compounds of 

 its own structure. These compounds afterwards serve as carbon 

 food for animals. 



If we grow a plant from the seed in such a way that it can get 

 no carbon except from the atmosphere, we find, nevertheless, that 

 its bulk increases far more than would be possible by the use 

 only of the material originally present in the seed (E., p. 183). It 

 must have obtained its carbon from the atmosphere. 



Another experiment which should be made is the following 

 (E., p. 183): Fill a vessel, under which a green plant is growing, 

 with expired air from the lungs. Take a sample of the air at 

 once and determine the percentage of oxygen and of carbon 

 dioxide in it by gas analysis The oxygen will be low, the carbon 

 dioxide high, as compared with atmospheric air. Expose to 

 sunlight for a day or two. Determine the composition of the 

 gas again. The oxygen will have increased, the carbon dioxide 

 decreased. Therefore, oxygen has been produced from carbon 

 dioxide. If the experiment be done in the dark, this will not 

 happen. In fact, if the gas analysis is very accurate, the opposite 

 will be seen to have occurred, namely, an increase of carbon 

 dioxide and a decrease of oxygen, just as in animals. Moreover, 

 if a colourless plant, such as a mushroom or other fungus, be used 

 instead of the green plant, there will be increase of carbon dioxide 

 and decrease of oxygen even in the light. 



Two things, therefore, are required light and the pigment 

 that gives the green colour to plants. It is clear that the energy 

 used in the process has come from light, and that the means by 

 which it is utilised is the green pigment, called ' chlorophyll" or 

 " leaf-green." This substance has several remarkable properties, 

 but that which concerns us most is its relation to light-energy. 



The enormous quantity of radiant energy that we receive from 

 the sun is transmitted in the form of transverse vibrations in the 

 ether of space. The rate of these vibrations is of a wide range. 

 The wave-length depends on the rate, when the velocity of propa- 

 gation is the same, being the distance one wave has travelled 

 before the next one follows it. The lowest rates, or longest wave- 

 lengths, are only perceived by us as heat when they strike on the 

 skin. Those of a certain medium wave-length are perceived by 

 the eye as light, differing in colour according to their wave-length. 

 The most rapid vibrations, or shortest wave-length, the ultra-violet, 

 are not perceived directly at all, but are capable of causing chemical 

 actions of various kinds to occur. Those rays, which we call 

 "light" can also produce chemical changes when they fall upon 



