INTRODUCTION 



phyll, the protoplasm of these cells has the property of decompos- 

 ing carbonic acid, liberating free oxygen, and combining the carbon 

 with hydrogen and oxygen to form starch. This is the critical step 

 in the interaction of chemical elements on the earth's surface, by 

 which life is at present determined. Were there no assimilation of 

 carbon from carbonic acid to form starch by the green plants 

 the whole fabric of the living world would tumble to the ground in 

 truth, become mineralised. All living matter breaks down, within a 

 short space of hours or days, to the resting or mineral condition of 

 carbonic acid and ammonia (or nitrates). Were the building-up 

 process, the raising to higher potentiality, not incessantly performed 

 by green plants a power which chlorophyll and chlorophyll alone 

 confers on them all carbon must pass from the reach of the organic 

 world and living matter come to an abrupt end. 



And this is equally true of nitrogen. The nitrogen present in 

 living protoplasm tends inevitably to the stable inert condition as 

 a nitrate, as ammonia, or as the pure dissociated atmospheric gas. 

 It is only by a subtle chemical process which occurs in the green 

 plant as a result of and in connection with the fixation of carbon 

 as starch that nitrogen taken up in water by the roots of the plant 

 as nitrate and as ammonia is brought into combination as part of an 

 " organic " compound or molecule. Thus in the ultimate history of 

 the chemistry of living things the animal depends for its necessary 

 food proteids, carbohydrates, and hydrocarbons on chlorophyll, 

 the " leaf-green " of green plants. Vegetarian animals swallow 

 and digest these substances built up by plants ; carnivorous animals 

 swallow and digest animals which have already profited by the 

 work of the green plant. No animal can take up even a fraction 

 of a grain of carbon or nitrogen from a stomachful of carbonates, 

 nitrates, and ammonia. 



There are, however, as exceptions plants which are devoid of 

 chlorophyll and depend upon the results of the constructive activity 

 of other plants and of animals, just as per contra there are ex- 

 ceptional parasitic animals which have no mouths or gut and live 

 in the diffusible nutritive juices elaborated by other animals, which 

 they absorb by the surface of their bodies. The chemical life of 

 those plants which are devoid of chlorophyll the fungi, the 

 bacteria, and a few others may be considered as corresponding in 

 character to that of those tissues or cell-groups of green plants 

 which lie within the green plant and are devoid themselves of 

 chlorophyll. Both these tissues and the autonomous fungi and the 

 saprophytes depend for their food on the products supplied to 

 them by the chlorophyll-holding cells of green plants. There are 

 minute filamentous and rod-like plants devoid of chlorophyll 

 (Bacteria and others) which can take their carbon as tartaric acid 

 and their nitrogen as ammonia. It is probable that ail such non- 



