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(with a few rare and unimportant exceptions) are able to manu- 

 facture their organic food from the carbon dioxide of the air ; 

 all other plants and animals must obtain their carbonaceous 

 food from materials which directly, or indirectly, have been 

 manufactured for them by green plants. Without green plants 

 then all life on the earth would eventually cense. These 

 organic compounds contain energy, or in other words capacity 

 for doing work, and they can be made to yield up this energy 

 which is stored within them for the use of living organisms, 

 in various ways. Some of them, for instance, such as wood and 

 coal, when burnt in a fire are made to give up their energy in 

 the form of light and heat. Others can be used as food and 

 made to give up their energy in the process known as respira- 

 tion. Whether these substances are slowly burnt off in living or- 

 ganisms as food, whether they are quickly consumed in a fire, 

 or whether they are slowly oxidised and disintegrated under the 

 chemical and physical actions of air and water, the end product 

 of the decomposition is carbonic dioxide. Now when green plants 

 first construct these organic materials large quantities of car- 

 bon dioxide are removed from the air ; a certain amount of 

 this is again returned by mankind and animals by utilising 

 organic matter as food or fuel. Large quantities of carbon, 

 however, still remain locked up in the dead bodies of plants 

 and animals which have not been utilised in this way, and if 

 these dead bodies were to remain unchanged, the available 

 supply of carbon dioxide in the air would eventually become 

 exhausted and all living beings would be driven out of exist- 

 ence. We know, however, that this is not the case, but that 

 all such dead remains rot, decay, or putrify, and are eventually 

 broken up into the elements of which they were formed, car- 

 bon dioxide being again set free. Ordinary oxidation, as 

 already mentioned, is responsible for a certain amount of this 

 decomposition, but we now know that living organisms (espe- 

 cially bacteria and fungi) which derive their carbonaceous 

 food from these dead bodies take an exceedingly active part 

 in this work and are hence indispensable for the maintenance 

 of life. Plants which thus derive their carbonaceous food 

 from the dead bodies of other organisms, or from substances 

 manufactured by other organisms, are termed saprophytes. They Saprophytes, 

 do not obtain their supplies directly from the living tissues of an 

 organism. Green plants then, we see, are continually building 

 up organic matter and thus not only provide a continual sup- 

 ply of food for animals and other plants but also prevent the 

 accumulation of poisonous quantities of carbon dioxide in the 



