CARBON IN PLANTS 29 



is not even a vestige of combustible matter, i.e. of 

 carbon. Strange as it may seem, the whole of the car- 

 bon in an ordinary plant, making about one half of 

 the weight of its dry substance, is obtained from the at- 

 mosphere. Ordinary air contains, in addition to nitro- 

 gen, oxygen, water-vapour and other gases, a very 

 small quantity of carbon dioxide. In ten thousand 

 parts of air there are only four of carbon dioxide, and 

 by weight only three-elevenths of this gas is carbon, so 

 that the amount of this element in the atmosphere 

 is comparatively very small. Still it has been proved 

 by careful experiments that it is from the air and 

 only from it that green plants get their carbon. 



All green parts are capable of taking in this carbon, 

 but it is the leaves that are chiefly concerned in this 

 work. The carbon dioxide passes in with the air 

 through the small holes stomas through which, as 

 we have already learnt water passes out in the form of 

 vapour, and, making its way through the spaces in the 

 tissues, is absorbed by the living substance of the 

 leaf. There it undergoes chemical change ; the carbon 

 is separated from the oxygen, and made to combine 

 with the elements of the water to form a carbo- 

 hydrate. 



This process is termed assimilation, and the sub- 

 stance first formed appears to be a kind of sugar. 

 But in most plants this is condensed, so to speak, 

 into starch, which, being insoluble in the water of 

 the leaf, occurs 'as distinct grains, too small however 

 to be seen except with the help of a microscope. 



That starch is formed you can prove in this way. 

 Take any thin leaf, such as that of a Tobacco 



