170 Leaves and their Work 



very general in all the rocks composing the earth's 

 crust, that it is almost impossible to find any soil quite 

 without them. 



But the iron may be taken away by artificial means, 

 and when this is done the leaf-green turns yellow, as 

 human beings do when their blood contains too few 

 red particles, and for precisely the same reason. Both 

 stand in need of iron. Iron oxide is reddish when 

 it contains the full amount of oxygen possible, and 

 green when it contains less. Give the plant iron 

 and keep it in the light, and the grains of leaf- 

 green at once begin to turn their proper colour, and 

 tiny grains of starch form within them. 



A very little light, barely enough to read by, will be 

 sufficient to make a plant begin to turn green, but not 

 sufficiently green to enable it to separate the carbon ; 

 and therefore in dim light no starch grains can be 

 formed. In ordinary daylight, however, whether the 

 sun be shining directly upon the plant or no, these 

 starch-grains are being continually produced ; but the 

 brighter the light the more briskly the manufacture 

 goes on, up to a certain point — provided, that is, that 

 the air contains carbon-dioxide wherewith to furnish 

 the necessary supply of carbon. If it does not, no 

 starch, of course, can be formed, no matter how bright 

 the light, or how green the leaf-green. 



But all air, unless artificially deprived of it, contains 

 enough to furnish what the plant requires in this 

 respect, thanks to the supplies which are being con- 

 stantly furnished to it. If the supply of carbon-dioxide 

 were not renewed, however, it would be exhausted, as 

 already mentioned, in about a hundred years by the 

 present vegetation of the globe. But if the world were 



