34 STRUCTURAL BOTANY 



to the presence of chlorophyll- corpuscles in their 

 cells. 



We have said above that one of the two main 

 functions of the shoot is to take up food from the air. 

 The great food-substance which green plants obtain 

 from the air is carbon. About half the solid substance 

 of plants consists of this element, which enters into 

 all the chemical compounds of which living things are 

 built up. The whole of this carbon is obtained by 

 ordinary green plants from the carbonic acid, or more 

 properly carbon dioxide (C0 2 ), which is contained in 

 small quantities in the air. In ordinary air the 

 amount of this gas is equal to about three parts in 

 ten thousand. The carbon dioxide is decomposed, the 

 oxygen given oft again, and the carbon retained, to 

 help in forming the various organic substances of the 

 plant. 1 Now this decomposition of carbon dioxide is 

 entirely performed by the chlorophyll granules, and 

 they can only do this work under the influence of 

 light. We see, then, that the chlorophyll granules 

 are of the greatest possible importance to the 

 plant, for they are the organs by which alone, 

 with the help of the sun's rays, it can obtain its 

 carbonaceous food from the air. Both parts of the 

 granule are necessary for the process ; the chlorophyll 

 itself is useless without the protoplasm of the granule, 

 and the protoplasm is quite unable to do the work 

 unless it contains chlorophyll. The process of nutri- 



1 We shall see presently that in all probability only half the oxygen 

 of the C0 2 is given off, and that the other half of the evolved oxygen is 

 derived from the simultaneous decomposition of H 2 0. 



