28 GENERAL BOTANY 



solid matter of plants. These are both colourless gases, 

 and smoke is black only because it contains solid 

 particles of carbon that are carried away by the draught 

 before they can be completely burnt. 



Now burning means oxidation, and the fact that 

 carbon dioxide and water-vapour are produced when 

 wood is burnt shows that it consists very largely of 

 the two elements carbon and hydrogen. The solid 

 framework of a plant is in fact formed entirely of a 

 substance called cellulose, which is a compound or 

 mixture of compounds of hydrogen, oxygen and carbon, 

 in which there are always eight parts of weight of oxy- 

 gen to one of hydrogen. Starch and sugar, which we 

 obtain only from plants, are also compounds of these 

 three elements (and of no others) and in them too the 

 oxygen and hydrogen are in the proportion of eight 

 to one, the difference being in the amount of carbon, 

 and in the arrangement of the atoms in the molecule. 



Because the hydrogen and oxygen are in the same 

 proportion as they are in water, these substances (and 

 others of the same kind) are called by the general 

 name of carbohydrates. 



Now where does the plant obtain these elements ? 



Hydrogen and oxygen it gets from the ground as 

 water, but the carbon cannot come from the ground. 

 We cannot burn earth, even that in which plants 

 grow best, and we have only to look at the soil of 

 a paddy or cornfield after the crop has been cut 

 and carried away, to see that except for the roots 

 left behind there is no carbonaceous matter in it at 

 all. Many plants indeed will grow quite well on the 

 sand by the seashore or in waste places where there 



