44 THE LIFE OF THE PLANT 



The carbohydrates are so called because hydrogen and 

 oxygen are combined in them in the same ratio as they 

 are found in water ; since they also contain carbon, 

 they seem to be composed of carbon and water. The 

 following substances belong to this group of carbo- 

 hydrates: common cane sugar, beetroot sugar, and 

 grape sugar, or glucose, which is found in old raisins ; 

 gums, such as the gum which oozes out of the stems of 

 cherry trees ; starch ; and, lastly, cellulose, the sub- 

 stance which forms the solid skeleton of the plant, its 

 cell-walls, and which is used in our cotton and linen 

 cloths, and in paper. The carbohydrate group is some- 

 times spoken of also as the sugars, because some of the 

 members of the group, as we have just seen, are actual 

 sugars, while others can be easily changed into sugar. 

 For instance, by treating starch with dilute sulphuric 

 acid starch sugar is obtained. Cellulose can also be 

 changed into sugar if treated with the same acid. The 

 same method will transform old rags into sugar. The 

 carbohydrates we have mentioned seem to fall into a 

 series : cane-sugar and glucose are easily soluble in 

 water and capable of crystallisation ; gums, like cherry 

 gum for instance, are soluble in water, forming a thick 

 viscous liquid, but are incapable of crystallisation ; 

 starch does not dissolve in cold water, but swells in 

 hot water, forming a sort of paste ; lastly, cellulose 

 neither dissolves nor swells in cold or hot water. 



Now let us see how we can detect the presence at 

 least of the chief of these substances. They are all 

 colourless, but we possess means of producing in them 

 certain characteristic colour changes. The colourless 

 liquid in this glass is a solution of grape sugar, the other 

 glass contains a bright blue liquid. I pour the colour- 

 less liquid from the first glass into the blue liquid 

 in the second, and slightly heat the mixture. It 

 becomes turbid, then turns a dirty green colour, and 

 finally forms a yellow precipitate which turns brown, 



