348 THE LIFE OF THE PLANT 



of the potato. There are many substances besides starch 

 which belong to this group, e.g. sugar, and cellulose, the sub- 

 stance which forms the solid skeleton of plants, from thin blades 

 of grass to the trunks of trees. AU these bodies have a similar 

 composition, and differ only in their greater or less density 

 and other physical properties. Sugar, for instance, dissolves 

 in water ; starch does not, it only swells and forms a kind of 

 semi-liquid paste ; cellulose scarcely swells at aU. In a sense 

 we can say that starch is condensed sugar, and cellulose con- 

 densed starch. Other carbohydrates can be easily derived 

 from starch. As a matter of fact sugar is artificially obtained 

 from starch in the manufacture of potato molasses. Cellulose 

 has not yet been prepared artificially, but certainly is derived 

 from starch in the plant : thus, for example, the starch of the 

 germinating seeds of cereals changes into the cellulose 6i which 

 the rootlet is built up. 



Albuminoids form the second predominant group of vegetable 

 substances after the carbohydrates. They are called albu- 

 minoids from their likeness to the albumen of an egg. Wheat 

 flour, taken as an example of vegetable food, contains something 

 like 17 per cent, of albuminoid matter, called gluten. There- 

 fore, if starch and albuminoids are subtracted from cereal 

 seeds, there will be only a small percentage left for aU other 

 substances. In addition to carbon, hydrogen, and oxygen, 

 nitrogen enters into the composition of albuminoid sub- 

 stances. 



Though starch, as we have seen, cannot be formed otherwise 

 than with the co-operation of Ught, the formation of albuminoids 

 in a plant does not require Ught, or any other external source of 

 energy. It depends instead upon the presence of ready carbo- 

 hydrates. If some plants are only provided with a carbohydrate 

 of some kind, say sugar, and some source of nitrogen, say ammonia, 

 they are sure to form albuminoids even in total darkness. Accord- 

 ing to exact experiment, without touching the problem unsolved 

 as yet by chemists, as to the relation which exists between 

 carbohydrates and albuminoids, we may say that plants are 

 able to form albuminoids from a carbohydrate and ammonia. 

 A physiologist can say to a chemist : give me sugar, ammonia, 

 and a cell, and I will give you in return as much of an albuminoid 

 as you wish. Its manufacture may certainly not always be 



