THE FOOD OF PLANTS 83 



coloured band which emerges is not continuous, certain 

 parts of it being blotted out. The spectrum, as the 

 band is called, is consequently crossed vertically by a 

 number of dark bands, corresponding to the position of 

 the missing rays. In the living cell these rays are 

 absorbed by the chlorophyll exactly as they are by the 

 solution used in the experiment, and it is from them the 

 plant derives the energy which is used. The rays which 

 are most active are a certain number of the red ones; 

 these correspond in position with a broad black band 

 which is one of those described. 



This process of sugar construction can only take 

 place at a moderate temperature. 



Another very important constituent of the plant's 

 food is protein, which differs from the group of food- 

 stuffs to which sugar belongs by containing nitrogen in 

 combination. Very little is at present known of the 

 processes by which protein is made. Compounds of 

 nitrogen, preferably nitrates of potassium, calcium, 

 magnesium, or ammonia, are absorbed by the roots, dis- 

 solved in the water which they take in. The changes 

 they undergo lead in some still unexplained manner to 

 the formation of much more complex nitrogen com- 

 pounds, which are generally though not strictly accur- 

 ately described as amides. Among them may be 

 mentioned asparagin, leucin, and tyrosin. 



By still further changes these are converted into 

 proteins, but the chemistry of the process is still obscure. 



A third constituent of the food of plants is fat or oil. 

 This is less widely distributed and appears for the most 

 part only in places of storage. It is formed directly 

 from the living substance there and does not appear to 

 be built up from simple substances in the plant. 



Sugar is a member of a group of substances which are 

 called carbohydrates. It is formed in large quantities 

 when the chloroplasts are properly illuminated; much 



