1 6 NATURE OF PROTEIDS 



photosynthesis. So nearly balanced are the rates of formation 

 and absorption, however, that the percentage existing in the air 

 does not materially vary. 



8. The Construction of Proteids. A second group of foods 

 formed by plants are called proteids. These differ from the car- 

 bohydrates in that they contain nitrogen in addition to carbon, 

 hydrogen and oxygen. They are more complex compounds than 

 the carbohydrates and may contain sulphur and phosphorus in 

 addition to the four elements mentioned above. Less is known 

 about their formation than of the carbohydrates. It is probable 

 that they are largely formed in the leaves and by a process simi- 

 lar to that of photosynthesis. The sulphates, phosphates, and ni- 

 trates absorbed from the soil are decomposed and the elements of 

 nitrogen, sulphur or phosphorus are united to simple carbon com- 

 pounds and complex proteids are the result. Light does not 

 co-operate directly in this construction although it may do so 

 indirectly. These foods are formed in much smaller quantities 

 than the carbohydrates but they are of the greatest importance 

 in the nourishment of the plant. This is especially true as regards 

 the living substance, protoplasm, which resembles somewhat in 

 composition some of the more complex proteids. 



9. The Distribution of the Foods. Let us now consider what 

 becomes of these foods. A small part is consumed on the spot 

 by the manufacturing cells themselves, a larger portion is trans- 

 ported ^hrough the vascular bundles to all the living and grow- 

 ing cells of the plant body, but as the plant approaches the com- 

 pletion of its annual growth a larger and larger part of the food 

 is transferred to special parts of the plant, such as buds, roots, 

 seeds. Here it is stored as a reserve food to meet the needs of 

 the plant at such times as it is not able to manufacture food. 

 This transfer of foods is slow and consequently the rapidly con- 

 structed sugars gradually accumulate in the chlorenchyma during 

 the day. This would result in the saturation of the cells with 

 sugar and so stop the work of photosynthesis were it not for the 

 fact that the chloroplasts quickly change the sugar to insoluble 

 starch, thus leaving the cells free to receive more sugar. If 

 chloroplasts of well-sunned, starch-forming leaves are examined 



