i6 NATURE OF PROTEIDS 



consequently these corn plants were manufacturing food at the 

 rate of 14 million a day. These figures represent only a small 

 portion of the work performed, since the grain is but a part of 

 the plant, and furthermore the larger portion of the land is 

 covered with forests and other forms of vegetation. The fact 

 must not be overlooked that this process of photosynthesis is of 

 vital importance to our welfare in another way. Owing to the 

 large volumes of CO, that are constantly formed by fires and the 

 respiration of animals such an excess of this gas would accumulate 

 in the atmosphere that all animal life would soon cease were it 

 not for its absorption in photosynthesis. So 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 properly 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 through 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 



