i6 NATURE OF PROTEIDS 



days, consequently these com plants were manufacturing food 

 at the rate of over 15 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 CO2 that are constantly formed by fires and the 

 respiration of animals such an excess of this gas would accumu- 

 late in the atmosphere that all animal life would eventually 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 

 nitrates absorbed from the soil are decomposed and the elements 

 of nitrogen, sulphur or phosphorus are united to simple carbon 

 compounds and complex proteids are the result. Light probably 

 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 



