16 NATURE I >F l'K< HI". IDS 



photosynthesis. So nearly balanced are the rates of formation 

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



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 mure complex compounds than 

 the carbohydrates and may contain sulphur and phosphorus in 



addition t< .» 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- 

 trate- 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 S< 



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 

 become- 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 t<» all the living and grow- 

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

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

 i- transferred to special parts of the plant, such a- buds, roots 



Mere it is stored as a reserve food to meet the needs of 

 the plant at such time- a- it i- not able to manufacture f 1 



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 bo stop the work of photosynthesis were it not for the 



•hat the chloroplasts quickly change the sugar to insoluble 



starch, thus leaving the cells free t" more Mi-ar. If 



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



