CHAPTER XVII. 



FORMATION OF PROTEINS. TRANSLOCATION 

 AND STORAGE OF FOODS. 



I. Within the body of a living plant a great variety of chemical 

 changes, which are collectively referred to as metabolic processes 

 or Tiietabolism, are always being carried on. Some of these 

 changes, like those discussed in the preceding chapter, result in 

 the formation of complex compounds from simpler ones ; such 

 constructive chemical processes are spoken of as anaboUsm, 

 the destructive chemical changes, such as those involved in 

 the respiration-process, which result in the breaking down or 

 decomposition of complex compounds into simpler ones, being 

 included in the term catabolism. 



The conditions under which the chemical reactions take place 

 within a living plant, are very much more complicated and 

 probably of a very different class from those met with in a 

 chemical laboratory, and our knowledge respecting the chemical 

 changes involved in the production of the many different organic 

 compounds present in plants is still very scanty and imperfect. 



2. Formation of proteins. — During the growth of green plants 

 there is not only the synthesis or construction of sugars and 

 other carbohydrates from simple inorganic food-materials, but 

 other organic compounds are built up, the chief of which are 

 those containing nitrogen, namely, amides and proteins. 



The natural sources from which green plants obtain the 

 nitrogen necessary for the production of these compounds 

 are : — 



(i) The free uncombined nitrogen of the atmosphere. 



