122 THE GREEN PLANT CELL 



hydrolysis of this to sugars again, form a very important 

 part of the metabolism of green plants. Sugar is not 

 only used by the plant in respiration, but it is also the 

 material from which the cell waUs, the skeleton of the 

 plant, are formed. For these reasons the carbohydrate 

 metabolism of the plant makes up much the greater 

 part of the whole- of its metabolic processes. But the 

 synthesis of proteins to form the basis of the new 

 protoplasm which is constantly being formed in meri- 

 stematic cells is clearly also of essential importance. 

 This appears to take place by the interaction of soluble 

 carbohydrates with the nitrogen and sulphur derived 

 from the nitrates and sulphates absorbed by the roots. 

 It seems that comparatively simple organic nitrogenous 

 substances called amino-acids are thus formed, and 

 that these are synthesised by successive condensations 

 (analogous to the condensations of sugar to starch or 

 cellulose) into more complex nitrogenous substances, 

 and eventually into proteins. These anaboHc processes 

 probably occur in the mesophyll cells of the leaf, whence 

 the nitrogenous substances are conveyed, in relatively 

 simple form, to growing organs and storage organs. 

 At least the final synthesis of the protoplasmic proteins 

 must also take place in the meristematic cells where 

 new protoplasm is actually being formed. It is not 

 possible to enter here into the details of these processes, 

 some of which are still largely obscure and others 

 unknown. Some of the simpler proteins — the poly- 

 peptides — can be made synthetically in the laboratory. 

 The Raw Materials oJ Plant Food. — The chemical 

 elements which are constituents of the molecules of the 

 most important organic substances involved in the 

 structure of organisms have been enumerated on page 37. 

 Of these the green plant obtains carbon from the carbon 



