THE METABOLISM OF PLANTS. 151 



in the shoots undergo decomposition, asparagin being one of the 

 products, and that the asparagin accumulated in consequence 

 of the absence of any appropriate non-nitrogenous organic 

 substance with which it could combine to form proteid. 



Taking all these facts into consideration, we may con- 

 clude that these amides are formed both synthetically and 

 analytically in the leaves. But in whatever way they may be 

 formed, they are used in the construction of proteid: hence 

 their presence or absence depends upon the supply of appro- 

 priate non-nitrogenous organic substance ; when the supply is 

 adequate, no trace of the amides can be detected, when it is 

 inadequate, they begin to accumulate. 



The synthesis of amides with non-nitrogenous organic 

 substance to form proteid is not, however, confined to the 

 leaf : probably every living plant-cell is capable of effecting it. 

 It has been found, for instance, by many observers that the 

 tissue of the growing-point, the primary meristem, never con- 

 tains any starch orsugar or asparagin, although these substances 

 are abundantly present in the rest of the plant. There can 

 be no doubt that proteid is being formed there in connexion 

 with the production of new cells. The only explanation 

 which can be offered is that the asparagin and the sugar with 

 which these cells are supplied are rapidly converted into 

 proteid. We shall have occasion to discuss these facts in 

 detail in a subsequent lecture. 



Strasburger and Schmitz have suggested, as already mentioned (p. 27), 

 that the nucleus is in some way connected with the formation of proteid 

 in the cell. This suggestion is based upon the following facts : that a 

 nucleus has been found to be present in the living cells of the vast 

 majority of plants ; that it is the last structure to disappear from the cell 

 when its death is approaching ; that it is present in cells which never 

 contain starch, and is therefore not directly connected with the production 

 of non-nitrogenous organic substance ; that it gives very marked proteid 

 reactions. 



3. The Function of Chlorophyll. 



We have seen that it is only the green parts of plants that 

 are capable of absorbing carbon dioxide and of exhaling 

 oxygen ; it is only in these, then, that a formation of organic 



