THE CONSTKUCTION OF PROTEINS 169 



the protein preponderates, and at some distance behind the 

 seat of growth it occurs alone, the acid having disappeared. 

 Certain fungi can utilise nitrogen-containing derivatives of 

 methane or benzol for the same purpose. It is possible, 

 therefore, that more than one pathway to the protein mole- 

 cule may yet be found in different plants. 



Probably the construction of protein is not confined to 

 any definite tissue or series of tissues in the plant. It is 

 certainly only connected indirectly with the chlorophyll 

 apparatus, and that in so far as carbohydrates are 

 necessary for its formation. At the same time, there is a 

 certain amount of evidence which points to its synthesis 

 being in the first place effected in the leaves. The fact that 

 nitrates can be traced towards these organs, and that they 

 nevertheless do not appear to be present in the mesophyll 

 cells, makes it probable that they are manufactured into 

 something else there. The occurrence of amido-acids in 

 the leaves is more in harmony with the view that they are 

 built up there than with the assumption that they arise 

 from the decomposition of already existing proteins, though, 

 no doubt, the latter is the case in the tissues of seeds, and 

 possibly of seedlings, which are being nourished at the 

 expense of materials stored in the seed. The proportion of 

 protein to dry weight of tissue has been stated to increase 

 gradually and progressively from the roots to the leaves, 

 in which it attains a maximum. Moreover, proteins are 

 continually being removed from the leaves. If, however, 

 the process does primarily go on in the leaves, it does not 

 take place under the same conditions as the construction 

 of carbohydrates. It goes on quite well in green cells in 

 darkness, so that it is not, as already mentioned, a process 

 of photosynthesis. It has recently been claimed that the 

 construction of protein in certain plants is favoured by 

 light, and more particularly by the ultra-violet rays, though 

 the luminous ones have a certain feeble effect. Whether 

 or no the energy for the construction is derived therefrom 

 is not, however, certain. 



