THE CONSTRUCTION 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. 
