174 VEGETABLE PHYSIOLOGY 
zortex which contain this acid. In those nearest the apex 
the latter occurs alone, but as they grow older, a little 
protein is found to be mixed with it. In still older ones 
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 never- 
theless do not appear to be present in the mesophyll cells, 
makes it probable that they are manufactured into some- 
thing 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 
