168 VEGETABLE PHYSIOLOGY 
compounds and carbohydrates, though of course only 
hypothetical, associates certain processes which apparently 
occur in nature. Its formation seems to involve the simul- 
taneous presence in the cells of some amino- or amido-acid, 
frequently asparagin, and some carbohydrate such as 
sugar. If shoots of plants which exhibit no accumulation 
of asparagin during normal growth are cut off and kept 
in darkness for some time, a gradual accumulation of the 
amido-acid can be observed. This in all probability is the 
expression of the decomposition of protein taking place during 
the life of the shoot, and is presumably a normal occur- 
rence. The reconstruction which would explain its non- 
accumulation during illumination is prevented by the non- 
formation of the needed carbohydrate in the darkness. 
The probability of a combination or interaction of these 
two classes of substance in the synthesis of proteins is 
supported by the fact that at the active growing points, where 
protoplasm is energetically formed, and where consequently 
abundant supplies of proteins are needed, neither sugar nor 
amido-acids can be detected, though they can be traced quite 
readily up to a short distance below the place where this 
active growth is proceeding. This fact is easily understood 
if we admit that protein is constructed there at the expense 
of these two constituents, supplemented, of course, by the 
necessary compound or compounds of sulphur. If either of 
these supplies ceases to be available, the growth of the plant 
at that point stops. 
Though we have seen reasons for fining that nitrates 
and amido-acids form two stages in the normal process of 
protein construction, we must not conclude that they in- 
variably do so. In one plant, Pangiwm edule, which was 
examined by Treub in 1894, the nitrogen needed for pro- 
tein construction appears to be supplied in the form of 
hydrocyanic acid. In the shoots of this plant, cells occur 
in the cortex 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 
