170 VEGETABLE PHYSIOLOGY 
Sachs held that the sieve-tubes of the fibro-vascular 
bundles of the axis of the plant are also the seat of the 
construction of protein. Though this is possible, it seems 
more likely that they are concerned in the transmission of 
organic nitrogenous material from the leaves to other 
organs. In whatever form protein material travels about 
the plant, which for the present we cannot discuss, it is 
almost certain that it passes by the sieve-tubes, and it 
may well be that too great an accumulation of the travel- 
ling form may be attended by its conversion into an 
insoluble condition, and its deposition in the cells. There 
is no conclusive evidence pointing to the sieve-tubes as the 
places where it is originally synthesised. 
The same considerations apply to the various growing 
points or zones. There is little doubt that protein is con- 
structed there, but it is probable that it is so built up from 
bodies which have resulted from the digestion or decomposi- 
tion of protein that has already been synthesised elsewhere, 
and which has undergone such decomposition solely with a 
view to transport or translocation. 
We judge it probable on all these grounds that the 
great seat of protein construction in a green plant is the 
leaves, and this not on account of the possession of the 
chlorophyll apparatus, but because of a property inherent 
in the cell-protoplasm. Whence the energy is derived is 
not clear, but many writers hold it to be supplied by 
accompanying chemical decompositions. 
The construction of protein by fungi is an additional 
proof that it is altogether independent of the chlorophyll 
apparatus, if not that it is unconnected with the access of 
light. 
The third group of foods, the fats or oils, are probably 
not directly synthesised in plants, but are products of the 
decomposition of proteins, or perhaps of the living sub- 
stance itself. 
