170 VEGETABLE PHYSIOLOGY 



Sachs held that the sieve-tubes of the fibre-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. 



