2 i 6 J. H. Priestley and R. M. Tupper-Carey 
massed together, their thin walls separated by no intervening air 
spaces. These walls are so thin that even if they were permeable, 
the passage of water through the tissue would be extremely slow, 
that is, assuming the protoplasts to be impermeable. But this 
impermeability of the protoplast, in view of the physiological 
function of a meristem, seems to be a necessity. Each protoplast 
forms an active centre of the complex metabolism by which amino- 
acids and the simpler proteins are converted into that dynamic 
equilibrium of complex proteins and associated molecules which 
functions as living protoplasm. We are almost completely ignorant 
of the chemical nature of the reactions involved in this complex 
synthesis, but in the light of our knowledge of the hydrolysis of 
proteins we may expect reversible reactions of the type there met 
with to be proceeding in the direction of synthesis. 
Such a synthesis involves condensation, a chemical synthesis 
proceeding with the elimination of water, which may be represented 
in the simplest manner by the following illustration of the conden¬ 
sation of two molecules of glycine to form glycyl-glycine. 
CH 2 —NH 2 —CO(OH + H)NH . CH 2 —COOH 
^ CH 2 —NH 2 —CO-NH—CH 2 —COOH + H 2 0 . 
There seems no doubt that these reversible reactions, with all 
possible intermediate stages, play an important part in the protein 
metabolism associated with protoplasmic synthesis. But unless 
provision is made for the rapid removal of the products of the 
reaction, this reaction will soon approach the equilibrium point and 
proceed no further in the direction of synthesis. Amongst the pro¬ 
ducts of reaction thus requiring removal is water. It is therefore 
essential that the synthetic reaction should proceed in the absence 
of water, a condition which is provided for in the closely massed, 
non-vacuolated and relatively impermeable cells of the meristem. 
The comparatively small amount of water formed during the re¬ 
action is probably reabsorbed into the active metabolism of the 
cell, but its accumulation, or the accumulation of any osmotic 
substance attracting water into protoplasmic alveolae, will probably 
be responsible for the subsequent vacuolation of the meristematic 
cells as they are left behind the growing point. 
That the concentration of water has a material effect upon the 
direction of such reversible chemical changes as are taking place in 
the meristematic protoplast is well known and attention is drawn 
to it by Bayliss(i) ( loc . cit. p. 239), but the important bearing of the 
