PERCENTAGE OF WATER 



'3 



affect the apical cells. When dilute solutions are suddenly applied, 

 however, the cases may be reversed. This is obviously due to the apical 

 cells being more readily permeable by glycerine than the older cells 

 derived from them. 



It seems, therefore, that the protoplasm of the embryonic cells at 

 actively growing apices is relatively rich in water, and if the force with 

 which the protoplasm imbibes water remains the same, or nearly the same 

 in adult cells, it must on the whole contain less water than that of very 

 young cells. The statement, therefore, that active rotation appears in 

 adult cells because their percentage of water increases, indicates not 

 a causal but an accidental relationship, and moreover, as far as the 

 evidence goes, the osmotic pressure rises and the percentage of water 

 in the protoplasm decreases slightly as the cell becomes adult. It is also 

 certain that the protoplasm can increase and decrease its own viscosity 

 independently of its percentage of water 1 , and probably such changes 

 are often the chief or even the sole factor in determining the appearance 

 or the cessation of streaming in particular cases. There are, however, 

 two instances in which an increased percentage of water in the protoplasm 

 is of primary importance. Firstly, when cells in which streaming has 

 ceased owing to immersal in strongly plasmolytic solutions are returned 

 to dilute solutions, so that the protoplasm can re-imbibe the water with- 

 drawn from it and recommence streaming. Secondly, when streaming 

 appears, as is often the case, in cells from which large stores of soluble 

 reserve materials are being removed. In both cases, however, other factors, 

 such as changes in the respiratory activity, &c., may come into play. 



Hauptfleisch considers the absence of streaming from so many adult 

 cells to be due to the presence of an infra-minimal or supra-maximal 

 percentage of water in them. Many of the instances quoted by Hauptfleisch 

 (1. c., p. 213), however, bear a different interpretation to that which he gives. 

 For example, rotation appears over an area of a leaf of Vallisneria or 

 Elodea exposed to air, not because the percentage of water in the proto- 

 plasm has fallen to the optimal amount for streaming, but because the loss 

 of water acts as a stimulus, which is propagated to neighbouring cells 

 in which the percentage of water is unaltered. Similarly when dilute 

 solutions of salt, glycerine, or asparagin induce or accelerate streaming, 

 they do so, not owing to any decrease in the percentage of water in the 

 protoplasm, but because they exercise an indirect stimulating action. 

 The latter is shown by the fact that the velocity usually undergoes 

 a further increase on returning to water, and that streaming will appear 

 if the cells are removed from the solution and replaced in water before 

 it has actually commenced. 



See Pfeffer's Physiology, 1900, Vol. I, p. 45, Eng. ed. 



