ABSORPTION AND MOVEMENT OF WATER 107 



solution in the vacuoles and outside the cell is evident from 

 the following. Under normal conditions the protoplasm is. 

 slightly alkaline, the cell-sap slightly acid! If the cytoplas- 

 mic membrane bounding a vacuole (the Vacuolenhaut, 

 as Pfeffer calls it) were permeable to all substances, and 

 equally permeable in both directions, this difference in 

 chemical reaction could exist only momentarily ; it could 

 not be the normal condition. Furthermore, so long as the 

 cells are alive, no color will pass from clean slices of beet or 

 of red or black cherry, or from other tissues composed of 

 cells containing colored sap in the vacuoles ; but if the cells 

 are killed by immersion in hot water or by steam, the 

 color will rapidly pass out into the water. Again, although 

 harmless coloring solutions will pass into and stain the 

 walls, the majority of such solutions, no matter what their 

 concentration, will not pass into the protoplasm or stain 

 any part of it, so long as the cells are alive. Some few 

 harmless stains, if applied in sufficiently dilute solutions, 

 may be employed to stain living protoplasm* and nuclei, t 

 or may be accumulated in the vacuoles t of living cells. 



From these experiments it is obvious that the living proto- 

 plasm, especially the structurally and physiologically differ- 

 entiated layers adjoining the cell- wall and bounding the vacu- 

 oles and nucleus, do exercise some control over the dissolved 

 substances adjacent. That this power is limited is shown by 

 the above experiments with staining agents, by daily experi- 

 ence in the laboratory with the various poisons employed 

 as fixing agents for histological purposes, and by the 

 sensitiveness of plants to the soluble substances by which 

 they are surrounded. If the cytoplasmic membranes could 

 exclude poisons, at the same time allowing nutritious solu- 

 tions to enter freely, the advantage would be great. 



* Pfeffer, W. Uber Aufnahme von Anilinfarben in lebende Zellen. Unter- 

 suchungen aus d. bot. Institut zu Tubingen, Bd. II. 



f Campbell, D. H. The staining of living nuclei. Ibid. 



Pfeffer, W. L. c. 



$ The living cell may, however, control the osmotic exchanges taking 

 place between itself and the solutions outside, not only by regulating the 

 composition of the cell-sap, but also by changing the permeability of the 

 cell-wall. See Nathansohn, Zur Lehre vom Stoffaustausch. Ber. d. D. 

 Bot. Gesellsch., XIX., pp. 509-13, 1901. 



