38 



are of great importance in connection with the absorption of 

 water by the roots of plants. To sum up, then : 



The cell-wall is a membrane which permits of the passage of 

 any dialysable substance through it. This may be easily shown 

 by placing the cell in currant juice or litmus solution, which 

 will not destroy its life, when we obtain the converse of 

 the experiment last detailed. The cell-sap and protoplasm do 

 not either of them become colored, but if the solution of coloring 

 matter used be tolerably concentrated, a plasmolytic condition 

 will be produced, and the intermediate space between the con- 

 tracted protoplasm and the cell-wall will become filled with the 

 colored fluid, which, though it has passed through the cell-wall, 

 yet does not pass into the vacuole through the protoplasm. 

 The cell-wall is also very elastic, since the cell is capable of 

 taking up into itself a considerable amount of fluid, and within 

 certain limits the cell-wall gives way to the strain so produced, 

 while, if conditions are present by means of which the fluid is 

 withdrawn again from the cell, the cell-wall contracts like an 

 elastic bladder, and returns again to its normal limits. In 

 plasmolysis the cell-wall maintains its form though it dimi- 

 nishes in size ; since the internal pressure which caused tension 

 has been removed, it in consequence has contracted, although 

 it still retains the same shape as before. The protoplasm has the 

 the power of resisting the passage through it of certain substances, 

 and hence we cannot apply to it the ordinary principles of dialysis, 

 though we can give expression to these physical principles in a 

 mathematical form for the cell-wall. It exercises a peculiar 

 principle of interference with, and regulation of, the passage of 

 substances from Avithin outwards and from without inwards. It is 

 also extensible, for the cell is capable of taking up a certain amount 

 of fluid, and as the amount of fluid in the vacuole increases, 

 the protoplasm lining the cell-wall expands also. It is not so 

 clear, however, that the protoplasm is elastic— i.e., an extensible 

 body returning to a state of rest when the strain is removed — 

 for in plasmolysis the protoplasm shrivels up, and only slowly 

 regains its former proportions. But in many cases, and probably 

 in all, the protoplasm is contractile, and in this respect it differs 



