PLASMOLYSIS 9 



concentration. This is due to the fact that such membranes, 

 whilst readily permeable to water, are for all practical purposes 

 impermeable to the dissolved substances. The hning layer of 

 protoplasm, or probably more correctly the plasmatic membranes, 

 exhibit these features. Consequently, when cells are surrounded 

 by solutions which are more concentrated than the cell-sap, 

 water passes out and the protoplast contracts. On the other 

 hand, when replaced in water, the cell-sap is the more concen- 

 trated solution, and the protoplast becomes distended until its 

 further expansion, and further increase in the volume of the 

 sap, is prevented by the cell-wall. In a healthy plant, supphed 

 with sufficient water, all the living cells are thus distended to 

 their utmost, that is to saj' they are turgid , a condition which 

 plays an important part in maintaining the stability of herba- 

 ceous organs (cf. F. & S\, p. 102). 



These phenomena plainly show that the cytoplasmic hning 

 is readily permeable to water, but not appreciably to the sub- 

 stances dissolved in the sap, nor to those in the solution around. 

 If it were permeable, the concentration inside and outside the 

 cell would rapidly become equal as a result of diffusion and 

 plasmolj'sis could not occur, nor would turgescence of the cells 

 be possible. 



The plasmatic membranes exhibit their semi-permeable char- 

 acter, however, onl}' so long as the protoplast remains alive ; 

 hence after death both the substances dissolved in the cell-sap 

 can pass outwards, and external solutions can diffuse inwards. 

 If cells of the Spiderwort, etc., mounted in water, are killed by 

 gently heating the slide over a spirit-lamp, the coloured sap 

 wiU be found to escape into the surrounding liquid ; moreover, 

 it wiU be found impossible to bring about plasmolysis or to 

 render such killed cells turgid. These results, apart from showing 

 that the semi-permeable character of the cytoplasm is altered 

 by death, demonstrate that the cell-waU is permeable to sub- 

 stances in solution. The permeabihty of the cytoplasm, when 

 dead, to the cell-sap may be exhibited on a large scale by placing 

 slices of Beetroot in hot water. 



A moment's thought, however, will show that the plasmatic 



1 I.e. Fritch and Salisbury, An Introduction to the Study of Plants, 

 which in the subsequent pages will always be briefly referred to in this way, 



