8 PLASMOLYSIS 



embedded within their substance (Fig. 4, p.) ', but these are more 

 evident after treatment with iodine, whereupon they become 

 blue, owing to the fact that each is surrounded by a layer of 

 starch. Pyrenoids are not found in the chloroplasts of the 

 higher plants, but are quite frequent amongst the Algae (see 

 p. 207). 



Returning to the cells in the hairs of the Spiderwort, we 

 will mount them in 2 per cent, natural or artificial sea-water 

 (see Appendix III) ; the resulting phenomena could be equally 

 well observed 'by using any cells with coloured sap, e.g. those 

 forming the lower epidermis of the Mother-of-Thousands (Saxi- 

 fraga sarmentosa), or those in the petal of a Pseony. In the 

 cells thus treated the lining layer of cytoplasm has contracted 

 away from the wall, so that a clear space is visible between, it 

 and the latter (Fig. 2, C). In this condition the cell is said to 

 be plasmolysed, and the phenomenon is spoken of as plasmolysis. 

 If the sea-water be replaced by ordinary water, the cell-contents 

 increase in volume, so that the plasmatic membrane regains its 

 original position in contact with the wall and the cell resumes 

 its normal appearance. By alternately substituting sea-water 

 and tap-water, this sequence of events may be repeatedly ob- 

 served. 



In the normal cell the pressure of the sap within the vacuole 

 keeps the lining layer of cytoplasm distended and forced against 

 the wall, in much the same way as the bladder of a football, 

 when inflated with air, is pressed against the leather cover. If 

 the air be allowed to escape, the bladder shrinks and a space 

 is left between it and the cover. Similarly, the contraction of 

 the protoplast of the cell, when surrounded by sea-water, must' 

 be attributed to a decrease in volume of the cell-sap owing to 

 leakage into the outside liquid. The recovery, when placed in 

 water, obviously implies an increase in volume of the sap, and 

 this can only be due to some of the water around having passed 

 into it. 



It is a well-known physical phenomenon (osmosis) that, when 

 two solutions of different concentrations are separated by certain 

 kinds of membranes termed " semi-permeable membranes " (which 

 may be of a fluid consistency) , there is a passage of water from 

 the weaker solution to the stronger until both attain the same 



