12 PERMEABILITY 



produce any visible plasmolysis (for equivalent pressures, see 

 Appendix V). 1 



Another method, in which the average osmotic pressure of a 

 whole tissue is determined, depends upon the curvatures con- 

 sequent upon the different stretching capacities of the walls of 

 the component cells. 2 For this purpose one can make use of 

 short lengths of the stalk of a Dandelion-inflorescence which 

 are split lengthwise into four portions. When placed in water 

 liquid is absorbed by the cells, but, since those towards the 

 outside of the stem have thicker walls than the inner ones, the 

 latter have a greater stretching capacity ; as a consequence the 

 strips curl up and form rings. A strength of sea-water that 

 causes neither increase nor decrease in the curvature will have 

 approximately the same osmotic pressure as the cell-sap. Stems 

 of many herbaceous plants can be utilised in this way. 



Attention has already been drawn to the fact that the proto- 

 plasm must be permeable to the compounds dissolved in the 

 soil-water or in a water-culture solution. The permeability of 

 the protoplasm can be demonstrated by placing young shoots 

 of the Canadian Pondweed in a solution of methyl blue, so weak 

 that it has but a very faint tint. If the cells be examined after 

 some days the sap is found to be of a deep blue colour, indicating 

 that the dissolved dye has passed through the cytoplasm in 

 considerable amount. If the methyl blue remained unaltered 

 on reaching the vacuole, only sufficient could have entered to 

 bring about a concentration equivalent to that of the solution 

 outside the cells. But the deep blue colour shows that the dye 

 has accumulated within the sap, and this is due to the com- 

 bination of the methyl blue with the tannin in the latter to form 

 a substance to which the plasmatic membrane is impermeable. 

 In this way the concentration of the dye which enters the cell- 

 sap is continually being reduced to a strength below that outside. 

 As a consequence more and more methyl blue diffuses in, and 

 thus the deep blue colour is gradually produced. 



A further demonstration of permeability of the cytoplasm is 



1 Very accurate determinations of osmotic pressure are made by indirect 

 means depending on the relation between osmotic pressure and the tem- 

 perature at which a liquid (e.g. the expressed sap of a plant-organ) freezes. 



3 Cf. F. & S., pp. 103, 104, see footnote on p. 9. 



