THE MOLECULAR FORCES IN PLANTS. 147 



different. The former is usually not permeable to colouring 

 matters, sugar, etc., while the cell-wall behaves, as regards these 

 substances, precisely like vegetable parchment. It is therefore of 

 interest to make some experiments which will afford us informa- 

 tion concerning the osmotic permeability of parch ment. 



We may use for the dialyser a wide glass tube, closed at the 

 bottom with parchment paper. -The apparatus I have used in my 

 experiments is constructed somewhat differently, for the sake of 

 greater convenience in manipulation. A thick glass tube, 80 

 mm. long and 40 mm. in diameter, is fitted at the lower end with 

 a brass ring, this being provided on the outside with a screw. 

 To the under surface of this ring is closely applied a piece of 

 vegetable parchment, and on this is placed a second, but rather 

 thin, brass ring (brass washer), over which we screw a brass cap 

 with a circular aperture 40 mm. in diameter. The exposed brass 

 parts of the dialyser are painted with a suitable varnish. We 

 place the apparatus on small blocks of glass in a crystal- 

 lising glass, into which we then pour distilled water, while into 

 the dialyser is poured a solution of the substance whose osmotic 

 behaviour is to be investigated. In experiments with extracts of 

 beetroot, solutions of sugar, or salt solutions (e.g. solutions of 

 Sodium chloride, Potassium nitrate, etc.), it is easy to determine 

 that the colouring matter, the sugar, and also the mineral sub- 

 stances, can traverse the membrane and pass into the water sur- 

 rounding the dialyser. 



It is now further of special interest, having in view the fact that 

 many substances which can traverse the cell-wall cannot make 

 their way into the protoplasm, to prepare artificially membranes 

 through which substances capable of traversing parchment paper 

 by osmosis cannot pass. We prepare I per cent, solutions of 

 Calcium nitrate and Bisodium phosphate. The latter is placed in 

 our parchment paper dialyser, while the former is used as the 

 outside fluid. A precipitation membrane of Calcium phosphate is 

 formed in the vegetable parchment, and if, after a few hours, we 

 add a drop or two of aqueous solution of methyl blue to the 

 Bisodium phosphate in the dialyser, we shall find that the colour- 

 ing matter does not pass over into the outside fluid. In my 

 experiments, for example, this was still quite uncoloured after 

 twenty-four hours. If we remove the coloured fluid from the 

 dialyser, fit the apparatus with a new piece of parchment paper, 

 replace the outer fluid by water, and pour back into the dialyser 





