112 SLOW DIFFUSION. [CH. V 



arises by which the brittle cell-wall is broken, but is 

 instantly mended by a fresh precipitate forming : as soon 

 as the wall is mended the pressure inside again increases, 

 and again ruptures the cell-wall, and thus by a series of 

 breaks, healed as soon as made, an apparently continuous 

 growth of the cell takes place. 



(143) Slowness of diffusio'n}. 



Fill a tall narrow jar with water and with the help of 

 a long funnel run in very slowly and carefully a stratum 

 of concentrated solution of potassium bichromate, which 

 accumulates at the bottom of the jar. 



It will be seen that the colour spreads, to the upper 

 stratum with extraordinary slowness. The chief physio- 

 logical interest of the result is that it serves to suggest 

 the value, to the living cell, of protoplasmic circulation. 



(144) Relation of membrane to diffusing fluids 



A dialyser made of vegetable parchment is filled with 

 a 1 per cent, solution of di-sodic phosphate coloured with 

 methylene blue, and is placed in distilled water; after some 

 hours the blue colour is visible in the water. If, however, 

 a precipitation membrane of calcium phosphate is pro- 

 duced in the wall of the dialyser, the methylene blue is 

 unable to pass. The precipitate is produced by immersing, 

 in 1 per cent, calcium nitrate, a dialyser filled as before 

 with 1 per cent, di-sodic phosphate coloured with methylene 

 blue. The importance of the experiment is to show that 



^ See de Vries, Bot. Zeitung, 1885, p. 1. 

 s Taken from Detmer's Praktikum, p. 96. 



