THE MOLECULAR FORCES IN PLANTS. 143 



57. Diffusion and Endosmosis. 



If substances in solution appear at any place in the protoplasm 

 or cell-sap of a cell, they can spread thence over the whole proto- 

 plasm or the whole cell-sap. In this process diffusion plays an 

 important part, but the rate at which diffusion takes place is by 

 no means always so great as we are often in the habit of thinking, 

 and it is certainly instructive to convince ourselves of this. We 

 place a tall glass cylinder filled with water on a table free from 

 vibrations, drop into the water a crystal of Potassium bichromate, 

 and cover the cylinder with a glass plate. The Potassium bichro- 

 mate dissolves, but even after several days the upper layers of the 

 fluid are only coloured faintly yellow, while the lower layers of 

 fluid exhibit the characteristic colour of saturated solutions of the 

 salt. In making our experiment, we have by no means excluded 

 all the conditions which might cause currents in the fluid ; and 

 the fact accordingly stands out so much the more clearly that the 

 distribution of dissolved substances by diffusion does not proceed 

 with special rapidity. 



This being so, any causes by which the distribution of substances 

 dissolved in the protoplasm and in the cell-sap is accelerated, 

 gain in importance, and among such conditions must in many 

 cases be regarded protoplasmic currents, and movements of plants 

 structures due to the wind. 



Putting aside diffusion, osmotic process.es play a most important 

 part in the distribution of substances in the plant, and we will 

 therefore direct our attention to them. 



A glass tube about 8 cm. long and 3 cm. wide is covered at 

 one end with a piece of pig's bladder. To make the closure per- 

 fectly tight, we first moisten the bladder, and then tie it firmly 

 over the end of the tube with string, or better still with elastic. 

 The tube is now completely filled with an almost concentrated 

 solution of cane-sugar, and its upper end is closed with a rubber 

 stopper, through which passes a long glass tube. We note the 

 position of the cane-sugar solution in this tube, and then dip the 

 lower end of the apparatus into distilled water. We find that 

 the fluid at once begins to rise in the tube. The water passes by 

 osmosis through the bladder into the sugar solution, and although 

 a certain quantity of the sugar solution also travels in the opposite 

 direction into the water, still the quantity of fluid flowing into the 

 apparatus is greater than that leaving it, and the volume of fluid 



