PERMEABILITY 13 



obtained when filaments of Spirogyra (and many other Algas) 

 are placed in a dilute (i per cent.) solution of caffein. If the 

 effect be observed under the microscope, a very fine greyish 

 precipitate is seen to appear in the vacuole, which, as it accu- 

 mulates, renders the cell more and more opaque. This precipitate 

 is again a consequence of combination between the tannin in 

 the ceU-sap and the caffein. 



Both examples illustrate a very important phenomenon, 

 viz. that plant-cells can absorb and accumulate considerable 

 quantities of various substances from very dilute solutions, pro- 

 vided that within the cell they are changed into some other 

 form which does not readily pass through the plasmatic mem- 

 brane. This fact is significant when it is remembered that many 

 compounds are only present in the soil-water in very small 

 amounts. The phenomena just discussed also explain the 

 accumulation of food-reserves (e.g. insoluble starch) in large 

 quantities in the cells of storage-organs. When these reserves 

 are utihsed, it is clear that they must be changed into a form 

 which can diffuse to the growing organs, starch, for example, 

 being transformed into sugar. This process can be simulated 

 by placing leaves of the Canadian Pondweed, whose cell-sap 

 has acquired a deep blue colour in the way above described, 

 in a very dilute solution of citric acid (i per cent.). The latter 

 passes into the cells and changes the blue pigment into a form 

 capable of diffusing through the plasmatic membrane. Since 

 its concentration inside the cell is greatly in excess of that outside, 

 diffusion takes place and the sap gradually loses its deep blue 

 colouration. 



Although, in the numerous processes of diffusion that occur 

 between the living cells of the plant, complex organic substances, 

 analogous to methyl blue and caffein, are often involved, the 

 mode of absorption of the simple inorganic compounds in the 

 soil-water is of primary importance. It is a well-known fact 

 that mineral salts bring about contraction of the protoplast of 

 plant-cells (including root-hairs), when used singly and in a 

 sufficiently concentrated solution. In recent j'ears, however, the 

 American botanist Osterhout has shown that many, perhaps all, 

 of these cases of so-called plasmolysis are due, not to imperme- 

 abilit}' of the plasmatic membrane to the compound in question, 



