122 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



submerged in distilled water in bottles which once contained methyl-blue, or violet, 

 will become distinctly coloured, and this affords a useful illustration of the amount 

 of care which must be taken in physiological experiments to insure absolute purity 

 and cleanliness. Owing to this pronounced power of selective absorption and 

 passive secretion, it becomes increasingly difficult to insure the entire absence of all 

 iron, or of potassium, or phosphoric acid, &c., when a small number of plants are 

 grown upon a large quantity of culture fluid. 



The special diosmotic properties of the plant and of the substances 

 absorbed, together with the changes which the penetrating substances 

 may undergo internally or externally to the cell, suffice to explain the 

 attainment of this physiological selective capacity. The diosmotic nature 

 of a substance determines in the first instance whether it can penetrate 

 the plant, either reaching the interior of the cell, or not passing beyond 

 the cell-wall, and for the outward passage of a substance, diosmotic 

 relationships are of equal importance. In either case, if the substance 

 can pass from cell to cell, it will continue to be absorbed (or evolved), 

 until a condition of osmotic equilibrium is reached. This is never attained 

 if the substance is continually being altered or removed ; and in this 

 manner the most minute traces of a salt present in a dilute solution may 

 be absorbed. 



Within the plant, accumulation of a substance is due to its 

 undergoing metabolic alteration as rapidly as it is absorbed, while the 

 substances which the plant rejects are removed by the surrounding medium. 

 The evolution of carbonic acid gas formed in a plant or of alcohol 

 produced in a yeast-cell continues as long as the accumulation of these 

 bodies in the surrounding air or fluid is prevented. The excrete di- 

 osmosing substances formed in a tissue-cell move away from a certain 

 centre of repulsion towards the excreting surface or region, translocation 

 being due to diffusion, imbibition, and diosmosis. Translocation of sub- 

 stances produced within the plant from one region to another is due 

 primarily to the fact that at the latter point the diosmosing substances 

 are being altered or used in metabolism. The changes which the trans- 

 located substances undergo may be of far-reaching nature or of only 

 trifling degree, for it is sufficient that an insoluble combination should be 

 precipitated either inside or outside the cell, or again that a soluble 

 but non-diosmosing compound should be formed in the cell-sap. Even 

 when no accumulation takes place, a plant or even a single cell may serve as 

 an attractive centre for a diosmosing substance, for if the latter is converted 

 into a different, but still diosmotic compound, which in turn diosmoses 

 from the cell, a continual current will be thus maintained, the one sub- 

 stance passing into the cell, the other away from it. Under appropriate 

 conditions and in certain cases, of which examples have already been 



