go THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



other scarcely at all. A continuous disturbance of equilibrium may be 

 maintained, if the absorbed substances at once undergo a more or less 

 marked chemical change or alteration into soluble or insoluble compounds 

 of different character. 



The power of determining the relative amounts of absorption (and 

 also of excretion) of particular substances is dependent upon the changes 

 and modifications, in the widest sense, which these undergo when ab- 

 sorbed, and hence upon the vital activity of the organism. Every 

 substance which accumulates in the cell must therefore necessarily enter 

 it in a slightly different form. In many cases, this may actually be 

 observed, and so far no other explanation appears to be necessary ; 

 although it is quite possible, that in certain cases the influence of the 

 protoplasm may be such as to cause particular substances to accumulate 

 within the cell in the same form as that in which they entered. 



The osmotic properties of the substances present in solution in the 

 cell-sap are such as to cause the protoplast, when in the form of a 

 primordial utricle, to be closely adpressed against the cell-wall, which 

 serves as a support for it, and which it stretches to a certain extent. The 

 preservation of this condition of turgidity by an internal regulatory 

 mechanism is essential for the continuance of life and growth, and in 

 turgid cells, the osmotic pressure may attain a value of from a few to 

 many atmospheres, whereas in gymnoplasts, the osmotic pressure is only 

 trifling and, indeed, could hardly be otherwise. The substances, to which 

 the osmotic pressure is due, are in general compounds incapable of 

 diosmosis, and a very dilute solution of a non-diosmosing crystalloid body 

 will suffice to maintain a comparatively high osmotic pressure. In an 

 isosmotic saline solution, this pressure is just equilibrated, whereas in 

 a more concentrated solution the protoplast becomes plasmolyzed, and 

 contracts, until a condition of isosmotic equilibrium is again reached. 



SECTION 16. The Diosmotic Properties of the Cell. 



Having now taken a general view of the subject, we may pass on to 

 a more detailed account of the osmotic exchanges of turgid cells, whose 

 cellulose walls arc readily permeable by water. The osmotic properties 

 of the protoplasm are of similar character, whether it is naked, or sur- 

 rounded by a cell-wall ; hence the general account given will apply to 

 both, and the importance of certain special properties of the cell-wall will 

 be mentioned later (Sect. 21). In order to reach the cell-sap a particle 

 of water, or dissolved substance, must diosmose first through the cell- wall (#) 

 and the plasmatic membrane (/>'), which is closely applied to it, and 

 finally pass through the internal limiting plasmatic membrane (/>"), which 

 bounds the vacuole (Fig. 4). Any substance dissolved in the cell-sap must 



