2 THE CAUSES OF FLUCTUATIONS IN TUKGESCENCE 



by the nature and amount of the products to which its osmotic properties are due. 

 "Where the former are unstable, the cessation or alteration of protoplasmic activity will 

 at once give rise to corresponding changes in the degree of turgidity; where they are 

 stable and present in considerable quantity, turgidity may remain for long, inappreciably 

 affected by death or even complete absence of protoplasm. So long as the osmotic 

 materials which are the direct cause of turgescence are present, the latter will persist 

 under conditions of sufficient water-supply, &c, whether protoplasm be present or absent, 

 and no action of living protoplasts, unless affecting the stock of osmotic materials, can 

 produce any effect on the turgescence of cells. 



It may perhaps have been observed that the statement in regard to the effects of 

 turgescence on tissues is a qualified one. The qualification is necessary, because it does 

 not invariably happen that turgescence of the protoplasts of a tissue implies turgescence 

 of the latter as a whole. So long as any protoplast retains its vitality, so long as it 

 continues to exercise its functional activities, it appears normally to be more . or less 

 turgid, and the coincidence of protoplastic and cellular turgescence, which we find 

 constantly prevailing in most tissues, is due to the relation which the protoplasts 

 bear to the cell-cavities within which they are situated, and to the fact that even 

 minimal normal turgescence in the former implies a certain amount of active internal 

 pressure in the latter. In any case turgescence is du c j to the presence of certain 

 products of protoplasmic activity, but so long as they are confined to the interior of 

 the protoplasm, either diffusedly or within accumulations of cell-sap, and the proto- 

 plasts are situated within cavities with rigid walls, it is clear that various degrees of 

 protoplastic turgescence may be present without any coincident turgescence of the cells 

 as a whole, and that depression of protoplastic turgescence below a certain degree 

 must give rise to phenomena of natural plasmolysis. The fall in turgescence in the 

 protoplasts implies a corresponding diminution in their bulk, and as the cell-walls are rigid 



this must tend to a separation of the protoplasm from them. In such cases we have 

 plasmolytic phenomena arising from intrinsic causes, whilst in experimental plasmolysis 

 they &re due to extrinsic ones; in artificial plasmolysis the exosmotic property of the 

 medium is raised ; in natural plasmolysis the endosmotic properties of the protoplasts 

 are lowered. 



In the great majority of cases, however, no such phenomena present themselves 

 under normal circumstances, and even minimal protoplastic turgescence implies a certain 

 degree of turgescence of the tissue as a whole. The interior of the cell-walls is thus 

 constantly subjected to a certain amount of pressure, which may or may not be accom- 

 panied by correspondingly active distention of the cell-cavities. The actual decree 

 of tension present at different times, however, varies greatly, the variations being deter- 

 mined by two perfectly distinct sets of factors— factors telling on the absorptive and 

 retentive properties of the cell contents, and factors affecting the supply of water 

 available for absorption and retention, or causing variations in the conditions of exter- 

 nal pressure to which the cells are exposed. The former are directly related to the 

 functional activities of the protoplasts ; the latter to the general loss and supply of fluid 

 dependent on atmospheric and telluric conditions, to conditions affecting the water- 



conducting elements of the tissues, or to conditions determining local alterations 

 pressure in the absorptive tissues themselves. , 



According to the relations which these two sets of factors bear to one another at 

 different times, variations in the degree of turgescence of the tissues must necessarily be 



