4 * THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



does not in itself determine any appreciable diminution in the mass of contents 

 of the latter, but at the utmost a mere rearrangement of them. Active contraction will 

 give rise to increased pressure in fluids contained within the substance of the contracting 

 protoplast, but as these are relatively incompressible, the utmost that this is calculated 

 to give rise to is a mere redistribution, a certain amount of fluid escaping from the 

 interior of the protoplasm and accumulating between it and the cell-wall in plasmolytic 

 fashion. In so far as alterations in the mass of cell-contents are concerned, a protoplast 

 contracting within a cell is practically the parallel of a mass of muscle contracting within 

 a closed vessel of water. The distribution of the muscular mass and of the surrounding 

 water is altered, owing to the change of form occurring in the former on contraction, but 

 the total bulk of content of the vessel remains practically unaltered. 



Mere protoplastic contraction being thus insufficient in itself to give rise to diminished 

 turgescence in cells, we have next to consider how far we have any reason to believe 

 that any redistribution of the cell- contents accompanying it may be capable of doing 

 so. Were turgescence in cells necessarily dependent on the presence of fluids included 

 within the substance of the protoplasm, there would be good grounds for supposing that 

 any discharge of these from the latter might lead to diminished turgescence; but as 

 we know that turgescence is directly related to the products of protoplasmic activity 

 and not to protoplasm itself, and that it may be present in high degree apart from the 

 presence of any living protoplasm whatever, we have no grounds for regarding any mere 

 redistribution of the cell-contents, apart from alterations in their osmotic properties, as 

 efficient causes for alterations in cell-turgescence. 



If contraction of the protoplasts implied increased pressure on the exterior of the 

 cell-walls, or were necessarily accompanied by change in osmotic properties of the cell- 

 contents, it might well be accompanied by diminished turgescence; but as it certainly 

 cannot lead to the former, and as there is no evidence that it is necessarily associated 

 with the latter, we are justified in regarding it as incapable of affecting cell-turgescence 

 either directly or indirectly. The essential factors which determine the normal fluctuations 

 in turgescence in vegetable cells are alterations in the osmotic properties of their contents 

 and alterations in the conditions regulating general supply and loss of fluid or local 



filtration; and it does not appear that protoplastic contraction per se is capable of 

 producing such chan 





o 



Because conspicuous massive movements occur much more frequently in the organ- 

 isms of the higher animals than in those of the higher plants, it has been assumed that 

 the mechanism whereby they are produced in the former must necessarily be that which 

 causes them in the latter ; but this is of course no matter of logical necessity. Move- 

 ments in an organism may be arrived at by two perfectly distinct paths— either by means 

 of alterations in form or alterations in bulk of its constituent elements ; in the case of 

 the higher animals the former, and in the case of the higher plants the latter, is that 

 which has been followed. 



The lines which evolution has pursued in the animal and vegetal series are divergent ; 

 in the former contractile and, in association with this, irritable function ; and in the latter 

 assimilatory function has been progressively highly specialised. The higher plants, ou. 

 to their great superficial extension and highly evolved assimilatory capacity, are enabled ... 

 obtain sufficient nutritive materials from those supplies which are generally diffused through- 

 out their environment ; consequently any special means for bringing them in condensed 



ing 



and massive form into immediate relation to the organism are unnecessary. In the 



