

42 THE CAUSES OF FLUCTUATIONS IN TUEGESCRNCE 



protoplasts in an intermediate condition, not nearly so highly contracted as they were 

 when the plant was in the white condition, but at the same time not so turgid as m 

 the diurnal green state, so that they do not fully occupy the cell-cavities and still leave 

 narrow peripheral spaces between their outer surfaces and the interior surfaces of the 



cell- walls. 





We have here not merely an illustration of the chromatic effects which may be produced 

 by fluctuations in tumescence, but of the effects of the latter in determining alterations 

 in bulk of protoplasts, and also of the fact that very considerable fluctuations in the 

 turgescence of the protoplasts of a tissue may occur without corresponding fluctuations 

 in turgescence of the tissue as a whole. Whenever natural plasmolysis has occurred in the 

 cells, and the peripheral portions of the cell-cavities have been occupied by air, the 

 internal pressure on the cell-walls must remain practically unaltered, however much the 

 protoplasts go on contracting and losing turgescence. The details which have been 

 already given regarding the structure of the fronds afford a ready explanation of the 

 exceptional phenomena occurring in the latter. The protoplasts of the superior epidermis 

 with their abundant chlorophyll content are specially liable to fluctuations in turgescence 

 connected with the variations in their functional activity — with the variations in osmotic 

 properties in the cell-sap related to the absence or presence of solar stimulation. Under 

 the influence of sunlight, assimilatory activity is stimulated; and, if this be associated 

 with properly regulated supply and loss of water, turgescence rises to a maximum. This 

 does not lead to any appreciable increase in volume of the cells as a whole ; for changes 

 in their bulk are rendered impossible owing to the fashion in which the tissue is stretched 

 over the resistent inferior epidermis and framed by the rigid margin of the frond, but 

 it causes the protoplasts to fill the whole of the cell- cavi ties . On the diminution and 

 final entire cessation of solar stimulation assimilatory activity gradually falls to a minimum, 

 and at the same time transpiratory loss for a time goes on unchecked. There is a loss of 

 osmotic property in the cell-sap, whilst conditions of general loss and supply of water 

 remain practically unaltered, and with this a great fall of turgescence in the protoplasts 

 ensues. Owing to the unalterable dimensions of the cell-cavities this leads to a process of 

 natural plasmolysis, and the vacuities thus established are filled by air, the entrance of which 

 into, as well as the escape of water from, the cavities being specially facilitated by the 

 peculiar pitted character of the walls presenting to the free- surface. Presently, however, 

 with the fall of atmospheric temperature, transpiratory loss diminishes, and, when the dew- 

 point has been arrived at, ceases altogether, whilst root-supply continues, and now the 

 whitening of the tissue also begins to diminish steadily until the fronds appear almost 

 as they do during the day. So long as active transpiratory loss goes on, turgescence 



of the protoplasts goes on falling below the level it would naturally sink to, simply 

 as the result of the loss of osmotic property connected with removal of solar stimula- 

 tion, and consequently the maximum of loss and of whitening of the tissue occurs. 

 On the cessation of evaporation the continued root-supply allows the cell-sap to satisfy 

 its osmotic capacities unfettered, and hence an increase in turgescence takes place. 

 This is sufficient almost entirely to do away with the whitening of the tissue, but does 

 not completely restore the protoplasts to their diurnal state of turgescence, which is 

 only regained when solar stimulation of assimilatory activity once more raises the 

 osmotic capacity of the cell-sap to a maximum. Precisely similar alterations in the turges- 

 cence of the protoplasts, and therefore in their relations to the cell- walls of the cavities in 

 which they are contained, may be artificially induced by exposing the fronds to the 



