COHESION MECHANISMS 557 



B. COHESION MECHANISMS. 272 



Hygroscopic mechanisms, as we have seen, depend entirely upon 

 the swelling power of the cell-membrane. Cohesion movements, on 

 the contrary (which have been studied by Steinbrinck, Schrodt and 

 Kamerling), are brought about by the water contained in the cavities 

 of the motor-cells, the cell- walls at most undergoing passive deforma- 

 tion, which may result in a condition of strain. 



The mode of action of a cohesion mechanism may be explained as 

 follows : As the water contained in the lumen of a motor-cell diminishes 

 in volume through evaporation, it draws the thin portions of the cell- 

 membrane inwards owing to the adhesion between the liquid and the 

 wet cell-wall. The traction exerted in this way, depending as it does 

 to a large extent upon the very considerable internal cohesion of 

 water, is sufficient to produce deformation of any specially thickened 

 cell-walls that may be present ; these are accordingly brought into 

 a condition of strain. As evaporation continues, there comes a time 

 when the cell-wall resists any further invagination or other deformation, 

 or, in other words, overcomes the internal cohesion of the contained 

 water ; the previously continuous water-column thereupon collapses, 

 or, it may be, breaks away from the cell-w T all and a vacuum is suddenly 

 formed in the cell-cavity. The cell- wall is in all probability almost, 

 if not entirely, impervious to air in the dry state, so that this vacuum 

 may persist for some time. As the membrane is readily permeated by 

 water, renewed access of water quickly causes the vacuum to disappear 

 again ; hence the entire process may be repeated again and again. 



What may be termed the primary movement, in such cases, evi- 

 dently depends upon contraction of the thin portions of the wall, the 

 thicker regions undergoing a much smaller amount of deformation. 

 This movement, of course, takes place quite gradually, its speed being 

 determined by the rate at which water is evaporating. When the 

 internal cohesion of the water has been finally overcome, the cell-walls 

 may, from causes which are still obscure, persist in their state of 

 tension ; more usually, however, the elasticity of the strained thickened 

 regions comes into play, and the collapse of the water-column is accom- 

 panied by an instantaneous release of tension, which results in a 

 sudden jerky secondary movement. 



The best-known illustration of a cohesion mechanism is provided by 

 an ordinary Fern-sporangium. In the Polypodiaceae, the single-layered 

 sporangium-wall is furnished with an annulus, which extends over the 

 back and apex of the sporangium, and as far as the middle of its 

 anterior face ; this annulus consists of a row of cells with very thick 

 inner and lateral, but with unthickened outer walls (Fig. 227 a). 



