MOTILE MECHANISMS IN HIGHER PLANTS 465 



rigid party-walls that separate the cavities of the consecutive 

 cells in the annulus. The general effect is to reverse the general 

 curvature of this structure and to cause it to bend over back- 

 wards. Of course this can only be carried into effect by 

 rupturing the lateral sporangial wall ; the rupture also extends 

 over the front edge just where a weak spot is always present. 

 Then the upper part of the sporangium is rather slowly curled 

 over and it consists of an opened bag containing the spores. As 

 the tensile stress increases in the annular cells the continuity of 

 the water is at last broken, whether by air passing in through 

 the walls or in other ways. Instantly the annulus springs back 

 to its original position, hurling out the spores from the sling-like 

 bag as it does so. The surprising feature of this and similar 



Fig. 3. — Fern Sporangium. 

 a, before dehiscence ; b, in the act of dehiscing ; c, after the ejection of the spores. 



mechanisms is the perfection with which the details of structure 

 are arranged so that the whole works like the mechanism 

 designed by a skilful workman for the fulfilment of a definite 

 and prearranged purpose. 



Another remarkable example of a cohesion mechanism is 

 furnished by the elaters which are found mixed up with the 

 spores of most liverworts. The structures in question are 

 spindle or eel-like cells pointed at each end and possessing one 

 or more band-like coils of spiral thickening on the inner surface 

 of their otherwise thin membranous walls. 



The mode of working varies in the different forms and the 

 degree in which it is due to water cohesion or to hygroscopic 

 qualities also differs ; we are chiefly concerned with the former. 



If one opens a capsule of an Aneura and watches a clump of 



30 



