558 



MOTOR SYSTEM 



When water is removed by evaporation from the cavities of the 

 annulus ceils, its cohesive power causes the thin outer walls to sink 

 inwards, with the result that the thick, rigid, lateral walls are drawn 

 closer together. The whole annulus consequently tends to straighten 

 itself, and in so doing brings about the dehiscence of the sporangium. As 

 water continues to escape, the annulus may gradually bend far back- 

 wards, so that the side which was formerly convex (i.e. the outer or 



Pig. 227. 



Dehiscence of a Fern-sporangium (Pterin, sp.). A. Ripe, but still intact, spo- 

 rangium. B. Sporangium dehiscing. Annulus curved outwards and about to recoil ; 

 most of the spores still unshed. C. After the recoil of the annulus ; all the spores 

 ejected. 



thin-walled side) becomes concave (Fig. 227 b). This primary move- 

 ment takes place quite slowly. The drawing-in of the outer walls, and 

 the deformation or lateral approximation of the radial septa, finally 

 become so pronounced that the cohesion of the water is no longer able 

 to withstand the elastic tension of the membranes ; when this stage is 

 reached, the water-columns in the cells suddenly collapse, the tensions 

 in the radial and outer walls are instantly released, the whole annulus 

 recoils with a violent jerk, and, striking against the surface of the leaf, 

 throws the sporangium bodily upwards, or to one side. It is this 

 sudden secondary movement, that actually brings about the dispersal of 

 the spores. Eepeated ejaculation may take place, if the water-columns 

 do not give way simultaneously in all the cells of the annulus. 



In the Hymenophyllaceae and Cyatheaceae, where the oblique 

 annulus completely encircles the sporangium, the mechanism is the 

 same as in the Polypodiaceae. For an account of the variations in 

 detail exhibited by other Pteridophyte-sporangia, the reader is referred 



