Lecture XX. 163 



forty-eight small brown spores. In their early stages they adhere 

 together in fours, showing that they have been developed as 

 tetrads in separate mother-cells. As the spores and the sporangium 

 ripen, the water filling the cavity of the cells of the annulus 

 diminishes in volume by evaporation through the outer thin walls. 

 The cohesion of the water and its adhesion to the walls draws 

 the walls together. The thin outer and side walls collapse under 

 this tension and are thrown into folds, while the U-shaped radial 

 and inner walls, resisting the pull more effectively owing to their 

 rigidity, are bent into a C-shape. This alteration of form of the 

 component cells leads to a shortening of the outer margin of the 

 annulus and changes it from being convex to being concave ex- 

 ternally. 



This distortion of the annulus tears the cells of the stomium 

 asunder and makes a rent in the sporangium which spreads 

 radially inwards across the wall. Thus the lower half of the 

 sporangium as it hangs from the under side of the leaf with the 

 spores lying in it, is bent backwards by the contraction of the an- 

 nulus, the rigid walls of each cell of the latter being compressed 

 like a C-shaped spring by the tension in the water. As evaporation 

 proceeds the tension mounts higher and higher till at last the cohe- 

 sion of the water is overcome and the water breaks. The tension 

 thus relieved releases the spring, which immediately recovers its 

 original U-shaped form, and in doing so jerks out the spores in the 

 sling-like portion of the sporangium attached to the annulus. The 

 tension at which the break occurs has been found to be about three 

 hundred atmospheres. If the break does not occur simultaneously 

 in all the cells of the annulus, the breaking of the water successively 

 in different cells communicates a series of jerks to the sling, and the 

 spores from each sporangium are ejected in small numbers at a 

 time. The efficiency of this mechanism for distributing the spores 

 at different intervals and in different directions will be realised 

 when it is remembered that the sporangia in the sori ripen 

 successively arid that they are oriented in all directions round the 

 placenta. 



The spores when ejected are tetrahedral in form with a brown 

 cuticularised outer layer to their cell-wall. They contain a nucle- 

 ated mass of protoplasm, including many chloroplasts. Once on 

 damp soil they absorb water and become spherical. The inner 

 layer of wall is forced out and breaks through the cuticularised 

 layer, thus forming a tubular outgrowth into which the cell-contents 

 pass. The chloroplasts elongate, divide, and multiply. The 

 tubular cell divides and a filament is formed. Sooner or later 



