300 DEVELOPMENT OF PROTHALLUS 



leads to rupture of the capsule-wall along the plane of greatest 

 weakness, viz. the junction between the transversely extended 

 cells of the stomium (Fig. 166, B). On the sudden release of 

 the tension the annulus, carrying with it a great part of the 

 capsule-wall and many of the contained spores, flies back beyond 

 the position of equilibrium, and, with the subsequent recoil, 

 the spores are shot out as from a sling ; they are further dispersed 

 by air-currents, and may thus be carried to a considerable 

 distance. The dehiscence of ripe sporangia can be observed 

 under the microscope by adding glycerine. 



Apart from the difference of arrangement, to which reference 

 has already been made (p. 296), the sporangia of the Bracken, 

 and most other British Ferns, agree in all essential respects with 

 those of the Male Fern. Other types occur, however, as, for 

 instance, in the Royal Fern (Osmunda regalis). Here the spo- 

 rangia are short-stalked and pear-shaped, and lack the charac- 

 teristic annulus and stomium of the other forms. Dehiscence 

 takes place by a vertical split starting from a group of thick- 

 walled cells, a little below the summit of the sporangium, and 

 extending over the top of the latter and some way down the 

 opposite face (Fig. 173, B). 



The spores of most Ferns retain their capacity for germination 

 for some time. In their thick walls three layers can often be 

 distinguished, of which the outermost, usually dark-coloured 

 and opaque (Fig. 166, A), is cuticularised, and constitutes the 

 principal protective covering, whilst the innermost is thin and 

 elastic. In germination (Fig. 166, F) the outer coats (Sp.) are 

 burst, and the innermost is stretched to form a long, colourless, 

 tubular outgrowth, which becomes separated off from the main 

 body of the spore by a transverse wall, and penetrates the 

 soil as the first rhizoid (r.) of the future plant. The remainder 

 of the spore-contents, enveloped by the innermost layer of the 

 wall, lengthen into a short horizontal filament (p.) whose few 

 cells contain chloroplasts. Within the terminal cell two inter- 

 secting walls soon arise, and the apical cell thus formed cuts off 

 segments on either side, so that the filament widens into a flat 

 plate. 



As the front margin broadens, two or more of its cells become 

 meristematic (Fig. 167, g.p.) and development proceeds rapidly ; 



