DIV. I 



PTERIDOPHYTA 



509 



On drying of the wall of the sporangium the cohesion of the remaining 

 water in the cells of the anuulus draws in the thin outer walls of these cells ; this 

 causes the annulus to shorten and determines the dehiscence of the sporangium by 

 a tranverse slit between the broad terminal cells of the annulus. When the pull 

 exerted by the cohesive power of the water suddenly gives way, the annulus returns 

 by its own elasticity to its original position, thus effecting the dispersal of the 

 spores. The sporangium remains open owing to the drying and contraction of 

 the thin cell walls ( 1I8 ). 



The form and insertion of the sori, the shape of the indusium when present, 

 or its absence, all constitute important criteria for distinguishing the different 

 genera. The sori of Scolopendrium (Fig. 472) are linear, and covered with a 

 lip- shaped indusium consisting of one cell- layer. They are so disposed in pairs 

 on different sides of every two successive nerves, that they appear to have a 

 double indusium opening in the middle. In the genus Dryopteris (Aspidium), on 

 the other hand, each sorus is orbicular in form and covered by a peltate or 



FIG. 473. Sporangia. A, Dryopteris (Aspidium) filij; mas; there is a glandular hair at the base. 

 B and C, Alsophila armata, seen from the two sides. D, Aneimia caudata. E, Osmunda 

 mjalis. (A-D x 70 orig. ; E x 40. After LURSSEN.) 



reniforrn indusium attached to the apex of the placenta ; a glandular hair is 

 frequently present on the stalk of the sporangium (Fig. 471). The sori of Poly- 

 podium vulgare are also orbicular, but they have no indusia. In the common 

 Bracken, Pteridium aquilinum, the sporangia form a continuous line along the 

 entire margin of the leaf, which folds over and covers them. 



Besides the Polypodiaceae the Ferns include other families, mainly represented 

 in the tropics, the sporangia of which differ in the construction of the annulus 

 and in the mechanism of their dehiscence. The sporangia of the Cyatheaceae, to 

 which family belong principally the Tree-Ferns, are characterised by a complete 

 annulus extending obliquely over the apex of the capsule (Fig. 473 B y C). 

 The Hymenophyllaceae, often growing as epiphytes on Tree-Ferns, have also 

 sporangia, with a complete, oblique annulus. The sporangia of the Schizaeaceae 

 and Gleicheniaceae, on the other hand, have a transversely-placed annulus which, 

 in the former (Fig. 473 D), is close to the tip and in the latter above the middle 

 of the sporangium, while in the Osmundaceae, of which the Royal Fern, Osmunda 

 regalis, is a familiar example, the annulus is represented merely by a group of 

 thick-walled cells just below the apex of the sporangium (Fig. 473 E). In the 

 three last-named families the sporangia open by a median split ; in the three 

 preceding families the dehiscence is transverse or oblique. There are thus two 



