SECT. I 



CRYPTOGAMS 



453 



own elasticity to its original position, tlms effecting the dispersal of the spores. 

 The sporangium remains open owing to the drying and contraction of the thin cell 

 walls ('-8). 



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 Scoloixndrium 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 Aspidium, on the other hand, each sorus is 

 orbicular in form and covered by a peltate or reniform indusium attached to the 



apex of the placenta ; a glandular hair 



is frequently present on the stalk of 



the sporangium. The sori of Poly- 



/ podium vulgare are also orbicular, but 



J they have no indusia. In the common 



/ Bracken, Pteris aquilina, the sporangia 



Fio. 399. — Trichomanes rlgidum. Portion 

 of a prothallus with an archegoniophore 

 {A) to wliich a young plant is still 

 attached. (Aftpr Goebel.) 



Fig. iOO. — Polypodimn vulgare. A, Mature, 



B, discharged antheridium ; p, prothallium 

 cell ; 1 and 2, ring-shaped cells ; 3, lid-cell ; 



C, D, spermatozoids. {A, B x 240 ; C, D 

 X 540.) 



form a continuous line along the entire margin of the leaf, which folds over and 

 covers them. 



Besides tlie Polypodiaceae the Ferns include other families, mainly represented 

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

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

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

 annulus extending obliquely over the apex of the capsule (Fig. 398, B, C). 

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

 sporangia, with a complete, oblique, or horizontal annulus. The sporangia of the 

 Schizaeaceae. on the other hand, have an apical annulus (Fig. 398, D), while in 

 the Osmundaceae, of which the Royal Fern, Osmunda reyalis, is a familiar example, 

 the annulus is represented merely by a group of thick-walled cells just below the 

 apex of the sporangium (Fig. 398, E). 



All the members of the Filices are homosporous. The prothallium has usually 

 the form of a flat, heart-shaped thallus (Fig. 398), bearing the antheridia and 

 archegonia on the under side. In certain Hymenophyllaceae {Trichomanes) the 



