566 FILICALES 



one side, an ill-defined lateral stomium being present, while the rupture 



is by a ragged lateral slit, opened by the straightening annulus (Fig. 316). 



i The sorus originates as a smooth upgrowth from the lower surface of the 



pinnule, opposite a nerve, a considerable number of cells being involved 



from the first ; no definite mode of segmentation 

 has been recognised (Fig. 317 F). As develop- 

 ment proceeds, the margin of the upgrowth 

 extends all round, as the overarching indusium 

 (/, /') ; this, undergoing a somewhat regular seg- 

 mentation by anticlinal walls, curves so as to 

 cover in the sporangia which arise below (s., Fig. 

 317 F) ; the indusium thus precedes the appear - 

 to^^ ance of the sporangia, as in many other indusiate 



Ferns. The sporangia originate from single cells, 



FIG. 316. i'ii i 



which have commonly a square base, though it 





may be a question whether this is always so. 

 Sew n ar3 a ) turally detached ' (Mter The segmentation is by walls inclined to one 



another; the first wall is usually on the side next 



to the leaf-surface, and meets one of the lateral walls of the parent cell ; 

 then follow three other inclined walls, and the segments thus produced 

 surround a central triangular wedge-shaped cell, from which finally the 

 cap-cell is cut off in the usual way (Fig. 317 E). 



The further segmentation of the central cell follows the course usual 

 for Leptosporangiate Ferns; a double tapetum is formed (Fig. 317 D) of 

 which the inner cells become greatly enlarged, and their nuclei, clustering 

 round the sporogenous group of cells, and undergoing fragmentation, 

 present an appearance very like that in Gleichenia ; the archesporium 

 divides into 1 6 spore-mother-cells, and the typical number of spores seems 

 to be 64 : countings of mature spores gave figures between 48 and 64 as 

 the produce of single sporangia. Sections of sporangia, when cut so as 

 to traverse the annulus throughout its course, show the wall as a single 

 layer, but composed of more numerous cells than is the case in many 

 of the Leptosporangiate Ferns (Fig. 317 D) ; this is also brought out 

 plainly in views of the mature sporangia from without (Figs. 317 A, B, c). 

 It may be noted further that the stalk, which remains very short, is 

 rather massive, and consists of a peripheral series of six or seven cells, 

 surrounding a central cell (Fig. 317 A), which corresponds to the structure 

 of the stalk in the massive sporangia in Gleichenia and Osmunda. 



The mature sporangium is a body of rather irregular and variable 

 form, owing apparently to pressures in the developing sorus. The annulus 

 is incomplete and variable in position ; it consists of a series of large 

 cells, 20 or more in number, which takes an oblique and sinuous course, 

 corresponding in the main to that in Gleichenia. The sporangia are 

 liable to be tilted right or left, as shown in Fig. 317 B, which represents 

 two sporangia in situ, as seen from the side facing the indusium. 



