556 FILICALES 
one of these only is present it usually occupies a central position. The 
orientation of these central sporangia is not constant. By the presence 
of these supernumerary sporangia the gap is bridged over within a single 
genus, between two well-marked types of sorus; on the one hand are the 
Marattiaceae, and most of the Gleicheniaceae, representing the ‘radiate 
uniseriate” type, with a single linear series. of sporangia, surrounding the 
periphery of the low receptacle; on the other hand are the Cyatheaceae, 
Dicksonieae, Loxsomaceae, and Hymenophyllaceae, with a more or less 
elongated receptacle covered to its apex with numerous sporangia. 
As in other genera where the sorus is circumscribed, so also in Géeichenia, 
fissions of the sorus may be found, chiefly in conjunction with branching 
of the veins. Examples of this are shown in Fig. 310 fg, 4%. 
The sporangia have an annulus, consisting typically of a single row of 
cells: it is complete round the head, with the exception of the region of 
dehiscence, which is on the side directed away from the lower surface 
of the leaf (Fig. 310 z-»). The position of the annulus is oblique, so 
that of the two thinner areas of the sporangial wall which lie on either 
side of it, the one faces obliquely towards the centre of the sorus, and 
away from the leaf-surface, the other obliquely away from the centre, and 
towards the leaf-surface. The former may be styled the acroscopic or 
central, the latter the basiscopic or peripheral face of the sporangium. 
There is considerable variation in size of the sporangia in the genus 
Gleichenia. Those species which have a small number of sporangia in the 
sorus, such as Gi. rupestris and circinata, have relatively large sporangia 
(Figs. 310 2, 7, 2); those which have more numerous sporangia in the 
sorus have them of smaller size, ¢.g. G/. dichotoma (Figs. 310 4, m, 2). Taking 
first the sporangia of the larger type, as seen in G/. arcinata, the form 
is almost that of a kettledrum; the “ peripheral” face is almost flat, and 
lies in apposition to th® leaf-surface, while the annulus runs round its 
margin; the “central” face is very convex. The stalk is short, and con- 
sists of a central group of cells, surrounded by a peripheral series; it is 
thus thicker than in ordinary Leptosporangiate Ferns. The sporangium 
of Gletchenia dichotoma is of much more elongated form, the stalk is 
thinner, and has no central group of cells: the annulus rises more 
obliquely from the surface of the leaf. Gleichenia flabellata holds a 
middle position between these two types as regards size and shape of the 
sporangium, but in the number of spores produced in each sporangium 
it is, as we shall see, an extreme type. 
Tracing the development in G. fladel/ata, the sorus first appears in the 
still tightly circinate pinnule ; it arises as a smooth outgrowth opposite a 
nerve (Fig. 311 a), a considerable number of cells being involved in its 
origin. Having grown to a height almost equal to the thickness of the 
pinnule, it becomes flattened at the apex; in those cases where the sorus 
is to be a simple rosette (Figs. 311 3, ¢), the convex margin begins to 
grow out as rounded processes, which develop into the sporangia. There 
