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 Gleichenia, 

 fissions of the sorus may be found, chiefly in conjunction with branching 

 of the veins. Examples of this are shown in Fig. 310^ g, Ji. 



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 i-n). 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 GL rupestris and tircinata, have relatively large sporangia 

 (Figs. 310 z, y, k}; those which have more numerous sporangia in the 

 sorus have them of smaller size, e.g. GL dichotoma (Figs. 310 /, m, n\ Taking 

 first the sporangia of the larger type, as seen in GL circinata, the form 

 is almost that -of a kettledrum ; the " peripheral " face is almost flat, and 

 lies in apposition to the 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 i 

 thus thicker than in ordinary Leptosporangiate Ferns. The sporangium 

 of Gleichenia 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. flabellata^ 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 b, g), the convex margin begins to 

 grow out as rounded processes, which develop into the sporangia. There 



