2TO GLEICHENIACEAE [CH. 



Platyzoina\\v^% together by its irregularities the Gleicheniaceous ype of sporangium with 

 that of the Leptosporangiate Ferns (Compare Ceratopteris, Thompson, Trans. R. S. E. 

 Vol. liii, ii, PI. vii). The sporangia of this Fern could be ranged after two types, large and 

 small, the latter being in the majority. The spore-number is far lower than in any other 

 Gleichenioid Fern. The large sporangia contain large spores numbering from i6 down 

 wards, the smaller contain small spores, from 32 downwards. The difference in size of the 

 spores is very great (see Vol. I, p. 265, Fig. 258), but intermediate types occur between the 

 extremes. The spores are tetrahedral, and are all similarly marked, and they have thick 

 walls. The facts appear to suggest an incipient state of heterospory, but there can be no 

 proof of this till the germination has been observed 



The dehiscence of the Gleicheniaceous sporangium is by a slit in the 

 median radial plane, but in Platyzoma it is variable, and often lateral with 

 a definite stomium. The annulus which is continuous all round, except 

 along the line of rupture, becomes gradually straightened on drying, the 

 whole sporangium thus widening laterally. Plainly this mode of dehiscence 

 requires lateral space, and it is thus ill-suited for a crowded sorus, though 

 it will serve well enough for lax sori, such as those of Eii-GlcicJienia, or 

 possibly for those oi Dicranopteris where the centre of the sorus is unoccupied. 

 Where the central space, usually vacant, is occupied by one or more short 

 stalked sporangia attached to the central receptacle, difficulties are apt to 

 arise: these are, however, lessened in G. linearis by the smaller size of the 

 sporangia (Fig. 486). But in G. pectinata the space is fully taken up: here 

 the sporangia are arranged in two tiers, the upper fully occupying the central 

 space, and so closely are the numerous sporangia packed that their sides 

 are flattened against one another, leaving little or no lateral room available 

 for the widening movement of dehiscence (Fig. 488). The ineffectiveness 

 of the result is shown by the fact that many sporangia may be seen fully 

 ripe, but with the spores still inside. There is in fact a deadlock, which 

 could only be relieved by some structural change: such as enlargement of 

 the receptacle, lengthening of the sporangial stalk, diminution of the 

 sporangia, or by adopting some other method of dehiscence. It will be seen 

 later that other Ferns have progressed along one or o^her of these lines: 

 but Gleichenia has stood still, and has adopted none of those alternatives. 



F'OSSILS 

 T\\(t flabellata-X.y'^Q of foliage is essentially Pecopterid (Fig. 476), but that 

 does not define affinity: nor do the continued apical growth and peculiar 

 branching of the leaf Both of these features are such as might well be shared 

 with plants of other relation, such as the Pteridosperms. The identification 

 must depend upon the conjunction of such characteristics with a soral structure 

 clearly Gleicheniaceous. The genus Oligocarpia, founded on Fern-like 

 fronds from the Coal Measures bearing circular sori with few sporangia, has 

 been quoted as proving the existence of Gleicheniaceae in Palaeozoic 



