. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. GLEICHENIACEAE sss whatever may be the doubt as to the proof of Palaeozoic Gleicheniaceae, their existence in the Mesozoic seems clear: certain of the fossils of that age have even been referred to the sub-genera of Gleichenia, as represented by living species. It will be seen that a detailed examination of the living species supports on comparative ground an early origin of the family, such as the fossil evidence suggests. Spore-Producing Members. The naked sorus consists of a low circular receptacle
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. GLEICHENIACEAE sss whatever may be the doubt as to the proof of Palaeozoic Gleicheniaceae, their existence in the Mesozoic seems clear: certain of the fossils of that age have even been referred to the sub-genera of Gleichenia, as represented by living species. It will be seen that a detailed examination of the living species supports on comparative ground an early origin of the family, such as the fossil evidence suggests. Spore-Producing Members. The naked sorus consists of a low circular receptacle bearing a variable number of sporangia. The sporangia are commonly quite separate from one another, though instances of synangia which resemble a fusion of two. Fig. 310. a-h sori of Gleichenia dichotoma, Willd. a-c show sori of radiate type, but with one or more sporangia in the centre of the sorus, usually in this genus vacant. f,g,h show degrees of fission of the sorus. a-hx about 14. i,j, £=sporangia of Gleichenia circinaia, Sw., seen respectively from the side, from the distal end showing the line of dehiscence (x), and from the proximal end showing the stalk. x 50. /, wz, »=sporangia of Gleichenia dickotoma, Willd, seen respectively from the side, presenting the peripheral face. Note the difference in size from G. circinata. X 50. sporangia, are not uncommon. The sporangia usually form a single row round the receptacle; their orientation is in this case constant, the longi- tudinal slit of dehiscence facing directly towards the centre of the rosette- like sorus. Where the number of sporangia in the sorus is more than five, single sporangia may be displaced, perhaps by lateral pressure, and point obliquely upwards. But in Gl. dickotoma, in which the number of sporangia in the sorus may be as high as ten, or even more, the central area of the sorus, which is usually vacant in other species, may also be occupied by sporangia. Figs. 310, a-e, show cases of the insertion of
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