GLEICHENIACEAE 555 
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 détailed 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 
Fic. 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. 4, ¢, 4 show 
degrees of fission of the sorus. @-2xabout 14. 2,7, 4=sporangia of Gleichenia circinala, 
Sw., seen respectively from the side, from the distal end showing the line of dehiscence 
(x), and from the proximal end showing the stalk. x50. 4% m, »=sporangia of 
Gleichenia dichotoma, Willd, seen respectively from the side, presenting the peripheral 
face. Note the difference in size from G. circinata. X50. 
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 Gé. dichotoma, 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 sporangia on the 
apex of the receptacle; the number of these sporangia may vary from 
one upwards, and they form a second tier above the basal rosette. When 
