GLKICHENIACEAE 



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 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-h X about 14. z, /, k = sporangia of Gleichenia circinata, 

 Sw., seen respectively from the side, from the distal end showing the line of dehiscence 

 (-r), and from the proximal end showing the stalk. X 50. /, m, n = sporangia of 

 Gleichenia dichotoma, Willd, seen respectively from the side, presenting the perigheral 

 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 GA 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 



