SECT. vm. POLYPODINEJE. 345 



being rigid and opaque, and usually dichotomously- 

 branched ; Trichomaninese, with the ring resembling 

 that of the Gleichenineae, but the sporangia lenticular, 

 numerous, clustered on an exserted receptacle, which is 

 a prolongation of the vein beyond the ordinary margin 

 of the frond, so that the sori become extrorse marginal, 

 or projected outwards as well as opening outwardly, 

 while the fronds are pellucid-membranaceous ; Schizse- 

 ineae, with the ring horizontal or transverse, situated 

 quite at the apex of the oval sporangia, which is, in con- 

 sequence, said to be radiate-striate at the appx ; Cera- 

 topteridineae, one or two aquatic species, the sporangia 

 sometimes furnished with a very rudimentary ring, 

 reduced, as in Osmundineae, to a few parallel striae, some- 

 times furnished with a very broad and more lengthened 

 ring ; and Osmundineae, with the spore-cases two-valved, 

 bursting vertically at the apex, the ring very rudimen- 

 tary, reduced to a few parallel vertical striae on one side 

 near the apex. In all but the last of these groups, the 

 sporangia are not valvate, and consequently, when they 

 open for the liberation of the spores, they burst par- 

 tially or irregularly ; but in the Osmundineae they split 

 at the top in two equal divisions. 8 



A large portion of the Polypodineae are either tropical 

 or subtropical. The genus Polypodium itself is one of 

 the most extensive and diversified genera of the group. 

 It is chiefly distributed over the tropical regions of the 

 western hemisphere, but four species are indigenous in 

 Britain, and of these the Polypodium vulgare, or Com- 

 mon Polypody, is abundant about the trunks of moss- 

 grown trees, on banks, rocks, and old thatched roofs. 

 The young fronds appear in May, and rise from five or 

 six to twelve or eighteen inches in height. They are 

 lanceolate, deeply pinnatifid, with obtuse, linear, lan- 

 ceolate, indistinctly serrated segments. The genus is 



8 Moore, in ' Treasury of Botany.' 



