THE FERNS AND THEIR ALLIES 9 



entirely stalkless. The sorus, as the group of 

 sporangia is called, is surrounded by an en- 

 closure from the leaf margin. In Trichomanes 

 this is cup-shaped, whilst in Hymenophyllum it 

 is bivalved. The popular name Filmy Fern 

 bestowed on the Hymenophyllums and allied 

 species has reference to the semi-transparent 

 nature of the fronds. In the case of Tricho- 

 manes the axis on which the sporangia are 

 inserted often projects beyond the cup in which 

 they are contained. This gives a curious spiky 

 appearance to the fertile frond, and hence the 

 name Bristle Fern. 



2. Polypodiacea. This is a very large family, 

 containing two or three times as many species 

 as all the rest of the Vascular Cryptogams put 

 together. Nearly all our native species, with a 

 few exceptions, belong to the family. A dis- 

 tinctive feature is the incomplete annulus of the 

 sporangium. Another point to notice is that 

 the spore cases are stalked. So large is the 

 family that it has been divided into a number of 

 sub-families ; the members of these are chiefly 

 characterized by the position of the sorus, the 

 cluster of sporangia on the back of the frond. 

 The different sub - families may be briefly 

 outlined. 



(a) Davalliacece. There are no British re- 

 presentatives of this family. In this case the 

 sorus is always near to the margin of the leaf, 

 and the indusium or covering is cup-shaped. 

 A familiar species is Davallia bullata from .the 

 East ; the rhizomes of this Fern are trained 

 into various shapes by the Japanese. 



(b) Pteridea. lte Bracken Fern (Pteris 

 aqmlina), the Maiden Hair (Adiantum capillus- 

 veneris), and the Parsley Fern (Cryptogramme 

 crispus) belong to this sub-family. A notable 



