8 HOW TO KNOW THE FERNS 



are important guides. The actual position of 

 the sorus on the leaf, the presence or absence 

 of a covering (indusium), are also distinctive 

 features, both in the families and sub-families. 

 When the individual sporangium is examined 

 it is found that there is often present an annulus, 

 a special ring of cells which plays an important 

 part in the rupturing of the case. The extent 

 of this ring or (as sometimes happens) its 

 absence will alike be a decisive factor in fixing 

 the family to which a species belongs. In 

 some families a prominent feature is the fact 

 that the sporangium has little or no stalk, 

 although this is the exception rather than the 

 rule. For a more complete description of the 

 sporangium of the Fern the reader is referred to 

 a succeeding chapter. It is certainly helpful to 

 a study of these beautiful plants to try to fix 

 in the mind the families, and their characters, 

 of the order Filices, In all there are eight 

 families belonging to the Fern tribe. These are 

 given in the order in which they occur in 

 technical books. 



i. Hymenophyllacea. The Filmy and Bristle 

 Ferns. This family includes some of the 

 simplest kinds of Ferns. There are only three 

 representatives in the United Kingdom. These 

 are Hymenophyllum tunbridgense, H. Wilsoni, 

 and Trichomanes radicans. The two former 

 species are fairly common on rocks which are 

 splashed with water, but the latter seems only 

 to occur in restricted districts in the South of 

 Ireland. All the species must have an abund- 

 ance of water, or the foliage quickly shrivels. 

 This is due to the fact that the leaves consist 

 of a single layer of cells and are, of course, very 

 thin. A distinctive feature in this family is the 

 bearing of the sporangia ; these are almost or 



