THE RUSHES, GRASSES, AND FERNS. 285 



these being formed of a single layer of cells. Fig. 

 215 shows at No. 1 the indusium, formed by the out- 

 growth and overlapping of the edge of the pinnule or 

 leaflet. As soon as this is effected, the fhecse appear 

 between it and the leaf as small green cells, full of 

 granular matter (No. 2). In a little while these 

 develop a cellular covering and a peculiar disposition 

 of cells, which afterwards form the spring by which 

 the spores are eventually liberated from the thecae. 

 As the thecse advance towards maturity, there 

 appear within them small round cells (No. 5) de- 

 veloped from the granular matter, these by sub- 

 sequent cell-division become the spores. In their 

 early stage the spores are transparent, but after- 

 wards become opaque arcd dark coloured. The 

 young spores of the Maiden-hair are represented at 

 No. 6 ; of the Oak Fern (Poly podium dryopieris), at 

 No. 7 ; and of the Narrow Prickly Fern (Polysticlium 

 lobatum) at No. 8, in which the formation of the 

 spore from the division of the round cell is very 

 evident. 



The genera into which our British ferns are 

 separated are characterised chiefly by the position of 

 the Kori and their coverings. Thus, those of the 

 Common Polypody (Polypodium vulgare) are naked, 

 that is, the sori are not covered over by any ex- 

 traneous protection. You cannot fail to find this 

 beautiful and commonest of ferns growing on old 

 tree trunks, or festooning the banks of the ditches 

 by the lane side. It is green, even in the winter 



