THE FERN TRIBE. 271 



give birth to no fewer than eighteen millions of young 

 plants. 



The form and situation of the sori is not, in other 

 genera, the same as in the Hart's-tongue ; on the 

 contrary, it is upon differences in those respects that 

 the genera have been established. For example in 

 Shield-ferns ( Aspidium) the sori are round and covered 

 with a kidney-shaped indusium ; in Polypody (Poly- 

 podium) they are round and have no indusium ; and 

 in the graceful Maiden-hair-ferns (Adiantum) they 

 are oblong bodies arising from the edges of the leaf. 

 The most curious arrangement of their parts is in the 

 Brake itself (Pteris) ; no matter at what time of the 

 year you examine the leaves of that plant you will 

 probably discover no trace of sori, and yet it would 

 be difficult to find a Brake-leaf in the autumn, which 

 does not abound with them. The truth is that in 

 this plant they occupy so singular a position, that one 

 could almost be tempted to believe that they were de- 

 signedly hidden where none but the curious Botanist 

 should find them. Look attentively at the under 

 side of the leaves : you will remark the margin to be 

 turned in and thickened, like the hem of a lady's 

 gown in which a cord in run ; there lurk the thecse 

 you are in search of. With the point of a knife lift 

 up gently the edge of the leaf, and you will at once 

 discover a ridge of thecse running all round it ; in this 

 instance the margin of the leaf acts the part of 

 indusium. 



Another singular form of Ferns is that in which 

 the whole of the segments of a leaf are contracted and 



