The Fern Tribe. 313 



possess of disseminating themselves. Hart's-tongue, owing to 

 its small size, is one of those in which the power resides in only 

 a small degree ; and yet a little computation will show even its 

 means to be prodigious. Each of its sori consists of from three 

 thousand to six thousand thecae; let us take four thousand and 

 five hundred as the average number. Then each leaf bears 

 about eighty sori ; which makes three hundred and sixty thou- 

 sand thecae per leaf; the thecae themselves contain about fifty 

 spores each ; so that a single leaf of Hart's-tongue may 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 Heart' s-tongue ; on the contrary, it is upon 

 differences in those respects that the genera have been estab- 

 lished. For example, in the Shield-ferns (Aspidium,) the sori 

 are round, and covered with a kidney-shaped indusium ; in 

 Polypody (Polypodium,) they are round, and have no indusium ; 

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

 oblong bodies arising from the edge of the leaf. The most 

 curious arrangement of their parts is in the Brake, (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 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 them designedly 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 is 

 run; there lurk the thecae 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 thecae running all around it. 

 In this instance the margin of the leaf acts the part of the in- 

 dusium. 



Another singular form of Ferns is that in which the whole of 

 the segments of a leaf are contracted and curled up round the 

 thecae, so as to lose entirely their natural appearance, and to 

 resemble a sort of inflorescence. A striking instance of this is 

 not uncommon in bogs, in form of a plant called the Osmund- 

 roijaly or Flowering-fern, (Osmunda regalis ;) a minute species 



