FERTILISATION AND PROPAGATION. 



109 



from a rhizome which, according to the species, either creeps on the surface 

 of the ground, runs below it, or rises upwards into the air. Without 

 exception, these fronds unfold in a spiral manner and are traversed by veins. 

 It is from the under surface of these veins that the " sporangia," which 

 contain the reproductive sporules — the equivalent for seeds in other plants — 

 are produced. They are dependent from the " sporangiferous receptacle," 

 which is a thickened point or lengthened portion of the ultimate " veinules." 

 This receptacle, when formed on the apices of the veinules, is called 

 "terminal"; it is termed "medial" when situated between their base and 

 their apex ; " axillary," when on the point of the forking ; " compital," when 

 disposed on points of confluence of two or more veinules ; and " amorphous," 

 when part of the disk of the frond is changed in texture, and closely occupied 

 by the sporangia. These sporangia, whose shape is either globose, pyriform, 

 or oval, are disposed on each receptacle in crowded masses, termed " sori," 

 which, in most genera, are each furnished with a membranous covering, called 

 an " indusium," which rises from a receptacle, and is of various forms ; or the 

 sori are naked, as in Polypodium — that is, without an indusium. The sori, 

 besides being of different forms and dimensions, occupy various positions, and, 

 according to these, are called " marginal," when situated close upon the margin 

 of the fronds or segments, as in Adiantum, Pteris, &c. ; " intramarginal," 

 when placed between the margin and the midrib, as in Dicksonia ; " costal," 

 when close to the midrib, as in Sadleria and Fadyenia ; and " transverse," 

 when elongated, either oblong or linear, and making a continuous or broken 

 line forming an angle with the midrib, as in Blechnum, Lomaria, Wood- 

 wardia, &c. The indusium is a membrane produced from the receptacle of 

 each sorus, of a plane, cup-shaped, or vaulted nature, which, as it becomes 

 replicate, generally drops off. When the indusium is produced from the 

 centre of the receptacle, as in Aspidium, it is central, and usually orbicular, 

 with its margin depressed and free all round, or it is inflated. When it is 

 lateral, or produced on one side, its shape is either oblong, oval, or reniform, 

 and it is attached by a point or by the base of one side, as in Nephrodium ; 

 or it may be linear, when its whole length is fastened on the one side of an 

 elongated receptacle, as in Aspknium, the other side being quite free. When 

 calyciform, its base is attached all round, and it is entire and globose, as in 

 Diacalpe ; but after a time it opens with an entire or laciniated margin, forming 



