638 



FILICES. 



times arise from the surface of the frond, while at other times they 

 spring from below, having a cutieular covering in the form of an 

 indusium or involucre (fig. 882). The clusters of thecas are called 



sori (fig. 883). The margin of 

 the frond sometimes is folded 

 so as to cover tlie thecse, and 

 at times the whole frond is 

 converted into clusters of 

 thecse. The spores when sown 

 give rise to a prothallus (pro- 

 thallium), which bears anther- 

 idia and archegonia (p. 280). 

 The antheridia are ia the 

 under surface of the prothallus, 

 and consist of cellular papillse, 

 having a central cavity. This 

 cavity contains free cellules, 

 within each of which there is 

 a ciliated spiral filament or 

 spermatozoid. These cellules 

 are discharged by a rupture at 

 the apex of the antheridium. 

 The spiral filaments then burst 

 the cellules, and being set free 

 reach the cellular body con- 

 taining the embryo-germ, or 

 archegonial cell, embedded in 

 the substance of the prothallus. The archegonia are larger than the 

 antheridia, and present a canal leading to the germ-cell, which canal 

 remains closed till the period when the spermatozoid is matured. 

 After fertilisation the archegonial cell enlarges, develops numerous 

 cellules, and forms the true sporangiferous frond of the fern. — Ferns 

 abound in moist insular climates. They characterise the New 

 Zealand Flora. They are elegant leafy plants, occurring chiefly in 

 moist insular climates, and abounding in the tropical islands. In 

 mUd and warm climates they occur in the form of large Tree-ferns, 



Fig. 881. Khizome of Scolopendrium vulgare, "with several fronds (leaTes), /',/", /'",/"", 

 in different degrees of development. In /' and /", the eireinate or gyrate vernation is seen. 

 In S"", the linear transverse sori or clusters of thecse are seen, having the appearance of 

 dark lines on the lower surface of the frond. Figs. 882-884. Frond and fractiflcation of 

 Lastrea (Nephrodium), to illustrate the natural order Filices. Fig. 882. Part of the 

 frond seen on the lower surface. ^, Two pinn^ covered with sori, s, having an indusium. 

 r, Rachis or central stalk of the frond. Fig. 883. One of the sori or clusters of thecse cut 

 vertically. %, The vein bearing it. i, Indusium or fold of the frond covering it. c, Thecas 

 or sporangia (spore-cases). Fig. 884. One of the thecse separated at tlie period of dehis- 

 cence, a. Incomplete annulus or ring, "which is elastic, and causes transverse dehiscence 

 of the theca. 2?, Stalk of the theca. s. Spores discharged. 



Fig. 881. 



Fig. 884. 



