PTERIDOPHYTA. 



203 



and may be one or, more rarely, several layers in thickness ; and 

 an intermediate one, the tapetum (Fig. 204 A, B, b ), which is rich 

 in protoplasm, and Avhose cells are dissolved so that the spores 

 float freely in the fluid thus provided. The spores arise as in the 

 Mosses (in tetrads), by the cross-division of the special mother- 

 cells, and according to the manner in which they are arranged 

 in the mother-cell have either a tetrahedral form, with a large 

 base resembling a segment of a ball, or are oblong (bilateral 

 spores). Their construction is the same as in the Mosses (p. 

 187). 



The spore-formation in its earliest commencement takes place in 

 the same way in the Isosporous and the Heterosporous Vascular 



Fr&. 204. Selaginella in&qualifolia. A A yonng sporangium, which may develope either 

 into a macro-, or a micro-sporangium. B A microsporangium. 



Cryptogams; but from a certain point, after the tetrahedral divi- 

 sion, a difference occurs with regard to the macrosporangia. All 

 the spores formed in the microsporangium may complete their 

 development ; but those which are formed in the macrosporangium 

 are generally aborted, with the exception of one or four, and these 

 consequently attain a much larger size (see Fig. 239. The series to 

 the left are microsporangia ; those to the right, macrosporangia). 



APOGAMY. In some Ferns (Pteris cretica ; Aspidiitm filix mas, var. cristatum; 

 A. falcatum; Todea africana) the young plant is not developed as a consequence 

 of fertilisation, but as a bud from the protballium. This is known as apogamy, 

 or loss of the power of sexual reproduction. The antheridia are generally 

 more or less developed ; archegonia are entirely wanting in Asp. filix max, var. 



