FILICINE^. 



471 



and in the mode of its development, it shows a still greater resemblance to the tissue 

 that fills up the embryo-sac of Gymnosperms, and even of Angiosperms. In Isoetes 

 the cavity begins to be filled with cellular tissue a few weeks after the escape of the 

 macrospores from the decaying macrosporangium ; the cells of this tissue are all at 

 first naked (without cell-wall); they appear to become enclosed in firm cell-walls 

 only when the whole cavity of the endospore is filled with them (Fig. 330). In 

 the meantime the endospore thickens, becomes differentiated into layers, and 

 assumes a finely granular appearance, phenomena which, as Hofmeister insists, are 

 exhibited in like manner in the embryo-sacs of Coniferae. The spherical pro- 



FlG. 331.— Germination ai Selaginella (after Pfeffer) ; /—///, S. Martejisii, A—D, S. catilescens ; I longitudinal section 

 of a macrospore filled with the prothallium and * endosperm,' d the diaphragm, e e' two embryos in process of formation ; 

 // a young archegonium not yet open ; /// an archegonium with the oospore fertilised and divided once ; A a microspore 

 showing the primordial cells ; B C different views of these divisions ; D the mother-cells of the antherozoids in the perfect 

 antheridium ; v, vegetative cell. 



thallium now swells up, the three convergent edges of the exospore burst length- 

 wise and thus form a three-rayed fissure, where. the prothallium is covered only 

 by the membranous endospore; this also peels off, and softens, finally exposing 

 the corresponding part of the prothallium. At its apex appears the first arche- 

 gonium ; if this is not fertilised, several others are subsequently formed at its side. 

 In Selagmella, even when the macrOspores are still lying in the sporangium, the 

 apical region is found to be clothed with a small-celled meniscus-shaped mass of 

 tissue which is probably formed, during the ripening of the spores, by the division 



