404 VASCULAR CRYPTOGAMS. 



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

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

 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 Selaginella, even when the macrospores are still lying in the sporangium, the 

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

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

 accumulation of protoplasm. This tissue afterwards produces the archegonia, and 

 is therefore the true prothallium ; but a few w^eks after the dissemination free cell- 

 formation begins beneath it in the spore-cavity, finally filling up the whole cavity, 

 and forming a large-celled tissue, which Pfeffer, supported by considerations with 

 which I also agree, compares to the endosperm of Angiosperms, and, following 

 this analogy, calls by the same name. At the period of fertilisation and of the form- 

 ation of the embryo, the macrospores of Selaginella contain, therefore, both a pro- 

 thallium and an endosperm. The formation of the archegonia begins even before 

 the rupture of the exospore, which occurs in this genus in the same manner as in 

 Isoetes. The first archegonium originates at the apex of the prothallium ; the 

 others arise, whether the first is fertilised or not, in centrifugal succession on the 

 free parts of the prothallium. 



In both genera the archegonium originates by division of a superficial cell 

 parallel to the surface ; the outer of the two new cells divides into four cells placed 

 crosswise, each of which splits by an oblique division into two, one lying over the 

 other ; in this way the neck is formed, consisting of four rows, each of two cells. 

 The low^er of the first two cells is the central cell, the protoplasm of which separates 

 into an upper smaller and a lower larger portion ; the former is the canal-cell, which 

 penetrates between the two rows of the neck which separate from below, becomes 

 converted into mucilage, and finally breaks through the neck, while the lower 

 portion of protoplasm becomes rounded off and forms the naked oosphere (Fig. 



303, //)• 



Finally, in Lycopodium iniindatiim ^ the germination of the spores has been 

 observed by De Bary. The endospore stretches, and protrudes as a nearly spherical 

 vesicle from the exospore which is split into three deep lobes ; it is divided by 

 an upper partition-wall into a hemispherical basal cell which does not undergo 

 any further changes, and an outer cell which continues to grow as the apical 

 cell and forms two short rows of alternating segments by walls inclined alternately 

 in two opposite directions. Each segment is broken up by a tangential wall into 

 an inner and an outer cell, so that the prothallium consists finally of four short 

 cells forming an axial row, surrounded by two rows of lateral cells, and by the 

 basal and the apical cell. De Bary was unable to follow the further stages of 

 development ; and it is therefore still impossible to form a judgment on the true 

 nature of this structure. [See, however, supra, p. 400.] 



The Asexual Generation. The mode of formation of the embryo is, as has 

 been said, known only in Isoetes and Selaginella. The first division of the oospore 



