REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



completely in their mode of germination. 

 After lying a certain time on a damp surface, 

 their inner cavities are found to be occupied 

 by a number of small spherical cellules, each 

 of which contains in its interior a spirally 

 coiled fibre (antherozoid). By the dehis- 

 cence of the valves of the external tegument, 



Fig. 179. 



Vahate dehiscence of microspore of Selaginella hel- 

 ret'ica, six months after sowing, showing escape of 

 antli'irozoids, 300 diam. 



the antherozoa are set free; and it is presumed, 

 that it is by their agency that the archegonium 

 is fructified, after the prothallium has been 

 laid bare by the bursting of the macrospore at 

 its apex. 



96. RhhocarpetB. In describing the mode 

 of reproduction of the Rhizocarpese, we shall 

 confine our attention to the genus Pilularia, 

 respecting \vhich the most exact researches 

 have been made. 



97. The macrospore of Pilularia is an egg- 

 shaped body, presenting an equatorial constric- 

 tion. It consists of an internal proper mem- 

 brane (endospore), the so-called embryo sac, 



Fig. 180. 



Section of macrospore of Pilularia some time after 

 germination, 50 diam. 



a, two layers of the exospore, the outer, vertically 

 striated, the inner, homogeneous; b, cavity of 

 endospore bounded superiorly by the prothallium, 

 the papillaeform summit of which projects through 

 the canal of the exospore. 



which is surrounded by a white coriaceous 

 exospore. This last exhibits two distinct 

 layers, of which the internal is colourless and 

 vitreous, without trace of structure ; while the 

 external appears to be formed of prismatic 

 columns fitting closely together, which are 

 more distinct at the lower end of the spore, 

 while they disappear entirely towards its 

 smaller end or apex, at which point the exo- 

 spore forms a papilliform projection open at 

 its summit. From this arrangement there 

 results a canal, which is immediately sur- 

 rounded by the thickened and dentate margin 

 of the vitreous layer, and leads to the apex of 

 the endospore. 



243 



98. The prothallium. The first indication of 

 the commencement of the germination of the 

 macrospore is the formation of a lenticular 

 accumulation of granular plasma, at the sum- 

 mit of the endospore, which had previously 

 contained only starchy, mucous, or oleaginous 

 granules. Soon after there appears in the 

 same position a delicate cell of similar form, 

 the upper surface of which is in contact with 

 the endosporal membrane, and is iramediately 

 subjacent to the aperture in the exospore. It 

 is in all probability from this cell, although 

 the earlier stages of the development have not 

 been clearly made out, that the prothallium 

 takes its origin. A day or two after germina- 

 tion it consists of a central cell, which is sur- 

 rounded by a single layer of others of smaller 

 dimensions. Four of these last are invariably 

 found interposed between, the upper surface of 

 the large cell and the spore membrane ; the 

 septa by which they are separated being per- 

 pendicular, and at right angles to each other. 

 Soon after the central cell itself divides by a 

 transverse septum into two ; of these the 

 upper, of globular form, contains a large ve- 

 sicular nucleus, the future germ-cell. The 

 lower, which is tabular, divides repeatedly 

 by vertical septa, so as to form a single layer 

 of cells which intervenes between the cavity 

 of the archegonium and that of the spore. In 

 the meantime the four cells which surmount 



Fig. 18K 



Vertical section of prothallium of the same at the 

 period of impregnation, which passes through the 

 axial canal of the archegonium, and exposes 



a, the germ-cell ; b, cavity of macrospore ; b, outer, 

 and c, inner layer of exospore, the apicial canal 

 of which ( 97.), it completely occupies. 



the germ-cell extend upwards in the form of 

 four papillae, separated from each other by an 

 axial canal, which burst through the proper 

 spore membrane, and finally project beyond 

 the aperture of the exospore. By the absorp- 

 tion of the central cell, its cavity becomes 

 continuous with that of the vertical quadri- 

 lateral canal above mentioned. 



99. The embryo. In consequence of the 

 presumed entrance of the antherozoids into the 

 cavity of the archegonium, the germ-cell en- 

 larges, and is transformed by repeated division 

 into an embryo, which is at first a somewhat 

 meniscus-shaped body, formed of minute cel- 

 lules. Soon after, a conical projection of its 

 upper surface presents itself, which rapidly 



R 3 



