80 THE MORPHOLOGY OF PTERIDOPHYTES 



later are microsporangial and are estimated to contain up 

 to a million microspores each. Finally, a few sporophylls 

 with abortive sporangia are produced late in the season. 



There is no special dehiscence mechanism and the spores 

 are released only when the sporophylls die and decay, as 

 they become sloughed off at the end of the season. The first 

 cell division within the microspore is an unequal one which 

 cuts off a small 'prothallial cell'. The other cell is called the 

 'antheridial cell' since, by successive divisions (Figs. 13M 

 and 13N), it gives rise to a jacket of four cells surrounding a 

 central cell from which four antherozoids are formed (Fig. 

 13O). These are spiral and multiflagellate (Fig. 13P) and are 

 released by the cracking of the microspore wall. As already 

 mentioned, this mode of development, where the prothallus 

 is retained within the spore wall, is described as 'endosporic'. 



The female prothallus hkewise is endosporic. Within the 

 megaspore, free nuclear divisions take place for some time, 

 i.e. nuclei continue to divide without any cross-walls being 

 laid down between them. Then, when about fifty such nuclei 

 have become distributed round the periphery of the cyto- 

 plasm, cross-walls are slowly formed, starting in the region 

 immediately beneath the tri-radiate scar, but gradually 

 spreading throughout. Meanwhile, the megaspore wall 

 ruptures at the tri-radiate scar and an archegonium is 

 formed in the cap of cellular tissue which is thereby exposed. 

 Stages in the development of the archegonium are illustrated 

 in Figs. 13Q-S. If fertiUzation does not occur immediately, 

 further archegonia may develop among the rhizoids that 

 cover the apex of the gametophyte. 



Stages in the development of the young sporophyte are 

 illustrated in Figs. 13 T-Y, in which the megaspore is sup- 

 posed to be lying on its side, as is commonly the case. The 

 first division of the zygote is in a plane at right angles to the 

 axis of the archegonium, or sUghtly oblique to it. That part 

 of the embryo formed from the outermost half, designated 

 'the foot', is indicated in the figures by obhque shading. As 



