lyO THE MORPHOLOGY OF PTERIDOPHYTES 



running down in the wall of the sporocarp. Arching over the 

 top, is a gelatinous structure sometimes called a 'sporo- 

 phore' (cross-hatched in the figures) which swells up at 

 maturity and drags the paired sori from the sporocarp, as it 

 dehisces (Fig. 26E). 



The sporocarp of Pilularia is similar in construction, 

 except that there are only four sori. 



The sporangia are typically leptosporangiate in origin, 

 the sporangium wall is very thin, and there is a tapetum of 

 two or three layers of cells. The microsporangia contain 

 thirty-two or sixty-four microspores but, in the mega- 

 sporangia, all but one of the potential spores degenerate. 

 On dehiscence of the sporocarp, the delicate sporangium 

 wall rapidly decays and the spores begin to germinate almost 

 at once. 



The male gametophyte (Fig. 26F) is extremely simple, as 

 in most heterosporous plants, consisting of nine cells only. 

 There is a single small prothaUial cell, and six wall cells 

 surround two spermatogenous cells (cross-hatched in the 

 figure) that give rise to sixteen antherozoids each. 



The first cross-wall in the germinating megaspore is ex- 

 centrically placed and cuts off a small apical cell, whose 

 further divisions give rise to a single archegonium (Fig. 26G) 

 with a short neck of two tiers of cells, one neck canal cell, 

 a ventral canal cell and a large egg cell. 



The first division of the zygote is longitudinal, and the 

 second transverse, giving four quadrants (Fig. 26H) of which 

 the outer two develop into the first leaf (1) and the first 

 root (r), while the inner two develop into the stem apex (x) 

 and the foot (f ). Meanwhile, the venter of the archegonium 

 grows and keeps pace, for a time, with the enlarging embryo 

 so as to form a sheath round it, from the underside of 

 which a few rhizoids may be produced. The first few leaves 

 in Marsilea are without a lamina and, therefore, closely 

 resemble the leaves of Pilularia. 



It is often claimed that this interesting group of ferns 



