134 THE MORPHOLOGY OF PTERIDOPHYTES 



is a crude kind of annulus of thickened cells, whose contrac- 

 tions pull the sides of the sporangium apart along a line of 

 dehiscence on the inner face (Fig. 19H). Those with synangia 

 have no such device ; instead, a thin part of the sporangium 

 wall dries and shrinks to form a pore through which the 

 spores can fall (Figs. 19K-N). The whole sorus in Marattia 

 is very woody and, when ripe, splits into two halves which 

 are slowly pulled apart, so as to expose the pores in each 

 sporangium (Fig. 19J). 



Germination of the spores is rapid, occurring within a few 

 days of being shed, and they develop directly into a massive 

 dark green thalloid prothallus, which is mycorrhizal and is 

 capable of living for several years. An old prothallus may 

 be several centimetres long and may resemble closely a 

 large thalloid liverwort (Fig. 19T). The prothallus is mon- 

 oecious but, while the antheridia occur on both the upper and 

 lower surface, the archegonia are confined to the lower 

 surface, where they occur on the central cushion along with 

 rhizoids. Both types of gametangia are sunken beneath the 

 surface of the prothallus and the antheridium is large and 

 massive. The archegonium (Fig. 19U) has a large ventral 

 canal cell (except in Danaea) and a neck canal cell with two 

 nuclei. The antherozoids are coiled and multiflagellate, as 

 in other ferns. 



The first division of the zygote is at right angles to the 

 axis of the archegonium, and the embryo is endoscopic. 

 Thus, since the archegonial neck is directed downwards, the 

 embryo is orientated with its shoot uppermost and, as it 

 grows upwards, it bursts its way through the tissues of the 

 prothallus. A minute suspensor is present in Danaea (Fig. 

 19Y) and in some species of Angiopteris, but Marattia, 

 Christensenia and most species of Angiopteris are com- 

 pletely without a suspensor. This lack of constancy is 

 paralleled in the Ophioglossales and has led to speculation 

 as to its phylogenetic implications. A suspensor is generally 

 held to be a primitive character and its presence even if not 



