144 THE MORPHOLOGY OF PTERIDOPHYTES 



as in leptosporangiate ferns, or it may be cubical, as in the 

 Eusporangiatae. The tapetum is formed from the outermost 

 layers of the sporogenous tissue, unlike that of the Euspo- 

 rangiatae, and there is also a layer of tabular cells, formed 

 from the same regions, which becomes appressed to the 

 inner side of the sporangium wall. For this reason, at 

 maturity, the wall appears to be two cells thick. There is a 

 primitive kind of annulus, formed by a group of thick-walled 

 cells, on one side of the sporangium and a thin-walled 

 stomium, along which dehiscence occurs, extends from it 

 over the apex of the sporangium (Fig. 21G). Relatively large 

 numbers of spores are released from each sporangium (e.g. 

 about 128 in Leptopteris and more than 256 in Osmunda and 

 Todea). The spores contain chlorophyll and must germinate 

 rapidly if they are to do so at all. 



The prothallus (Fig. 2tC) is large, fleshy and dark green, 

 resembhng a thalloid liverwort, up to 4 cm long. The 

 antheridia (Fig. 21D) project from the surface, as in Lepto- 

 sporangiatae, but are larger, have more wall cells and pro- 

 duce a greater number of antherozoids than do most of 

 them. The archegonia (Fig. 21E) are borne along the sides 

 of the midrib; they have projecting necks and differ from 

 those of leptosporangiate ferns only in the number of neck 

 cells (six tiers, instead of the usual four). 



The embryology of the young sporophyte, too, shows 

 some features which distinguish the Osmundales from the 

 Leptosporangiatae. Not only is the first division of the 

 zygote vertical, but so also is the second. It is the third 

 division which is at right angles to the axis of the arche- 

 gonium, instead of the second. Subsequent divisions are 

 somewhat irregular and the embryo remains spherical for a 

 relatively long time. Ultimately, however, a shoot apex, 

 cotyledon, root and a large foot appear, but there is some 

 irregularity in their derivation from the initial octants. 



Despite the marginal position of the sporangia in 

 Osmunda, as compared with their superficial position in the 



