THE ANGIOSPERMAE 1395 



carpel altogether. It has directed attention to a much earlier and more 

 primitive stage of evolution, and has given fresh meaning to the third or 

 siii generis theory. 



That the ovule includes a megasporangium may be taken as common 

 ground, but there is obviously more to it than that. There is normally only 

 one fertile megaspore, and we know that in Lepidocarpon and in Selaginella 

 apoda a single megaspore may be enclosed by a megasporangium and ferti- 

 lized in situ. The enclosing nucellus is therefore generally regarded as the 

 megasporangium. Chadefaud has, however, made another suggestion. 

 Going back to the Ferns, he points out that the leptosporangiate sorus 

 bears a number of fertile trichomes (or telomes, as we would now say), but 

 that in the eusporangiate sorus these telomes have been reduced to one and 

 that one is sunk into the basal cushion, or soral placenta. The ovule, he 

 argues, is a basisporangiate sorus, like that of the Eusporangiatae, and the 

 nucellus is thus the equivalent of the soral placenta, sometimes with rem- 

 nants of vascularization, and with one embedded sporangium. 



This attractive idea, in spite of some difficulties, may be applied readily 

 enough to crassinucellate ovules, but what of tenuinucellate ovules? Are 

 they all the result of secondary reduction or are they truly " leptospo- 

 rangiate ", as indeed Warming called them? If so, then we are back again at 

 the same position as before regarding distinct lines of descent for nearly 

 related Angiosperms. Following the principle of economy of hypothesis, the 

 view that the nucellus is the megasporangium itself is to be preferred, at 

 least for the present. 



The integument surrounding the nucellus in Pteridosperms shows many 

 signs of being a compound structure built up from a fusion of units. What 

 these units may have been morphologically is far from clear. The original 

 hypothesis of Benson was that they were sterilized megasporangia, forming 

 the outer zone of a sorus, surrounding the single central sporangium which 

 had remained fertile. On the other hand Chadefaud considers that the 

 sorus was primitively amphisporangiate. In the male sori of Pteridosperms 

 the central megasporangium is supposed to have disappeared and only the 

 outer ring of microsporangia (sometimes fused together) remain. In the 

 female sori the central megasporangium has developed and the integument 

 represents a fused ring of sterilized microsporangia. A third possibility, sug- 

 gested by Yarravia (see under Paleobotany in Volume III), is that the 

 megasporangium was terminal on a telome and that the units of the integu- 

 ment were a group of associated sterile telomes. There is little definite 

 evidence, one way or the other, and the answer must await further dis- 

 coveries. 



The origin of the outer integument is even more puzzling. The cupule 

 which surrounds the ovule in many Pteridosperms has been brought into 

 consideration. It too shows indications of being formed of fused units, 

 generally regarded as having been pinnae of the megasporophyll. It is not, 

 however, a constant feature in Palaeozoic Pteridosperms, although the 

 Mesozoic Pteridosperms appear to have been all cupulate. Again, the 



