30 GYMNOSPERMS 



Besides the undoubted seeds of the CycadofiHcales and the Cor- 

 daitales, seed-like structures are well known in the Carboniferous. 

 After the spores become differentiated into microspores and mega- 

 spores, there was doubtless much difference in the stage of develop- 

 ment reached before the shedding of the megaspore, just as in living 

 forms. In I socles, the megaspore is shed at the uninucleate stage; 

 while in Sdaginella it germinates and the female gametophyte may 

 reach the archegonium stage before shedding occurs. By keeping 

 strobili of Selaginclla a pus on moist filter paper in a Petri dish, so as 

 to avoid complete dehiscence, development may be forced still far- 

 ther, so that fertilization occurs, the embryo is formed and even 

 breaks through the wall of the sporangium. It thus satisfies the defi- 

 nition of a seed. In other strobili, on the same plant, the sporangia, 

 allowed to develop naturally, shed their megaspores. The same plant 

 is thus, by accepted definition, a Pteridophyte and a Spermato- 

 phyte. 



We believe that the seed habit was attained by the Cycadofilicales 

 in the same way. It is a natural tendency in evolution. At first, 

 there would be little difference in the two kinds of spores. As the 

 megaspore became larger, it began to germinate inside the sporan- 

 gium, and as the development of the female gametophyte proceeded 

 farther and farther, the megaspore was shed later and later, until it 

 finally became permanently retained within the sporangium, and the 

 seed condition was achieved. After the retention of the megaspore 

 became permanent, the thick protective coats, no longer necessary, 

 became thinner and thinner. They were still prominent in the Cyca- 

 dofilicales, thinner and thinner as the evolution of the gymnosperms 

 progressed and finally, in the angiosperms, became unrecognizable 

 as spore coats. 



As the early seed plants emerged from the heterosporous fern 

 condition, we should expect the seeds to be small, with several in a 

 six)rangium; but, as evolution progressed, the number would be re- 

 duced, until only one remained, and the size would increase. There 

 would naturally be increased complexity of the sporangium as it 

 became a more permanent structure, taking over the protective func- 

 tions which had been performed by the thick spore coats of a mega- 

 spore adapted to shedding. The further development of the seed 



