CYCADOFILICALES 35 



heterosporous pteridophytes. The homosporous ferns, in the natural 

 course of evolution, gave rise to the heterosporous ferns, which, in 

 turn gave rise to the Cycadofilicales. 



SPECULATIVE RECONSTRUCTION 



The splendid Carboniferous swamp-forest reconstruction in the 

 Field Museum makes us wonder what we might find if we could go 

 back to those ancient times, collect living material, and get complete 

 life-histories of all the forms. Many, even at that far away time, 

 would be so advanced that we should speculate as to what their De- 

 vonian and even Silurian ancestry might be. In the swamp-forest 

 reconstruction, there was so much material available that the view 

 is probably much the same as greeted the eyes of Archeohlattina and 

 Diplovertebron. In a reconstruction of finer details of gross structure, 

 and especially in the microscopic anatomy, not so much material is 

 available, except in the harder woody parts which are very well pre- 

 served. Spore coats and impressions of seeds are preserved in great 

 abundance, but our knowledge of the internal structure of spores 

 and the gametophytes is very fragmentary. 



For the reconstruction, we have not followed any living form in 

 detail, although certain features of Azolla and Trichomanes have had 

 some influence. It does no t seem necessary to reconstruct any homos- 

 porous type or even the earliest beginnings of heterospory, since the 

 development of the sporangia of all the living heterosporous genera 

 is well known, and all of them show very clearly their homosporous 

 ancestry. So let us take a sorus terminating a leaf, as the seed, Tri- 

 gonocarpus, terminates the leaf of Neuropteris; and then, without ref- 

 erence to any particular forms, extinct or living, let us trace the 

 evolutionary course through the heterosporous condition to the seed 

 stage. 



In fig. 27 the oldest sporangium is at the top of the sorus. We shall 

 assume that this sporangium continues its development while the 

 sporangia below it disorganize, giving up their material to the termi- 

 nal sporangium. The early stages of development up to the spore- 

 mother-cell stage, and even through the two reduction divisions, are 

 still those of a homosporous type. Then some of the spores disor- 

 ganize, and give up their food supply to their more successful neigh- 



