6oo 



STRUCTURE AND LIFE HISTORIES 



519. Ancestors of the Angiosperms. — Just as the 

 Cycadofilicales indicate the ancestry of the cycads, so 

 fossil types of Cycadophyta have been discovered which 

 are interpreted by some paleobotanists as ancestors of the 

 modern angiosperms. Other investigators, however, dis- 

 sent from this view and consider that we have not yet 



Fig. 421. — To the left, Cycadeoidea dacotensis MacBiide. Longitudinal 

 section of a silicified specimen of a bisporangiate cone (unexpanded flower), 

 so taken that the pinnules of the microsporophylls on both sides of the 

 central axis, or receptacle, are successively cut throughout their entire 

 length. The lines indicate the planes of various sections through the cone, 

 published in Wieland's "American Fossil Cycads." To the right Cycado- 

 cephalus Sewardi Nathorst. Bisporangiate cone, natural size, preserved 

 as an impression on a flat slab. From a fossil-bearing bed of the Trias, at 

 Bjuf, Southern Sweden. (Both figures from Wieland.) 



sufficient knowledge of fossil forms to be justified in 

 designating the ancestors of the Angiosperms. This 

 difference of opinion is largely due to the meagerness of 

 the available evidence. As one writer has stated it, 

 "A trayful of flowers may be all the record of the Pterido- 



