Botany— 1038 Meeting 10 < 



A New Cycadophyte and its Relatives* 



James M. Schopf 



Illinois State Geological Survey, Urbana 



Recently a new type of Medullosa stem has been discovered in coal 

 balls from the Clarkson Mine, (coal No. 6) at Nashville. This stem may 

 best be compared with the three species of Medullosa which have been 

 described from the English lower Coal Measures. These are Medullosa 

 anglica, M. centrofilis and M. pusilla and the beds from which they are 

 derived are considered roughly equivalent to the Pottsville series of the 

 lower Pennsylvanian of this country. The new stem from Nashville con- 

 sequently is younger geologically than the related European forms. In the 

 accompanying drawings the author has attempted to reconstruct cross sec- 

 tions to scale as the stems would appear if uncrushed or undistorted. All 

 three English species possess three separate main steles. No doubt the 

 English forms are most closely related but their primary structures of 

 determinate growth differ so much in size that they must all be considered 

 valid species. M. centrofilis has a small centrally placed accessory stele 

 which further distinguishes it. A band-like internal periderm layer between 

 the stelar region and the cortex is consistently found in all three English 

 forms. 



Instead of three steles our new Nashville Medullosa has but two. The 

 stem is somewhat larger than Medullosa anglica as may be noted on the 

 plate since the drawings are approximately on the same scale. The de- 

 current petiole bases are essentially similar although their arrangement 

 on the stem is different. Details of histology also differ showing that this 

 form from Nashville is distinct and not simply a distelar aberration of a 

 typically tristelar species. Perhaps the most noteworthy single feature 

 is the extreme asymmetry of the two steles in this new species. The sec- 

 ondary wood is strongly developed on each vascular member toward the 

 center but only slightly developed on the portion facing the stem exterior. 

 Scott (1) has pointed out that this also occurs in Medullosa anglica and 

 M. pusilla. Examination of De Frame's figures of M. centrofilis show the 

 same condition exists for it too, but in none of these species is the asymmetry 

 of growth so extreme as in the Nashville stem. Whatever the cause for 

 the preponderant secondary growth toward the center of the stem may be, 

 its occurrence in all four of these species indicates that it has some definite 

 significance. A band-like internal periderm layer is present enclosing the 

 stelar tissues of the Nashville Medullosa as in the English stems. 



Sutcliffia insignis has been suggested by De Fraine as representing a 

 primitive type among the Medullosaroip from wlr'ch the p^iystelnv forms 

 could have been evolved. A reconstruction (more tentative than the other 

 sketches) obtained by combining features illustrated by Scott (2) and by 

 De Fraine (3) is also included on the plate. Sutcliffia also possesses de- 

 current petioles, and a less well defined internal periderm band. There is 

 but a single main vascular cylinder around which are a number of smaller 

 stelar units. These latter structures give rise to the multitudinous small 

 leaf trace bundles and on this account De Fraine regards them more as 

 large foliar traces than as structures homologous with the central stele. 



It might be inferred that the Nashville Medullosa is derived more 

 directly from a Sutcliffia type of stem than any of the other species on 

 ac-rumt of its fewer steles, notwithstanding that our new form is of some- 

 what younger geologic age than any of the others mentioned. However it 



Published by permission of the Chief. Illinois State Geological Survey. 



