24 



GYMNOSPERMS 



long, and resembles the seed of a cycad. The leaf, which has been 

 found in organic connection with the stem, is described under the 

 name Sphcnoptcris clegans. 



Medullosa. — The stem of Medullosa has a polystele, transverse 

 sections usually showing three steles, somewhat as in Selaginella 

 wildcnovii; but each primary stele is surrounded by its own secondary 

 wood and phloem. The primary xylem is mesarch. Its protoxylem 



Fig. 21. — Heterangium grievii: transverse section of stem; .v, primary wood; x\ sec- 

 ondary wood just beginning to develop, just outside the secondary wood, the phloem, 

 mostly broken down; ic, inner cortex; oc, outer cortex; //, leaf trace; r, base of adventi- 

 tious root; X (in the inner cortex), a group of sclerotic cells; pet, petiole; about Xs- — 

 After Scott. 



consists of spirally marked tracheids surrounded by a few sclariform 

 or reticulate tracheids; but most of the primary wood and all of the 

 secondary wood has multiseriate bordered pits. In the secondary 

 wood the pits are becoming restricted to the radial walls. Sieve 

 plates, resembling those of living gymnosperms, are found on the 

 lateral walls. 



Ncuropteris. — Beautiful impressions of this fernlike leaf have long 

 been familiar. Immense seeds, seeds more than 6 cm. in length, be- 



