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MORPHOLOGY OF GYMNOSPERMS 



The oldest known species of Medullosa, and the simplest in its 

 stem structure, is M. anglica, from the Lower Coal-measures of 

 England, and described in detail by Scott (22). The stems 

 average 7 to 8 cm. in diameter, and are distinctly polystelic 

 (fig. 15), resembling Pteris (fig. 5) in this regard. The vascu- 

 lar^bundles are usually three in number and are concentric, each 

 one of the xylem strands being surrounded by a zone of secondary 

 wood. This combination of polystely and secondary wood is the 

 chief anatomical feature of the group, and is said to distinguish it 

 from all living plants. The secondary xylem elements resemble 



Fig. 15. — Medullosa anglica: transverse section of stem showing three large leaf 

 bases (a, h, c); ah, he, position of next leaf bases above; st, the three steles; //, leaf 

 traces; an, accessory rings of wood and bast; pd, periderm, forming a ring around 

 the steles; r, adventitious root; oc, hypodermis; sclerenchyma band between leaf 

 base and stem; slightly reduced. — After Scott (22). 



those of Heterangium and Lyginodendron, being radially arranged 

 tracheids with bordered pits on the radial walls. In fact, each one 

 of the several steles of Medullosa so closely resembles the single one 

 of Heterangium, that the Medullosa type is believed by some (36, 72) 

 to have been derived directly from the Heterangium type; and to 

 have given rise in turn to the Lyginodendron type by the organization 

 of the polystele into a compact cylinder (siphonostele), accompanied 

 by the degeneration of the internal vascular tissues. The opposing 

 view claims that the polystelic condition of Medullosa is derived from 

 the siphonostelic, as in ferns {Pteris, 31); and that the Medullosa 

 type is to be regarded as a modification of the Lyginodendron type, 



