156 MEDULLOSEAE [CH. 



extrafascicular strands and cylinders are a characteristic feature 

 and these may well have arisen independently of the central stele 

 in response to physiological requirements. From such a type 

 as Sutcliffia evolution may have proceeded along two lines; in 

 one direction new types were produced in which increasing com- 

 plexity, as represented by a multiplication of steles, was an 

 outstanding feature. These forms, illustrated by Medullosa 

 anglica and other more elaborate species, proved inefficient and 

 were unproductive. Along another line the protostelic condition 

 was maintained though in some cases extrafascicular strands or 

 cylinders and cortical steles were superadded: it was this line 

 that led to the recent Cycads. 



General considerations suggested by the anatomical features 

 of Medullosa. 



A comparison of the stems described under the generic name 

 Medullosa reveals a considerable range in the grosser anatomical 

 features superadded to certain fundamental characters denoting 

 a common origin 1 . If additional data were available giving us 

 a fuller knowledge of individual plants differences between species 

 would be more clearly denned and would provide adequate grounds 

 for the institution of new genera for some of the types now included 

 in the comprehensive genus Medullosa. A proposal by Dr Lotsy 2 

 to adopt the names Pecopteromedullosa and N europteromedullosa 

 rests primarily on the relatively unimportant difference between 

 the fronds associated with certain Medullosan stems and, as Scott 3 

 points out, this two-fold division if applied to such a species as 

 Medullosa Leuckarti would result in its separation from species 

 which anatomically are clearly of the same generic type. In 

 Medullosa anglica, one of the oldest and simplest types, there 

 are three steles of equal importance, and each of them is practi- 

 cally identical with the single stele of Heterangium. Each stele- 

 as indeed all Medullosan steles consists of a strand of primary 

 xylem enclosed by secondary xylem and phloem, and it is obvious 

 that the development of a constantly increasing cylinder of 



1 For additional figures of the principal types of Medullosa, see Miss Bancroft's 

 paper (14). 



2 Lotsy (09) p. 719. 3 Scott (14) p. 998. 



