440 STUDIES IN FOSSIL BOTANY 



this appears to be the only case in which this structure 

 has been recognised in the root of a fossil plant. A 

 very characteristic feature of these roots, with the 

 exception of the youngest specimens, is the presence 

 of a wide zone of large-celled periderm, forming the 

 external covering of the organ. Developmental stages 

 have been observed, proving that the periderm was an 

 internal one in origin, arising by division of the 

 pericyclic cells, and thus causing the exfoliation of the 

 whole cortex. A small amount of phelloderm was 

 formed on the inner side of the cork-cambium. 1 In 

 all respects the roots of Medullosa anglica agree with 

 those of recent Gymnosperms. 



The larger roots reached a diameter of about half 

 an inch. They branched pretty freely, giving off 

 rootlets of various orders. The xylem of the rootlet 

 was always inserted on an angle of the primary wood 

 of the main root, in the way normal for these organs. 



If we now sum up our knowledge of the vegetative 

 organs of Medullosa anglica, we find that it had a poly- 

 stelic stem, each of the steles, usually three in number, 

 resembling, as regards both the primary and secondary 

 tissues, the single central cylinder of a Heterangium. 



Externally, the stem was clothed by the bases of 

 the large, spirally disposed leaves, which were repeatedly 

 compound, and no doubt presented the external 

 characters of an Alethopteris, with which they agree in 

 anatomical structure. 



Each leaf received from the stem a large number of 

 vascular bundles, derived from the repeated subdivision 



1 Arber, loc. cit. p. 428, Plate xx. Fig. j.. 



