OH. XXX] MEDULLOSA 93 



the separation of a leaf-stalk. The stem-cortex and the ground- 

 tissue of the petioles consist of parenchyma with numerous 

 secretory canals, not sacs only as in Heterangium, and are 

 abundantly supplied with scattered vascular bundles of collateral 

 and exarch structure. 



The leaf-traces are furnished by the peripheral tracheal groups 

 at the free surface of the primary portion of each stele: each 

 trace is at first concentric and consists of primary xylem with 

 one or more protoxylem strands near the outer surface and is 

 completely or partially enclosed by secondary xylem and phloem. 

 In the course of its passage to the leaf a leaf-trace loses its secondary 

 tissues, which were added by the cambium during the traverse 

 of the zone of secondary wood, and divides into small collateral 

 bundles consisting mainly of spiral and scalariform tracheids. 

 The collateral bundles accompanied by some narrow fibres are of 

 the Myeloxylon type (fig. 420), the xylem being wholly centripetal. 

 In the behaviour of the leaf-traces and in the vascular system 

 of the petioles Medullosa differs from Heterangium and Lyginopteris. 

 Each leaf-base is supplied by sets of vascular strands which pass 

 into it from the stem at different levels ; a large leaf-base reaching 

 4 cm. in diameter receives as many as 70-80 bundles. The 

 hypoderm is Kke that first described in the French species Myelo- 

 xylon Landriotii^ and often spoken of as the Sparganum type of 

 hypoderm. The branching of the rachises points to a compound 

 frond, and the occurrence of numerous linear pinnules with 

 revolute margins (fig. 420, D) in association with the stem suggests 

 that the ultimate segments were of the Alethofteris form. This 

 inference receives confirmation from the occurrence of petrified 

 specimens of undoubted Alethofteris rachises with the structure 

 of Myeloxylon. It is practically certain that the leaves borne 

 on the stems of Medullosa anglica are those long known as 

 Alethopteris lonchitica (Vol. ii. A, p. 553, fig. 364). 



An interesting feature in the stems is the occurrence of cortical 

 vascular strands (fig. 416, A, a, b), reaching a diameter of 7 mm., 

 containing scattered tracheids in a parenchymatous core surrounded 

 by secondary xylem and phloem. These cauline bundles are 

 almost identical both in structure and distribution with the 



1 Renault (76) B, 



