XXX] MEDULLOSA 99 



small collateral bundles which enter the petioles. The piece of 

 stem shown in fig. 416, G, illustrates the exit of leaf -traces from 

 the stele and their subsequent division into several small bundles, 

 -y, which are scattered in the cortex with strands of sclerenchyma. 

 In a specimen identified with Medullosa stellata, Schenk 1 found 

 part of a leaf-base attached to the stem : its vascular system was 

 of the Myeloxylon type, the bundles being identical with those 

 in the cortex of the stem seen in fig. 416, G. 



In some stems of M. stellata the outer, centrifugally developed, 

 portion of the main stele is very much broader than in the example 

 represented in fig. 416, D. The diagrammatic sketch reproduced 

 in fig. 416, F, represents a section of a Chemnitz specimen in the 

 British Museum 2 in which the axial region containing several 

 star-rings is almost enclosed by an inner zone of secondary xylem, 

 and beyond the narrow primary xylem (black in the sketch) the 

 rest of the block consists exclusively of secondary xylem 5-5 cm. 

 broad. This example illustrates a common tendency in Medullosa 

 towards a large excess of centrifugal over centripetal secondary 

 vascular tissue. A similar specimen of Medullosa stellata is 

 figured by Mougeot 3 from the Vosges showing a considerable 

 development of centrifugal xylem comparable with that in the 

 British Museum stem. Weber and Sterzel 4 describe stems of 

 Medullosa stellata showing slight periodic swellings which it is 

 suggested, though there is no evidence in support of the opinion, 

 may be connected with reproductive organs. 



Medullosa stellata var. corticata 5 . The specimen referred to this 

 variety, represented in fig. 416, G, has already been quoted as 

 affording data with regard to the origin and behaviour of the leaf- 

 traces. In this type of stem the outer portion of the main stele 

 is narrower than in M. stellata var. typica and the stele never forms 

 a complete tube. The star-rings in the centre of the stem are 

 more numerous than in the type-species of the genus. In the 

 axial region of some stems included in the form-cycle to which 



Schenk (82). See also Zeiller (90) B. p. 286. 



No. 13767, probably identical with M. stellata var. lignosa Weber and Sterzel. 



Mougeot (52) A. p. 36, PI. ra. figs. 810. 



Weber and Sterzel (96) B. p. 108. I 



Ibid. p. 56. 



72 



