XXX] MEDULLOSA 159 



of the cylinders of centrifugal vascular tissue. These abnormal 

 developments are considered by Worsdell^ to be relics of the inner 

 portions of concentric steles possessed by the MeduUosan ancestors 

 of recent Cycads. This interpretation afEords a means of bringing 

 into closer relationship the polysteUc MeduUoseae and the mono- 

 stelic Cycadaceae, the apparent simphcity of the latter being the 

 xesult of the progressive loss of centripetal xylem and phloem, 

 the normal cycadean cyhnder being therefore regarded as a one- 

 sided remnant of a concentric Medullosan stele. In other words, 

 the Cycads are descended from polystehc ancestors. As further 

 evidence in support of this view Worsdell points to the occurrence 

 of concentric steles in the cortex of Cycas and their occasional 

 presence in the pith of other genera. Matte^ has shown that in 

 the seedling stem of Encephalartos Barteri (fig. 396, K) there are 

 three concentric steles each similar to a normal Medullosan stele : 

 at a higher level in the axis the steles become 'unrolled' and 

 assume the form of one-sided cyhnders of centrifugal xylem 

 and phloem. 



In the peduncles of some recent Cycads, e.g. Stangeria, there 

 is a tendency towards a somewhat irregular orientation of the 

 collateral bundles that constitute the vascular cyhnder, and 

 tracheids occasionally occur internal to the protoxylem of the 

 individual bundles*. Worsdell regards these features as evidence 

 of a Medullosan ancestry. If the sinuous plate-rings of a stem 

 of Medullosa Letickarti (fig. 416, H) were broken up into separate 

 portions and wholly or in part deprived of the centripetal xylem, 

 the result would be an arrangement of bimdles comparable with 

 that in a Stangeria peduncle*. The scattered centripetal tracheids 

 discovered by Scott in Stangeria and other cycadean peduncles 

 are interpreted by Scott and by "Worsdell as rehcs of some 

 ancestral centripetal xylem, but with this important difference in 

 the point of view ; Scott beheves that they represent the almost 

 completely aborted centripetal xylem of a single stele hke that of 

 Lyginopteris, while Worsdell sees in them fragmentary vestiges of 

 the central primary xylem of two or more Medullosan steles. 



1 WorsdeU(96); (98); (00); (06) etc. 



2 Matte (04); (08). See also Bancroft (14); Dorety (09). 

 ' Scott (97). " WorsdeH (06). 



