88 MEDULLOSBAE [OH. 



Medullosa is always polystelic : the plan of the vascular system 

 varies considerably as regards both the number and form of the 

 steles, but there is a uniform type of structure within the limits 

 of each stele that recalls the single stele of Heterangium. The 

 steles consist of a central region composed of primary xylem, 

 originally surrounded by phloem, which in its mesarch or exarch 

 structure agrees with the vascular tissue of some species of 

 Gleichenia or Lygodium. To this central region a cambium 

 added secondary xylem and phloem either in the form of a cylinder 

 of uniform breadth, or more frequently the centrifugally developed 

 xylem exceeded in amount the secondary conducting tissue added 

 to the inner side of the primary region. Apart from anatomical 

 details a Medullosan stem with its several steles, each with 

 secondary tissue, embedded in parenchymatous ground-tissue 

 resembles the stems of some Dicotyledonous climbers such as 

 Thinouia scandens, species of Serjania and Paullinia^. 



Anatomically the main features of the stelar system of Medul- 

 losa, neglecting the secondary xylem and phloem, are in closer 

 agreement with the stems of Ferns than with those of any other 

 plants. It has been shown that the genus Heterangium bears 

 a close resemblance to Gleichenia in the structure of the primary 

 stele (fig. 418, C) : one of the oldest types of Medullosa, M. anglica, 

 may be described as a Heterangium with three steles and may be 

 compared with a dictyostelic Fern in which the irregular vascular 

 framework is made up of three main strands. In certain types 

 of Medullosa (fig. 416) the ground-plan of the vascular system 

 recalls that of a solenostehc Fern, while in others the greater 

 complexity suggests comparison with such Ferns as Matonia, 

 Angiofteris, Psaronius, or Cyathea; 'it is as though Nature were 

 at the Carboniferous moment in the midst of a series of amazing 

 engineering experiments, most of which were either buried deep 

 in Palaeozoic obhvion, or permitted to survive only as vestigial 

 rehcs and atavistic ghosts' 2. Though many Medullosae resemble 

 Ferns there is an important difierence between the two groups 

 in the origin of the various plans of Medullosan stelar systems: 

 in Ferns the leaf is the determining factor in the evolution of 

 stelar arrangement, while in Medullosa the occasional interruption 



1 Sohenck (93) B. Pis. i— v, etc. ' White, D. (05^) B. p. 389. 



