30 CYCADALES [CH. 



pericyclic parenchyma occur between adjacent extrafascicular 

 cylinders of xylem and phloem. In a stem of Cycas media 35 cm. 

 in diameter examined by Worsdell there were 12 concentric 

 cylinders. Matte 1 and Miss Dorety 2 have described partially 

 flattened arcs of extrafascicular xylem and phloem in the hypo- 

 cotyl of Ceratozamia mexicana. Worsdell 3 first drew attention 

 to the occasional occurrence of short tracheids on the inner edge 

 of the secondary wood and to the spasmodic development of 

 cambial arcs in the tissue between the extrafascicular cylinders 

 forming strands of inversely orientated xylem and phloem. More 

 recent work by Matte gives support to Worsdell's comparison 

 between Medullosean stems and those of recent Cycads with 

 inversely orientated arcs or concentric vascular cylinders. The 

 French author draws attention to the close resemblance between 

 the seedling stems of such species as Encephalartos Barteri 

 (fig. 396, K) and Cycas siamensis (fig. 396, L) with their polystelic 

 type of structure and the adult stems of Medullosa*. In the 

 stems of Dioon, Microcycas, Stangeria, and Zamia no extrafasci- 

 cular cylinders are recorded. Two main vascular bundles enter 

 the cortex from each leaf-base and in most stems these diverge 

 right and left and more or less completely encircle the stele before 

 passing through the medullary rays and joining the inner portion 

 of the xylem of the stele either as double or single bundles. These 

 girdle-bundles (fig. 396, H) first described by Karsten and Mettenius 

 form a very characteristic cycadean feature 5 . Adjacent girdles 

 are joined by connecting cortical bundles and, in addition, there 

 are cauline collateral bundles in the cortex which form an 

 anastomosing system. In some cases, e.g. species of Macrozamia 

 and occasionally in Stangeria, the female peduncle of a Cerato- 

 zamia, and in seedlings of Bowenia and Cycas revoluta 6 , the leaf- 

 traces pursue a direct course from petiole to stele as in stems of 

 Bennettitales. It is noteworthy that in seedlings of Microcycas 7 , 

 a genus characterised by a large number of male gametes 

 presumably a primitive feature the leaf-traces are of the girdle- 

 type. The two bundles at the base of a petiole by repeated 



1 Matte (08). 2 Dorety (08 2 ); (09). 3 Worsdell (96). 



4 Matte (04) especially pp. 185202. See also Worsdell (00); (06) etc. 

 6 Matte (04) p. 210. 6 Worsdell (98 2 ). 7 Caldwell (07). 



