158 MEDULLOSEAE [CH. 



long and narrow plate-ring and is then rolled into a hollow 

 cylinder like that in fig. 416, D. A modification of the tubular 

 type of stele is seen in Medullosa stellata var. corticata (fig. 416, G) 

 in which the vascular cylinder is broken up into two or more curved 

 plate-rings, a change superficially similar to that by which a 

 dictyostele is produced from a solenostele, but in Medullosa it 

 is not the overlapping of leaf-gaps that is the cause of the change. 

 A striking feature in the stem of Medullosa stellata represented 

 in fig. 416, D and F (especially F), is the inequality in breadth 

 of the centrifugal and centripetal xylem: this inequality is an 

 expression of the difficulty caused by the presence of an internal 

 as well as an external addition of secondary conducting tissue. 

 A limit is set to the production of secondary centripetal xylem by 

 the space available for extension, whereas there is unlimited room 

 for increase in the case of the centrifugal tissue. This tendency 

 to a greater development of xylem and phloem on the outer side 

 of the primary portion of the steles is illustrated also in M. Solmsi 

 var. lignosa (fig. 416, L) where the outer of the two series of 

 peripheral steles has a much larger proportion of centrifugal 

 xylem. In M. anglica the secondary xylem on the inner side of 

 the steles is not infrequently broader than the corresponding 

 tissue on the abaxial side 1 ; but this is exceptional in the genus. 

 A further development of centrifugal conducting tissue without 

 any corresponding development of centripetal tissue is effected 

 in certain cases (fig. 416, K, L) by the addition of concentric 

 cylinders of centrifugal xylem and phloem beyond the original 

 concentric steles. The type illustrated by Medullosa Solmsi var. 

 lignosa and M. stellata var. gigantea (fig. 416, L, K) at once suggests 

 comparison with stems of Cycas, Macrozamia, and Encephalartos, 

 the chief difference being the presence in Medullosa of an inner 

 series of concentric steles and a central ground- tissue containing 

 star-rings, though the latter may be regarded as corresponding 

 to the medullary system of bundles in Macrozamia. In Macro- 

 zamia the central region of the stem is considered to be the pith 

 of a monostelic stem, whereas in Medullosa the stem is polystelic. 

 In recent cycadean stems it is not uncommon to find patches of 

 inversely orientated xylem and phloem internal to one or more 



1 Scott (99) p. 89. 



