XXX] MEDXTLLOSA 161 



gnetalean seeds is alluded to elsewhere. To the statement that 

 the presence of concentric and inversely orientated steles in 

 Welwitschia are reminiscent of the polystehc Medulloseae, Mrs 

 Thoday adds the quaUfying remark that the occurrence of four 

 concentric groups of vascular tissue in the hypocotyl of Welwitschia 

 is not sufficient to justify the conclusion that the ancestral type 

 was polystelic. This reservation accords with the contention of 

 Scott and other botanists, that the occasional occurrence in 

 cycadean seedlings and adult stems of anatomical features sug- 

 gestive of polystely does not in itself furnish an adequate reason 

 for doubting that the apparent monostely of Cycads is phylo- 

 genetically what it seems to be, namely, an indication of monosteUc 

 ancestry. This brings us to the question of a possible mono- 

 stehc ancestor. It may be that the Upper Carboniferous genus 

 Sutcliffia affords a clue to the problem of the origin of the polystelic 

 type illustrated in various forms by Medullosa. The protostele 

 of SiUcliffia bears a close resemblance to each of the three steles 

 of Medullosa anglica; the fact that Sutcliffia is exarch and that 

 Medullosa anglica has mesarch xylem is of secondary importance, 

 particularly as exarchy is represented within the genus Medullosa. 

 The extrafascicular strands of xylem and phloem and the accessory 

 strands are points in which Sutcliffia and Medullosa anglica agree 

 and, as Miss de Fraine^ adds, the meristeles of Sutcliffia may be 

 homologous with the leaf-trace strands of Medullosa. Scott^ 

 gave expression to the characters shared by these two types by 

 describing Sutcliffiu as the most primitive of the Medulloseae. 

 It is suggested that the protostelic axis of Sutcliffia may be regarded 

 as the starting-point of the, monostehc Cycads, the central mass of 

 tracheal tissue being replaced by a parenchymatous pith, while 

 the extrafascicular and accessory strands arose independently 

 of the central stele in response to increased physiological demands 

 consequent on the increase in size of the stem. From the same 

 starting-point evolution may have progressed along another fine 

 through such a type as Medullosa anglica leading to the more 

 complex Permian species of Medullosa. Chodat's view^ that the 

 Medulloseae are Protocycadaceae, if we include Sutcliffia as well 



1 de Fraine (12). " Scott (06) p. 64 



3 Chodat (08) B. p. 38. 



s. in 11 



