46 PTEKIDOSPERMEAE [CH. 



Williamson^ and similar occurrences are more fully dealt with 

 by Williamson and Scott^ who consider that the perimedullary 

 cambium may represent an internal extension through a leaf-gap 

 of the normal cambial cyhnder. In the stem represented in fig. 

 403 there are two perimedullary xylem strands to the left of the 

 bottom of the V-shaped gap in the secondary xylem-cylinder, r, 

 and on the inner face of one of these, as shown in fig. 405, C, 

 there is a narrow arc of internal secondary xylem, c, between the 

 xylem-strand and the outer edge of one of the sclerous nests. 

 The sporadic occurrence of arcs of inversely orientated secondary 

 vascular tissue affords an interesting parallel with a similar mor- 

 phological feature in some recent Dicotyledonous genera such 

 as Tecoma and lodes. As Williamson and Scott point out, this 

 similarity affords 'a striking warning against the indiscriminate 

 use of even conspicuous anatomical characters^.' While admitting 

 the necessity of guarding against the danger of attaching impor- 

 tance to occasional and abnormal characters they may have some 

 significance as collateral evidence in comparisons of different 

 types of stems. It is conceivable that these anomalous arcs of 

 secondary tissue on the inner side of the primary xylem strands 

 may, as Worsdell* maintains, be reversions to an ancestral character 

 and in this sense comparable with the strands of inverted vascular 

 tissue in some recent Cycadean stems. The question of relation- 

 ship of Lyginopteris and allied types to recent Cycads and the 

 Palaeozoic Medulloseae is considered in a later chapter. 



In 1902 Lomax* described two branching specimens of Lygin- 

 opteris, and more recently two others have been discovered at 

 a locahty near Bacup in Lancashire which have been thoroughly 

 investigated by Miss Brenchley* who constructed models from 

 drawings of serial sections'. One specimen shows six leaf-bases 

 in a length of 4| inches and branches spring from the axils of five 

 of them: some of the branches show secondary ramifications. 

 The phyllotaxis of the leaf-bases on a branch is always the reverse 

 of that on the main stem, a divergence to which no parallel was 



' Williamson (90) PI. xm. fig. 3, 6. 



2 Williamson and Scott (95) PI. xxm. fig. 8. a jjj(j_ p 722. 



* WoradeU (06) pp. 140 et seq. ^ Lomax (02). « Brenohley (13). 



' For a description of the method, see Salisbury (13). 



