PALMOBOTANY AND EVOLUTION. 117 



type ". The occurrence of pitted tracheids, the collateral 

 arrangement of the xylem and phloem hi the leaf-trace 

 bundles in their course through the stem tissues, and the 

 differentiation of the peripheral stelar tissues into distinct 

 leaf-trace strands, are features peculiar to Heterajighwi as 

 compared with recent ferns. No great stress should be 

 laid on the much more considerable development of 

 secondary tissue in the fossil genus than in living ferns. 

 Botiychhun and Hcluiinthostachys both illustrate to a 

 certain extent the power of recent ferns to increase by 

 secondary thickening, and the comparison of other Palaeozoic 

 types with their nearest living allies has taught us not to 

 attach any great importance to secondary thickening as a 

 character of primary taxonomic value. 



It is of interest to note, that in Hetcrmigium the re- 

 semblance of the secondary vascular tissues to those of 

 Cycads extends to details of the sieve-plates.^ 



Williamson and Scott have clearly shown that in dif- 

 ferent parts of their structure the two genera Heterangium 

 and Lyginodendron reveal distinct points of agree- 

 ment with GleicheinacecE, OsniiLndacccc, Marattiacc^, 

 OphioglossecE, and Cycadacca;. Without suggesting that 

 these Palaeozoic types are the actual ancestors of existing 

 Cycads, the authors regard them as " derivatives of an 

 ancient and ' generalised ' Fern Stock, which already shows 

 a marked divergence in the Cycadean direction ", The 

 two plants described above, with other genera previously 

 known, clearly point to the existence of a fossil group of 

 plants " on the borderland of ferns and Cycadacccs'" } 



It would be easy, did space permit, to discuss the 

 structure and affinities of several of the Palaeozoic plants 

 belonging to this generalised group. The genera so ably 

 and lucidly dealt with in Williamson and Scott's splendid 

 memoir, aifford excellent examples of the data which palaeo- 

 botany is able to contribute towards the solution of develop- 

 mental problems. The value of such work as this can 

 hardly be overestimated ; the evidence which is in this 



1 Williamson and Scott, p. 766. 

 '^ Ibid., p. 769. 



