180 MEGALOXYLEAE [CH. 



and broke up into several collateral strands, a suggestion based 

 on the behaviour of the leaf-traces in Medullosa anglica. 



The most striking characteristics of Megaloxylon are : (i) the 

 structure of the primary xylem, particularly the unusual form 

 of the majority of the metaxylem tracheids, a form obviously 

 correlated with storage rather than with conduction of water; 

 (ii) the gradual spreading of the leaf-traces and their absorption 

 as they descend into the main mass of the xylem ; (iii) the exarch 

 structure of the primary xylem. Confining our attention to the 

 primary region of the stele; a comparison is at once suggested 

 with Heterangium. In Megaloxylon the peculiarities are the 

 substitution of the large storage-tracheids for the normal xylem- 

 elements ; the greater irregularity in the groups of metaxylem ; 

 and an exarch instead of a mesarch structure. In these last 

 features the primary xylem agrees with that of recent species 

 of the Schizaeaceous Fern Lygodium and the external protoxylem 

 is a character shared with Rhetinangium. The occurrence of 

 short tracheids similar to those of Megaloxylon in the inner portion 

 of the stele of the Osmundaceous Fern Zalesskya gracilis (Eich.) 1 

 may be quoted as an example of parallel modification but, as 

 Scott 2 points out, the resemblance has no phylogenetic signi- 

 ficance. The secondary xylem though less parenchymatous than 

 in recent Cycads agrees more closely with the manoxylic than 

 with the pycnoxylic type. 



Megaloxylon affords an interesting example of a combination 

 of primary stelar anatomical features, comparable in the exarch 

 position of the protoxylem with the stele of Lygodium, and 

 secondary wood similar to that of Lyginopteris and Heterangium. 

 The large metaxylem tracheids may be regarded as derivatives 

 of elements which in some ancestral type were structurally fitted 

 for the role of water-transport and made up the xylem of a 

 Lygodium-like stele with little or no secondary xylem. As the 

 cambial activity increased and centrifugal xylem became a 

 prominent feature, usurping the function of the centripetal xylem, 

 the latter became modified and fitted for a new service. 



1 Vol. ii. p. 326. 



2 Scott (09) B. p. 476. 



