324 FOSSIL PLANTS OF THE COAL-MEASURES. 



Passing to the second genus Ht'icram/hwi, there are many points 

 of resembhance to the preceding type, but at the same time some 

 striking differences in the structure of both stele and cortex. 

 Instead of the central pith of Ly</inode7vlrun, we have in Hetermuiium 

 an axial group of primary xylem trachea? interspersed with bands 

 of conjunctive parenchyma. At the periphery of the stele occur 

 the smaller elements of the wood. In most examples there is a 

 ring of secondary xylem and phloem surrounding the central 

 primary xylem, and beyond this a pericycle, and an inner cortex 

 containing groups of characteristic radially elongated bands of 

 sclerous cells. The more external part of the cortex consists of 

 alternate plates of parenchyma and sclerenchyma as in Ly;/ino- 

 dendron. At the peripheral part of the primary xylem region there 

 are definite groups of tracheids similar to those of the primary 

 xylem strands of the preceding genus, and here too the protoxylem 

 elements are situated internally in each xylem-strand. The term 

 mesarch has been adopted for xylem groups in which the protoxylem 

 occupies this internal position. The course of these peripheral 

 strands, which are continuous with the leaf-traces, differs some- 

 what from that of the foliar bundles in Lj/fjinodendron, but it is 

 impossible to deal with the two genera in any detail. In one 

 species, Heteraufjium tilceoides AVill., the tissues of the secondary 

 phloem have been preserved in unusual perfection, the sieve-plates 

 being very clearly shown on the lateral walls. The leaves and 

 petioles agree fairly closely with those of L)j(iiiiodi'ndrnn, and a 

 smiilar correspondence is found as regards the adventitious roots. 



Having thus sketched in rough outline some of the characteristic 

 features of these two genera, we must in conclusion draw attention 

 to their affinities with recent forms. In neither case have we as 

 yet any satisfactory evidence as to the nature of the reproductive 

 organs, and are thus left to discuss the question of systematic 

 position on the evidence of vegetative structures alone. Hctcranfjium 

 is the more fern-like of the two forms ; the structure of the stele 

 agrees in essentials with that of a monostelic fern such as Gleichenia, 

 In both fossil genera the leaves and petioles are distinctly of the fern 

 type. On the other hand, the secondary wood of the stem resembles 

 that of a Cycad. The primary xylem bundles of each genus are 

 practically identical with those of a recent cycadean petiole, in the 

 position of the protoxylem elements. The roots, again, are fern- 

 like in character, and have been compared with those of the 

 MarattiaccoE. " We have seen how extraordinary a combination of 

 characters belonging to various groups these genera present. In 

 different parts of their structure they have been found to present 

 points in common with Glcicheniacece, Osmundacefe, Marattiacea;, and 

 Cycadece. The view of the affinities of Lyginodendron and Heter- 

 amjium which we desire to suggest is, that they are derivatives of 

 an ancient and 'generalized' fern-stock, which already show a 

 marked divergence in the cycadean direction." .... "We think 

 the existence of a fossil group on the borderland of Ferns and 

 Cycadece is now well established. The relation of these forms to 

 those very ancient Gymnosperms, the Cordaitem, is a difficult and 



