1872.] 



Plants of the Coal-measures. 



89 



some of the cells o£ the central part of the axis underwent rapid fission, 

 and thus developed a distinct cellular medulla, which forced the medullary- 

 vessels outwards where at first they constituted a ring, but which ring 

 soon broke up into the detached vascular masses already referred to as 

 adhering to the inner surface of the exogenous zones. 



The enlargement of the exogenous woody cylinder by the peripheral 

 intercalation of new radiating vascular laminae, and the repeated sub- 

 division of these laminae by a corresponding intercalation of new medul- 

 lary rays, demonstrates the close resemblance between the growth of the 

 ligneous zone in these plants and that of ordinary exogenous stems. A 

 fine series of specimens collected by the Eev.H. Higgins, of Eainhill, near 

 Liverpool, and which exhibit various modifications of the type figured 

 by the late Mr. Grourlie under the name of Lyginodendron Landsburghii, 

 are shown to be merely casts of the exterior surface of the bark of some 

 species of Dictyoxylon. They may actually belong to D. Oldhamium; but 

 this is not yet proven. 



Dictyoxylon Grievii. — This plant has many points of affinity with D. 

 Oldhamium ; nevertheless it has very distinct features of its own. Its 

 central or medullary axis is very large in proportion to the thickness of 

 its exogenous ring ; the former consists of cellular parenchyma, through- 

 out which are scattered numerous bundles of exquisitely reticulated 

 vessels unprovided with any special sheaths. The largest vessels are 

 nearest the centre of the axis, the peripheral ones becoming smaller, more 

 numerous, and grouped in more continuous masses. Immediately sur- 

 rounding this vasculo-cellular axis is a thin ring of similar vessels, but 

 arranged in radiating laminae, separated by well-defined medullary rays. 

 This zone is generally of unequal thickness on opposite sides of the plant, 

 and contains some barred vessels amongst its reticulated ones ; the 

 medullary rays are composed of mural cells. 



The bark consists of three very distinct layers. The innermost one is 

 very thin, consisting of delicate parenchyma, but which nevertheless has 

 formed a very clearly defined flexible layer; outside this is a thick 

 stratum of coarser but regular parenchyma subdivided in the transverse 

 section into vaguely defined areas by thick wavy lines of condensed cells. 

 The peripheral outline of this zone is very irregular, frequently projecting 

 outwards in large angular masses. It is bounded by a prosenchymatous 

 external layer, which is a dwarfed representative of the corresponding one 

 of D. Oldhamium. In the transverse section it exhibits dark radiating 

 bands of fibres, longitudinally disposed, alternating with similar bands of 

 parenchyma ; but it differs from D. Oldhamium in the narrowness of the 

 latter, and consequently in the more linear form of the cellular areolae of 

 the outer bark. In longitudinal sections of the bark its innermost layer 

 appears as in transverse ones. The middle parenchyma, on the other 

 hand, exhibits remarkable differences from its aspect in the transverse 

 section : its cells are arranged in vertical columns ; but these are inter- 



