1872.] 



Plants of the Coal-measures. 



87 



shire, which he called D. Grievii, after its discoverer, D. Grieve, Esq. A 

 detailed exposition of the organization of these two plants is given in the 

 memoir. 



Dictyoocylon Oldhamium. — This was a stem composed of the three divi- 

 sions of pith, wood, and bark. The pith consisted of regular parenchyma 

 without divisions or cavities of any kind. In very young plants it was 

 surrounded by an irregular ring or medullary cylinder of reticulated 

 vessels, not arranged in radiating laminae. This cylinder broke up at an 

 early period into several detached vascular bundles, which, as the stem 

 enlarged, became widely separated from each other, the intervening space 

 being occupied by medullary parenchyma. But before this change was 

 completed, the true ligneous zone appeared as a thin ring of vessels 

 arranged in radiating vertical laminae, separated from each other by large 

 and conspicuous medullary rays, composed of mural cellular tissue. 

 Additions were made to the exterior surface of this zone by the agency 

 of a delicate cellular layer of cells, which constituted the innermost 

 layer of the bark. These additions demonstrate their exogenous nature 

 in several specimens in which the vessels of the outermost zone have 

 not attained to half their normal size, resembling in this respect some of 

 the Lepidodendroid plants described in the author's last memoir (Part III.). 

 Through these successive exogenous growths the vascular axis of the stem 

 ultimately became arborescent. One specimen is described in which 

 such a vascular axis, though imperfect and waterworn, is fully six inches 

 in diameter, independent of the bark ; other specimens have been ob- 

 tained intermediate in size between the above example and the small 

 stems more usually met with. The vascular laminae increased in thick- 

 ness as they proceeded from within outwards, and then subdivided, in 

 the ordinary exogenous manner, through the intercalation of new medul- 

 lary rays. These rays are remarkable for the great vertical range of each 

 one, as well as for the large number of cells which enter into their com- 

 position. In tangential sections they appear as elongated lenticular 

 masses of parenchyma. 



The Baric. — This organ is separable into three, if not four layers. The 

 innermost one is a delicate parenchyma closely investing the ligneous zone, 

 its cells being continuous with those of the medullary rays. At its outer 

 surface this tissue passes gradually into another parenchymatous layer of 

 greater thickness than the inner one. Both of them have patches of 

 dark-coloured cells scattered through their tissues. But the most re- 

 markable part of the bark is the third or prosenchymatous layer, which 

 presents very different features according to the aspect in which it is 

 regarded. In the transverse section it consists of radiating bands of 

 parenchyma alternating with narrower and very dark-coloured ones of 

 woody prosenchyma, the latter looking very like the Soman numerals 

 upon the face of a clock. Tangential sections show that the black bands 

 are fibrous laminae, which pursue an undulatory course as they ascend 



