Royal Society. 227 



PROCEEDINGS OE LEARNED SOCIETIES. 

 ROYAL SOCIETY, 



Dec. 19, 1872.— Sir George Biddell Airy, K.C.B., President, followed 

 by Mr. Busk, Yioe-Presideut, and Dr. Sibson, Vice-President, 

 in the Chair. 



" On the Organization o£ the Eossil Plants of the Coal-mea- 

 sures. Part IV. Dictifoxylon, Lyginodendron, and Heterancjium." 

 By "W. C. AViLLiAMSON, E.E.S., Professor of Natural History in 

 Owens College, Manchester. 



In 1866 Mr. Binney gave the name of Dadoccyhn OldJiamium to 

 a fossil stem of a plant from the Lower Coal-measures of Lancashire, 

 believing it to belong to the same class of -^Jymnospermous Exogens 

 as the Pinites of Witham and the Dadoxylon of Endlicher. In 1869 

 the author pointed out that the reticulated markings upon the 

 surface of its vessels were modifications of the spiral fibre of fibro- 

 vascular tissue, and not the disks of what is often designated glandu- 

 lar fibre. He consequently separated the plant from the Dadoxylons 

 under the name of Dictyoxylon OldJiamium. At the Edinburgh 

 Meeting of the British Association in 1871 he gave a brief account 

 of the structure of this plant, as also of what appeared to be a second 

 species from the Lower Coal-measures of Burntisland in Fifeshire, 

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

 detailed exposition of the organization of these two plants is given 

 in the memoir. 



Didyoxylon Oldhamium. — This was a stem composed of the three 

 divisions of pith, wood, and bark. The pith consisted of regular 

 parench}Tna without diAdsions or cavities of any kind. In very 

 young plants it was surrounded by an irregular ring or medul- 

 lary 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 lamina>, 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 additioiis demonstrate their 

 exogenous nature in several specimens in which the vessels of the 

 outermost zone have not attained to half iluir normal size, resembling 

 in this respect some of the Jicpidodendroid plants described in the 

 author's last memoir (Part III.). Through these successive ex- 

 ogenous 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 dia- 

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



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