196 Prof. W. C. Williamson and Dr. D. H. Scott. [June 13, 



I. LYGINODENDRON. 



Lyginodendron oldhamium, Will.,* is one of the commonest fossils 

 preserved in tbe calcareous nodules of the Lancashire and Yorkshire 

 coal-measures, and has also been found in those of Germany and 

 Austria. A renewed investigation, with the aid of numerous addi- 

 tional specimens, has enabled us to clear up many doubtful points in 

 the structure of the plant, and to give for the first time a complete 

 account of all its vegetative organs. 



A. The Stem. 



1. General Structure. The middle of the central cylinder or stele 

 is occupied by a parenchymatous pith. Surrounding this is the 

 primai'y wood, which usually forms a ring of from five to eight distinct 

 strands. Beyond this we find, in all but the youngest specimens, a 

 broad zone of secondary wood, then the cambium, and next the phloem. 

 The whole stele is bounded by a well-marked pericycle. The inner 

 cortex is mainly parenchymatous, while the outer zone consists of 

 alternating strands of fibres and parenchyma, constituting the well- 

 known " dictyoxylon cortex " of Count Solms-Laubach. 



The pericycle and cortex are traversed by the leaf-trace bundles, 

 which alternate with the perimedullary xylem-strands. 



2. Course of the Vascular Bundles. We have obtained direct proof 

 that the perimedullary strands of xylem form the downward continua- 

 tion of the bundles which pass out into the leaves. Thus the entire 

 bundle-system of the stem is built up of the leaf-traces. Each leaf- 

 trace extends through at least ten internodes ; five internodes are 

 traversed while it is passing through cortex and pericycle, and five 

 more after it has reached the periphery of the pith. On entering the 

 pith the trace turns aside in the kathodic direction, and unites with 

 the adjacent perimedullary strand on that side. We thus see that 

 these strands are sympodial bundles, made up of the united lower 

 portions of adjacent leaf-traces. 



In the upper part of its course, each leaf-trace consists of two 

 bundles, which unite into one in passing through the pericycle. 



The phyllotaxis was usually two-fifths, but in the smallest stems 

 was probably one -third. 



3. Structure of the Vascular Bundles. The preservation is so good 

 that we have been able to determine with certainty that the bundles 

 in the stem were normally collateral, having xylem on their inner, 

 and phloem on their outer side. As they passed out into the leaves 



* See Williamson, "Organisation of the Fossil Plants of the Coal-measures,. 

 Part IV," ' Phil. Trans,,' 1873; Part VI, 'Phil. Trans./ 1874; Part VII, 'Phil. 

 Trans.,' 1876 ; Part XIII, ' Phil. Trans.,' 1887 ; Part XVII, ' Phil. Trans.,' 1890. 



