42 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



When the stem of Lepidodendron has attained to the condition 

 described, certain important changes take place by the formation of 

 exogenously developed tissues. At some point of the circumference 

 of the primary bundle a development of secondary xylem 

 commences which is formed from a cambium ring. At first the 

 secondary xylem appears as a crescent-like growth on the outside 

 of the primary bundle ; this gradually extends all round, and at 

 length the original bundle is surrounded by an exogenously 

 developed secondary zone of xylem, whose radial thickness is only 

 limited by the death of the plant. The elongated scalariform 

 tracheides forming the secondary xylem are radially arranged and 

 separated by medullary rays. 



A second source of increase in the girth of the stem arises from 

 the formation of a cork cambium or phellogen, which appears 

 immediately outside of the so-called middle cortex. The tissue 

 developed on the inner side of the phellogen consists of long 

 thick-walled prosenchymatous fibres, which add rigidity to the 

 stem. If any true cork is developed in the outer margin of the 

 phellogen, it is only to a very small extent. 



The leaf-bundles spring from the small outer tracheides of the 

 primary bundle, and, bending upwards and outwards, pass through 

 the zone of secondary xylem into the leaves. In their course 

 outwards they are accompanied by a parenchymatous sheath 

 derived from the inner bark. The leaf-trace in passing through 

 the inner cortex is further accompanied by a tract of parenchyma 

 derived from the middle cortex. This tract immediately before 

 passing into the leaf divides into two parts and forms the ^9a?•^c^- 

 nos, one lying on each side of the foliar bundle. 



Minor modifications of type occur in different species of 

 Lepidodendra, some never possessing a pith cavity, but it is 

 probable that in all a secondary zone of xylem was developed, 

 though in some at a later period of their growth than in 

 others. 



It is true that in the plants originally described as Lepidoden- 

 dron fidiginosum, Will., and Lepidodendron Harcourtii, Witham 

 (^Will.), no secondary zone of xylem has been observed,^ but 

 the first is ,'certainly a Lepiidophloios, and there is strong evi- 



■ See following Dote. 



