1889.] On the Fossil Plants of the Coal-measures. 439 



lignified scalariform tracheids are seen, enclosed within an outer 

 series of perfectly lignified ones. Here we have obviously an example 

 of the centripetal development of a vascular bundle, reminding us of 

 what is the normal mode of growth amongst the bundles of all recent 

 Lycopods. 



A fifth new species, Lepidodendron parvulum, is also described ; 

 after which the author points out the differences between the mode 

 of development of the cellular medulla of these exogenous Cryptogams, 

 and that of the representative organ amongst the Dicotyledonous 

 Exogens. Amongst the ordinary Exogens the growing tip of a stem 

 or branch is a mere aggregation of cells, w.hich mass is soon, sepa- 

 rated into two zones, in addition to the formation of the epiderm, by 

 the development within it of a ring of vascular bundles. The cells 

 enclosed within this ring become the medulla or pith, and those 

 external to it constitute the cortex. In this instance the cells about 

 to form the medulla exist, prior to their becoming defined as a medulla 

 by the first development of the vessels which enclose it, and which 

 vessels will ultimately grow into a woody, or zylem, zone. Such a 

 pith subsequently undergoes but a very limited enlargement. In most 

 cases a time arrives when it grows less with age, and ultimately 

 almost disappears ; but in the Lepidodendra, though the tip of each 

 growing stem was, in the first instance, also a cellular mass, what is 

 designated an axial solid bundle of vessels was developed in the centre 

 of the new growth almost at its very commencement. But it was 

 only after this growth had made some progress, and the twig had 

 become clothed externally with numerous leaves, that the first traces 

 of a medulla began to appear in the centre of the bundle. It is thus 

 clear that the medulla of these Carboniferous Lycopods is not geneti- 

 cally homologous with that of an ordinary exogenous flowering plant. 

 But the stage of growth of the stem at which this medulla first 

 appeared has differed remarkably in various species of Lepidodendron^ 

 a remark equally applicable to the first formation in them of a true 

 exogenous zone. 



The axial vascular medullary bundle expanded into a hollow 

 cylinder under the internal pressure of the growing medulla, 

 which latter not only attained to considerable dimensions, but was 

 a persistent organ. This ring enclosing the medulla, supplied 

 the vascular bundles going to leaves and branches. The author 

 demonstrates that the branches are supplied with such bundles in two 

 ways. When the growing stem divides dicbotomously, which it does 

 as amongst living Lycopods, the medullary vascular cylinder splits 

 into two equal halves. But, besides this mode, the author shows that 

 very frequently comparatively small segments are cut completely out 

 of the vascular cylinder, in which a gap is thus left where the bark 

 and the medulla meet. The angular segment thus detached develops, 



VOL. XLV. 2 a 



