LEPIDODENDREAE. 



221 



lumen, are thick-walled, roundish in shape, and irregular in their dis- 

 position. From the arrangement of the cells in the outer layer, and still 

 more from the circumstance that having been at first feebly developed they 

 increase so much in size in old specimens and surpass the inner unaltered 

 cells, we may conclude that the outer layer is formed by the constant 

 activity of a meristem. We may in fact satisfy ourselves from specimens 



FIG. 23. Transverse section of the stem of Sepidodendron selaginoides, Will. In the centre is the concentric 

 central .vascular bundle-strand, and next it on one side is a thin crescent-shaped layer of secondary wood. The inner 

 rind is destroyed, the thick cylinder of the middle rind is traversed by radial fissures containing the foliar bundles. 

 On the outside are still to be seen some remains of the subepidermal zone to which the leaf-cushions belong. After 

 a preparation in my collection from the calcareous nodules of the English Coal-measures. 



in a particularly good state of preservation, that near the outer edge of the 

 layer there is such a hollow cylinder of meristematic compressed cells, 

 which may be compared to some extent with the phellogen of the rind in 

 recent plants ; this zone gives rise to a considerable amount of phelloderm 

 on its inside, while the phellem is produced in small quantities only, and 

 usually comes away with the outer cylinder in the form of a thin layer 





