222 LEPIDODENDREAE. 



of radially disposed cells. As this phelloderm consists of prismatically 

 elongated cells, it is termed by Williamson the ' outer or prosenchymatous 

 layer of the bark,' the primary tissue being with him ' the middle paren- 

 chymatous part of the bark.' When Renault regularly calls this phelloderm- 

 portion of the middle cylinder the 'assise sube*reuse,' he means to call 

 attention to its connection with the peridermal system. For we cannot 

 imagine any formation of real cork either in the phelloderm or in the 

 phellem ; the main growth in thickness of the tree devolves on the phellogen, 

 and if this were to produce anything but normal parenchyma-cells on the 

 phellem-side, the tissue-cushions of the leaf-bases would at once die away 

 and disappear, but this is by no means the case. On the contrary, we find 

 them still preserved on stems in which the periderm has already reached 

 a considerable development 1 . 



It was said above that the type of Lepidodendron vasculare, Binn. is 

 further distinguished by the appearance of a secondary woody body. 

 Lepidodendron-stems of this kind have often been described under other 

 names ; such are Anabathra pulcherrima 2 and Diploxylon cycadoideum 3 , 

 which are one and the same form, as Brongniart 4 was the first to perceive. 

 He and Renault do not admit that they belong to Lepidodendron, and 

 usually speak of them by Corda's names. Stems of this kind are termed 

 in Binney Sigillaria vascularis, and are figured in great numbers in the 

 publications just cited. The secondary growth of wood begins to develope 

 on the boundary between wood-strand and bast-portion ; its inner side 

 is in contact with the former and the emerging leaf-trace-bundles are 

 inclosed in it ; the bast and the formative cambium are more and more 

 thrust towards the outside. In many cases the new growth does not appear 

 simultaneously on the whole of the circumference (Fig. 23), but is developed 

 in a one-sided manner, and may be of great size on one side, while it has 

 not even begun to be formed in the opposite quarter. It consists of 

 elongated scalariform tracheides, which normally are disposed in radial 

 rows and increase in breadth towards the outside. It is traversed by 

 numerous parenchymatous medullary rays, which are seen on the tangential 

 section to be of two kinds ; one narrow and formed of one cell-layer, 

 shallow and one to a few cells in depth, and this kind is very abundant ; 

 the other formed of several layers, but like the first of small depth, fusiform 

 and inclosing the emerging foliar bundles at the broadest part. As against 

 Carruthers's 5 views it may be observed that all these medullary rays agree 

 perfectly with those of the roots of recent trees. The size attained by the 

 secondary wood here described varies much. This variation may be partly 

 due to specific differences, and is usually confined within moderate limits, 



1 Binney (3), t. 35, if. 5, 6. Witham (1), p. 74 ; t. 8, f. 7. 3 Corda (1), t. lo. 



Brongniart (7). 5 Carruthers (13 . 



