FOSSIL PLANTS. 53 



structural details suggestive of an endodermis. 1 The leaf- 

 traces, probably collateral in form, are followed in their 

 course from the xylem cylinder to the outer cortex ; and 

 it is shown how the tissues composing the inner cortical 

 zone become gradually more lax towards the periphery, 

 and finally give place to a wide space in which the 

 leaf -traces appear as isolated groups of tissue. The cells 

 of the inner cortex surrounding each trace become 

 elongated, and gradually separate from one another in 

 the form of trabecular as we pass to the more loosely 

 arranged part of the cortex. In the middle cortex the 

 bundles appear to be surrounded by a few layers of 

 parenchymatous cells, and have the form of isolated 

 strands, in consequence of the formation of a wide 

 space in the middle zone of the cortex. Towards the 

 apex of the strobilus, the median cortical space becomes 

 narrower and bridged across by trabecular, which stretch 

 from the outer and inner tissues of the cortex to the several 

 leaf-trace bundles. Bower's figures show a very striking 

 resemblance between the central stele of Psilotum and that 

 of Lepidostrobtts Brownii ; the parenchyma of the pith in 

 the fossil specimen is more like the corresponding tissue 

 occupying the axis of Tmesipteris, than the sclerenchymatous 

 elements in the pith of Psilotum, thus affording "a most 

 interesting example of cross correspondence ". There is 

 also a detailed comparison instituted between the cortex of 

 Lepidostrobus Brownii and other species of Lepidodendron 

 with various forms of Lycopodium and Selaginella ; it is 

 stated that the several types of structure met with among 

 fossil forms may be "matched by similar characters in 

 living forms of close alliance".' 2 After speaking of the 

 trabecular tissue in the cortex of extinct and recent forms, 

 Bower adds, with reference to Selaginella, that " the 

 trabecular development in Selaginella is a specialised and 

 more definite example of that lacunar development which 



1 Cf. Hovelacque, M. Soc. Linn. Normand/e, vol. xvii., 1892, figs. 

 1 1 and 12, pp. 50 and 52. 



2 Annals Bot., vol. vii., p. 348. 



