112 BRITISH EOCENE FLORA. 



able group of extinct Carboniferous plants wliich united the characters of the Lycopodiaceae 

 and other Cryptogams with true exogenous stems. Though not in the direct lines of 

 evolution, for the truly Gymnospermous Dadoxylon equals them in antiquity, they 

 probably reveal the lines through, which the passage from Cryptogams to Gymnosperms 

 has taken place. Believed by Brongniart, Grand'Eury, Renault, Saporta, and Marion 

 to have been Gymnospermous trees, they have been the subjects of prolonged discussion 

 and research. The laborious investigations of Prof. Williamson had gradually made it 

 clear that their peculiar exogenous growth was shared by the undoubtedly cryptogamous 

 Lepidodendron, and the confirmation by M. Zeiller of Goldenberg's discovery that 

 Sigillaria had spore-bearing strobili, has now confirmed Williamson's contention that 

 they must be classified with Cryptogams. 



As the earliest connecting links between Cryptogams and Phanerogams their morpho- 

 logy is peculiarly interesting, and the exquisite preservation of many of their silicified 

 or calcified stems permits the minutest details of their structure to be studied. 



That Cryptogams reached a far higher stage of development in the Palaeozoic time 

 than exists in any surviving representatives has not been disputed. One of the best known 

 of these is Lepidodendron, the vigorous and splendid growth of which formed one of the 

 culminating developments of the Lycopodiacea. The complex organisation possessed by 

 them, even to the minutest points of internal structure, is very remarkable. " They formed 

 large trees with acicular or falcate, perhaps, at a late period, deciduous leaves, and bore 

 cones at the extremities of the branches, which differed exteriorly but little from those of 

 some Gymnosperms. The expanded bases of the scales or bracts bore the sporangia, those 

 containing the macrospores being nearest the base of the cone. In all the species the stem 

 consisted of several layers. In most the centre consisted of a parenchymatous pith. In 

 some this pith was only represented in its young state by one or two isolated cells, but these 

 rapidly multiplied, developing into a conspicuous medulla. In one or two others these 

 medullary cells were replaced by a solid rod of scalariform or barred Tracheids. In a third 

 type barred Tracheids were developed within the cellular medulla — but at the periphery 

 of the latter they rapidly coalesced into a vascular cylinder. In another type this cylinder 

 was developed pari passu with the medulla, the boundary-line between the two being 

 sharply defined. In all cases the vascular bundles proceeding to the leaves were composed 

 of barred Tracheids derived from this primary vascular cylinder, the vessels of which were 

 never arranged in radial lines. Even in its youngest state the bark investing this primitive, 

 non-exogenous, vascular zone exhibited three layers — an inner, often delicate paren- 

 chymatous one, a median prosenchyma, which ultimately attained to great thickness, 

 and a permanently thin superficial parenchyma. 



" In most Lepidodendra there was developed, sooner or later, a second and more external 

 vascular zone, the vessels of which were arranged in radiating laminae and which was 

 unmistakeably an exogenous development from a Cambium layer. 



" Lepidodendron, like the greater part, if not all, of the Palaeozoic Flora, became extinct 



