114 BRITISH EOCENE FLORA. 



the " Progymnosperms," consisted of numerous woody wedges separated from each other, 

 in the younger states, by peculiar vertical prolongations of the pith, to which Prof. 

 Williamson assigns the name of primary medullary rays, while his secondary medullary 

 rays separate the constituent vascular laminae of each wedge as in recent Exogens. The 

 wedges extended vertically from node to node, and their apices or inner faces originated in 

 a vertical duct or canal. Investing this woody zone, and very rarely preserved, was a 

 cellular layer without vessels, the structure of which is, as yet, but imperfectly known. 

 In the young state it consisted of a thin layer of parenchyma composed of cells of various 

 sizes. At a more advanced stage of growth these developed a thick internal prosen- 

 chymatous layer like that of Zepidodendron and Siyiltaria. The outer surface appears to 

 have been smooth, not fluted longitudinally, and the articulations to have been incon- 

 spicuous. The exogenous wood thus surrounded the pith, and somewhat resembled, in 

 its arrangement, the first year's shoot of a recent Conifer. The rootlets (formerly called 

 Primularia) grew from near the nodes and were branching. The arrangement of the 

 appendicular organs on the young trunks was verticillate, and the leaves or bran chlets were 

 distributed at regular distances on the line of nodes, which were pretty close together, but 

 the branches became few and irregular in older stems. Though there is no direct proof that 

 such is the case, Saporta and Marion hazard the opinion that the foliage known as Arc/ieeo- 

 calamites and Borniu, which consists of repeatedly dichotomosing or acicular leaves, 

 arranged in verticels around nodes, on slightly striated stems, belongs to Catamodendron, 

 together with a male inflorescence born in catkins something like those of Taxece. Sir William 

 Dawson states that he has found, on the other hand, the leaves attached to the stem in five 

 species of Catamites, and in such relations as to give satisfactory proof as to their nature; and 

 has shown that they are similar in form and external markings to the so-called branchlets 

 of modern Equiseta. The fact is there is much difficulty in determining the true 

 relations which the verticillate leaves of Aster ophyltites, Sphenophyllum, and Annularia 

 bear to their several stems. 1 Prof. Williamson has described a homosporous strobilus 

 which he thinks belonged to Catamites ; whilst he has found both homosporous and hetero- 

 sporous ones which belonged to other allied Asterophyllitean plants. Saporta and Marion 

 call attention to the resemblance between the leaves of Bornia and those of Trichopitys and 

 Bryon, which are true Satisburiea ; 2 for, though the former are verticillate and the latter 

 spiral in arrangement, the possibility of an easy transition from one to the other is 

 shadowed forth in some Lycopods, and both dispositions occur together in existing Cupres- 

 sinea and the young Abietinea. 



Another remarkable Carboniferous stem with exogenous wood is described by Prof. 



1 Prof. Williamson remarks that " the structure and true relations of the roots and leaves are, as yet, 

 the least known part of the history of Calamites." Very different looking organs occurring in sM at 

 Saint-Etienne are figured as roots of Catamites and Calamodendron respectively, ' Mem. de l'Acad. des 

 Sciences,' 2nd series, vol. xxiv. 



2 [Salisburiece — a family proposed to receive the numerous extinct as well as the solitary living species 

 of Ginkgo.] 



