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 Lejpidodendron and Sigillaria. 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 branchlets 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 Archao- 

 calandtes and Bornia, which consists of repeatedly dichotoraosing or acicular leaves, 

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

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

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

 species of Calamites, 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 Eqimeta. The fact is there is much difficulty in determining the true 

 relations which the verticillate leaves of Asterophyllites^ Sphenophyllum^ and Atinularia 

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

 which he thinks belonged to Calamites ; 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 Saluburiea ; ^ 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 Abietinece. 



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



^ 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 situ at 

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

 Sciences,' 2nd series, vol. xxiv. 



2 \SalisburiecB — a family proposed to receive the numerous extinct as vfcU as the solitary living species 

 of Ginkgo.] 



