GYMNOSPERMiE. 113 



during the Permian period, leaving Selaginella and Isoetes as its humble surviving 

 representatives. 



" Sigillaria closely resembles Lepidodendron in its organization. As in the latter, its 

 exogenous vascular zone is richly supplied with medullary rays through some enlarged 

 and symmetrically arranged forms of which the foliar vascular bundles, wholly derived 

 from the inner non-exogenous vascular cylinder, pass obliquely upwards and outwards, on 

 their way to the leaves." ' 



Professor "Williamson describes the prosenchymatous and the parenchymatous struc- 

 ture investing the woody zone as a bark, and remarks that, although not divisible into 

 layers identical with those of the Phanerogams, the enormous development of the elongated 

 prosenchymatous fibres, or bast-tissue, in the interior of the fossil stems is a manifest 

 foreshadowing of the presence of that same tissue in the bark of living exogens, especially 

 the Cycads. There is no difference of opinion as to the exogenous nature of the second 

 or outer vascular cylinder, though it bears a relatively small proportion to the diameter of 

 the stem, nor as to the presence of representatives of medulla or pith, and bark ; but while 

 the French School, including Adolphe Brongniart, B. Renault, Saporta, Marion, and 

 Grand'Eury, class Sigillaria, in consequence of its exogenous wood, as a low form of 

 Exogen, Mr. Carruthers, Sir J. D. Hooker, Prof. Williamson, and many of the German 

 authors have always regarded it as a highly-developed Cryptogam. It appears from William- 

 son's long-continued researches that the exogenous wood is not always developed in the 

 young stages, either of Sigillaria or Lepidodendron, and that there is a gradual passage 

 from one to the other. Sir W. Dawson, a great authority on the subject, now believes 

 that some Lepidodendra are exogenous, and Prof. Williamson considers that eventually 

 all may be found so. 



The roots of these plants are known as Sligmaria, and are looked upon by the French 

 School as rhizomes, capable of bearing leaves as well as roots, but as merely roots with 

 rootlets by other observers. The erect and cylindric Sigillarian stems were crowned with 

 a mass of long and linear leaves, whose scars have impressed their complex and beautiful 

 tesselated designs on the trunks. 



The next type of exogenous stem is still more remarkable, and its importance as one 

 of the connecting links between the Cryptogams and the Gymnosperms cannot be over- 

 estimated. Calamodendron, of Brongniart, but which Carruthers and Williamson affirm 

 is merely another name for Calamites, possessed a hollow stem with verticillate leaves, 

 somewhat resembling a gigantic Equisetum. This was filled in solid with pith or cellular 

 parenchyma when extremely young ; but soon becoming hollow with age, the fistular 

 interior consisted at last of a vertical series of oblong chambers, separated from each other 

 by transverse diaphragms, and lined with a very thin film of cells. The exogenous zone, 

 the presence of which has led to Calamodendron being classified by French authors with 



1 This description of the structure of the stems of Lepidodendron and Sigillaria has heen kindly 

 furnished by Prof. Williamson, to whom I am deeply indebted for it. 



