122 



BRITISH EOCENE FLORA. 



exogenous, composed of a woody interior and of a bark, these being most distinctly 

 separated from each other by the reproductive cambium layer, which produces annually a 

 new layer of wood on the one side, and of bark on the other. As in the Cycads, the true 

 vessels are confined to the periphery of the greatly reduced central pith. The fibrous 

 wood is arranged in concentric zones without any mixture of intercalated bark, and is 

 uniformly composed of punctated fibres, radially disposed and traversed by medullary 

 rays. The wood is destitute of true vessels and only contains an insignificant proportion 

 of woody parenchyma, except in a few Conifers of relatively rapid growth. The great 

 advance is the permanence of the cambium layer and the consequent regular increase of 



Fig. 40. — Leaves and male organs of Ginkgo. A. Lateral branch and male catkin, on the point of 

 emitting the pollen, surrounded by young undeveloped leaves. At the base are two bilobed bracts. 

 B and c. Detached flowers. J>. Pollen grain greatly magnified, showing a three-celled interior, sur- 

 rounded by extine. e. Leaf, two thirds natural size, of the wedge-shaped and deeply-cleft variety. 

 — After Saporta and Marion. 



the stem, a growth exclusively confined to the Coniferse and to Dicotyledons, and the 

 complete separation of the bark from the wood. The bark is composed of three elements, 

 the fibrous inner bark or liber, the parenchymatous second layer, and the outer or 

 suberous bark covered by the epidermis. The leaves are always simple, generally narrow, 

 and most often seated on more or less decurrent cushions, provided with longitudinal 

 veins, solitary or several parallel, simple or forked, but never anastomosing or even joined 



