LEPIDODENDREA E. 219 



Carr. Williamson) is very exactly known ; the fullest account of it will 

 be found in Williamson 1 . Binney's 2 fine figures of this form should also 

 be consulted ; the text which they accompany is not it is true of equal 

 value, and should be read with great circumspection. If we look first at 

 the primary structure, we find in the centre of the stem a woody cylinder 

 slightly developed in proportion to the thickness of the rind, circular on 

 the transverse section, and composed of scalariform and reticulately-thick- 

 ened tracheides, between which parenchymatous cells in tolerably large 

 quantity and increasing in number towards the centre are interspersed 

 singly or in groups. The longitudinal section shows that the tracheides are 

 of two kinds, that sometimes they are elongated and tubular, and again short 

 and isodiametric, and with their cross walls usually showing particularly 

 beautiful reticulate markings. The latter kind is found chiefly in the middle 

 of the bundle, the former is present everywhere and is the sole constituent 

 of the periphery. The narrowest elements of the bundle are found on its 

 outermost margin. The rather broad ring of bast which surrounds the 

 wood-cylinder has generally disappeared up to its innermost layer ; it is but 

 rarely that its delicate tissue is perfectly preserved. It is traversed by the 

 xylem-strands of the leaf-traces, the transverse sections of which are found in 

 great numbers in the immediate neighbourhood of the edges of the xylem 

 of the central strand, and are surrounded by the remains of bast-tissue. 

 In the well-preserved preparations before me I see only one homogeneous 

 bundle of tracheal elements on the transverse section of the leaf-trace. 

 The protoxylem-groups I am unable to distinguish with the needful cer- 

 tainty; there ought to be two of them present as in Ferns, according to 

 Renault : \ who however relies for this point on Corda's 4 extremely doubtful 

 figures. How much uncertainty still remains with respect to these points is 

 further shown by van Tieghem's 6 account, who ascribes a collateral structure 

 to the leaf-traces of Lepidodendron and finds their initial strand on the 

 outer borders of the xylem. He then makes the homogeneous central 

 cylinder of the stem be formed by the union of several such bundles with 

 their xylem-portions. The state of preservation of the associated bast- 

 portion being so unfavourable, I do not venture to decide whether we have 

 a concentric bundle before us o. a collateral, although for various reasons, 

 to which I shall presently have to return, I incline to the view that the plan 

 of structure is of the latter kind. Moreover this collateral structure occurs 

 at the present day, according to Russow and Janczewski, in Isoetes. And 

 here we may draw attention to a point which will have to be noticed often 

 again, namely how little we can rely on distinguishing the protoxylem- 



1 Williamson (1), II, in, XI. 3 Binney (1), III, 2 and 3. Renault (2), vol. iii, Introd. 



p. n ; t. 10. 4 Corda (1), (Lomatophloios crassicaulis, t. 3, f. 8). 5 van Tieghem (2), 



p. 1305- 



