346 SPHENOPHYLLEAE. 



triarch xylem-strand, the only one which appears at first, is afterwards 

 inclosed by a secondary growth of quite peculiar structure, after the 

 manner observed in growth in thickness in roots. These stems appear, 

 according to Williamson, to be comparatively rare in the calcareous nodules 

 of Lancashire, but they are much more abundant in those of Langen- 

 dreer, from which I have a considerable number of specimens before me. 

 In Autun also, and especially in the pebbles of Grand' Croix, they have 

 been repeatedly found by Renault and Grand 1 Eury 1 . The material from 

 Grand' Croix has enabled Renault 2 to prove that the stalks in question 

 have really belonged to Sphenophyllum. A plane of fracture in a favourable 

 direction showed him an undoubted leaf of that genus, with the characteristic 

 transverse section of the stalk in connection with it. He has repeatedly 

 figured 3 transverse sections of the stalks, which are surrounded by the 

 leaves still in the original position (Fig. 48, 2). 



If we first examine the young stem as yet without growth in thickness, 

 we find it to consist simply of a thick rind inclosing the three-winged bundle. 

 If the transverse section has exactly hit the node, we also get the leaf-trace- 

 strands passing through the rind and running outwards almost at a right 

 angle to the axis. In the leaf-trace may be distinguished an inner bast- 

 layer, which in such young stems is almost always destroyed and replaced 

 by an annular vacant space, and outside the bast Renault 4 maintains that 

 he has sometimes found the protecting sheath consisting of thick-walled 

 elements closely united laterally with one another. The sheath is sur- 

 rounded by a thick mantle of primary cortical parenchyma, the hypodermal 

 cells of which are said to be fibre-like. The form of the transverse section 

 of this cortex is peculiar (Fig. 48, 2). Generally three-angled approaching 

 to circular in shape, it shows on the outer side at each angle a longitudinal 

 furrow which divides the angle into two ribs lying in pairs side by side. 

 Each pair of ribs lies in front of one of the surfaces of the xylem-strand, 

 the flat sides between the ribs are in front of its angles, and consequently 

 the outer surface does not repeat the form of the central strand. The 

 section is less frequently simply cylindrical 5 , without any angles and furrows. 

 As the ordinary impressions all belong to the outer surface, we see again 

 how little comparison there can be between any trace-points visible on it 

 and the prominences on the substance which replaces the pith in Calamitae, 

 though such a comparison has been attempted by Stur 6 . The central 

 xylem-strand consists chiefly of tracheides with bordered pits, which are 

 broadest in the middle and gradually diminish in size towards the angles, 

 and here too are replaced by scalariform vessels. At the extreme edge of 



1 Grand' Eury (1), p. 50. 2 Renault (20), p. 288, t. 9, f. 12. 3 Renault (16), t. 4, 



ff. 5, 6 ; (20), t. i, f. 2 ; (2), vol. ii, t. 15, f. 2. * Renault (2), vol. iv, Introd. t. A, f. 2. 



5 Renault (2), vol. iv, Introd. t. A, f. i. 6 Stur (5). 



