220 LEPIDODENDREAE. 



strands when we only know the mature state of the bundles, and how 

 dangerous it therefore appears to draw conclusions in this direction from 

 the relative size of the elements, as has often been done by Renault, though 

 at other times he lays great stress and rightly on this point. On the outer 

 limit of the bast-portion is a sheath of stout parenchyma, which, consisting 

 only of a few cell-layers, is often preserved in cases where both the bast 

 and the inner layer of the cortical tissue lying outside the bast are entirely 

 destroyed. 



The rind is in general very thick, and separates into three cylinders 

 one within the other, which may conveniently be distinguished in the 

 following remarks as the outer, middle, and inner cylinders. The inner 

 cylinder, which is evidently composed of loose spongy tissue, is almost 

 always entirely destroyed ; a broad circular space filled with crystalline 

 carbonates and detritus which has floated into it takes its place (Fig. 23). 

 Remains of this tissue have been preserved in a few specimens, for example 

 in a transverse section figured by Williamson 1 . The outer cylinder readily 

 comes away from the middle one with a sharp circular line of separation 

 and is therefore often wanting, and may occur detached and variously 

 curved and rolled up. Its outer margin formed by the epidermis is rendered 

 uneven by crowded protuberances, the transverse sections of the leaf- 

 cushions, and these sections necessarily vary much in size and form since 

 they cut the cushions at different elevations. This outer cylinder is com- 

 posed of stout-celled parenchyma, the cells of which beneath the epidermis 

 add continually to the thickness of their walls, and assume the character of 

 sclerenchyma. The epidermis itself is often removed, and then the surface 

 bears some resemblance to a Bergeria-cast. Lastly, if the whole of the 

 outer cylinder is wanting, we then have the stems in the well-known state in 

 which they so frequently occur, covered with small flat protuberances, in the 

 usual phrase, stripped of their rind. The middle cylinder, at least as thick 

 as the outer and inner cylinder put together and usually much thicker, 

 is traversed on the transverse section by radiate stripe-like lacunae (Fig. 23). 

 In each of the lacunae runs, longitudinally or a little obliquely, the upper 

 horizontal portion of the emerging foliar bundle, of which the xylem-portion 

 only is usually preserved. The entire tissue-mass of this cylinder is paren- 

 chymatous, but two essentially different layers may always be distinguished 

 in it, the relative thickness of which changes with time ; in young stems 

 with a small transverse section the inner layer is the thicker and the outer 

 is often only a very narrow zone, while in older pieces of stem it is the outer 

 zone which attains to very considerable thickness. In this zone the some- 

 what thin-walled parenchyma-cells are rectangular on the transverse 

 section a::d arranged in radial rows, on the other they have a much broader 



1 Williamson (1), xr, t. 52, and Binney (3), . 35, f. 5. 



