LEPIDODENDREAE. 



217 



separates into three layers. The innermost, which consisted probably of 

 spongy parenchyma, has entirely disappeared. The outer layer is a stout 

 parenchyma, and contains the leaf-cushions which are cut through at 

 different elevations. The third layer, which is the boundary on the outer 

 side of the cavity formed by the disappearance of the innermost layer, 

 consists of a few layers of right-angled cells forming regular radial rows. 

 Renault was also able to investigate an older bit of stem some five centi- 

 metres in thickness. Here in the middle of the solid central xylem-strand 



FIG. 22. Diclyoxylon-structure of the rind, as it occurs in some Lepidodendrae and Sigillariae, and in Lyginoden- 

 dron, etc. A transverse section of the outer rind of Lepidodendron rhodumnense, B. Ren., showing the anastomosing 

 plates of sclerenchyma which bound the meshes filled with parenchyma ; the longitudinal section is quite similar, only 

 the sclerenchymatous elements are seen in elongated form. B impression of the inner side of such a Dictyoxylon-rind 

 separated from the stem. The ribs are more prominent owing to the disappearance of the parenchyma and answer to 

 the furrows. The rhombic cushions fill the depressions caused by this disappearance of tissue in the meshes. A after 

 Renault ', B after Williamson *. 



is an irregular fissure-like gap. The character of the rind, which had lost its 

 original outer surface, is peculiar. The parenchymatous fundamental tissue 

 is traversed by plates of sclerenchyma, which run on the whole in a radial 

 direction, but are so curved and undulated that they cut one another at 

 regular distances at an acute angle, and thus the parenchyma is seen on the 

 transverse and tangential sections to be divided into fusiform segments 

 (Fig. 32, A). Since these segments are of nearly the same length, zones 

 are formed at the places where the plates cross one another, and as the 

 sclerenchymatous tissue predominates in the zones, the unaided eye sees on 

 the transverse section a system of circular bands. This peculiar arrange- 



1 Renault (1). 



Williamson (1), iv. 



