STRUCTURE AND AFFINITIES OF A LEPIDODENDROID STEM. 917* 



The phelloderm tissue is continued to the exterior of the fissured surface shown in 

 the lower part of the photograph in PL I. fig. 1. In the upper part of the stem this 

 secondary tissue was succeeded by the phellogen layer, and this again by the parenchyma 

 of the leaf-cushions ; probably this outermost tissue had been thrown off from the older 

 portion of the stem during the life of the tree. # 



Some secretory strands in a state of development are met with also in the neigh- 

 bourhood of g in fig. 2, and beyond this region the phelloderm cells are less clearly 

 preserved, and are characterised by walls of considerable thickness, the lumen being in 

 some cases almost obliterated, and what remains of it is filled with a black substance. 

 It is often difficult in dealing with petrified tissues to distinguish thick-walled tissue 

 from tissue of which the cell-walls have been thickened by mineral deposition or by 

 swelling of the membrane during mineralisation. In the present instance the cell-walls 

 were probably fairly thick, but the thickened mass shown in the cell of PI. IV. fig. 27 

 would seem to be the result of swelling of the wall, and this explanation is rendered 

 probable by the fact that the cells of neighbouring rows are more or less flattened by 

 pressure. The outermost portion of the phelloderm is characterised by many of the 

 cells being tangentially elongated, and the occurrence of newly-formed radial walls in 

 some of these elements points to the stretching being the result of growth, and not of 

 mechanical or secondary origin. In a tangential section of the outermost phelloderm 

 there are seen narrow bands of an opaque black substance forming an irregular 

 anastomosing system ; these dark lines extending through the imperfectly preserved 

 tissue probably represent resinous or some other mineralised secretion. 



There is another point of some interest brought out by the examination of the 

 phelloderm connected with the leaf-traces. Reference has been made to the occurrence 

 of two partially disorganised leaf- traces in the inner part of the outer cortex (region a, 

 fig. 2, PI. I.). In the peripheral part of the outer bark no indication of any leaf-trace 

 xylem elements has been found, but in a radial section of the phelloderm one leaf-trace 

 was met with in the region of the phelloderm indicated by d in fig. 2, PI. I., passing 

 outwards in an approximately horizontal direction ; the xylem portion is crushed and 

 imperfect, but sufficiently clear to enable one to easily recognise the tracheids (PI. II. 

 fig. 14 tr). The xylem is accompanied by a large patch of parenchyma composed of 

 cells of varying shape, some of the elements having a hyphal or filamentous form. This 

 mass of parenchyma is in marked contrast to the regular radial rows of phelloderm 

 through which it is passing ; there can be no doubt that we have here a portion of the 

 middle cortex which accompanied the leaf- trace as the so-called parichnos (jpr) of 

 Bertrand and other writers. The parichnos tissue affords an indication of the character 

 of the lost middle cortex of which it formed a part. In tangential sections of the more 

 external portion of the phelloderm, the mass of fibrous cells of which it consists is 



* In a longitudinal section of Lepidophloios in the Williamson Collection (No. 1963) there appears to be an 

 indication of an absciss layer of tissue in course of formation at the base of a leaf, suggesting that the leaf-fall was 

 accompanied by the usual phenomenon familiar in the case of recent trees. 



