446 MR WM. T. GORDON 



While there is no distinct abscission layer in the leaf-base, it breaks away at this 

 parenchyma, which is therefore not present in stems denuded of their leaf-bases. 



Histology. 



In describing the various tissues from the centre outwards, the pith naturally 

 comes first. The stems examined are all of approximately the same diameter, and 

 yet the pith varies greatly in amount. One stem, with primary xylem of 3 '3 mm. 

 diameter, has only a few pith cells in patches (fig. 2, p) ; another, with xylem 4*3 mm. 

 diameter, has a pith 2'5 mm. diameter; while yet another, with xylem 3 '4 mm. 

 diameter, has a pith 1*6 mm. diameter. But even in the same specimen the pith varies 

 greatly ; the example referred to above as showing only a few pith cells in one part 

 (fig. 2), had, in another part (about 10 cm. further down), a pith of 2 mm. diameter, and 

 5 cm. further up from where the section (fig. 2) was cut, the pith was 1*4 mm. diameter. 

 In certain sections from other individuals there does not seem to be any pith at all. The 

 pith, then, is very variable both in extent and in occurrence ; its elements are slightly 

 elongated vertically, and are arranged in vertical rows, the ends of each element being 

 flattened horizontally. Secondary oblique walls also occur in places. The walls of these 

 elements are thin and soft, and do not show any secondary thickening. In fig. 2a, p — the 

 cell with no secondary thickening, but being bounded by what is the central lamella in 

 the tracheides — is a pith cell. 



Scattered through this soft parenchyma are rows of short tracheides ; they occur in 

 greater abundance towards the periphery, but are of no greater diameter than the other 

 parenchyma cells of that tissue. These tracheides seem to form an irregular layer 

 between the primary wood and the soft pith. They are thickened in an irregularly 

 scalariform manner, but in some the thickenings have a tendency to assume a reticulate 

 arrangement, whose structure is very similar to those figured by Brongniart in the 

 description of the Sigillaria which he in error referred to S. elegans. 



Occasionally also long tracheides occur isolated in the pith, and are easily distin- 

 guished from the others by their length and the scalariform thickenings on their walls. 

 The isolated and the short tracheides are somewhat similar to those found in Lepido- 

 dendron vasculare (Binney, sp.) and in Lepidodendron HicJcii (Watson). 



The zone of primary wood surrounding the pith is seldom broken even when 

 branches are given off. In one branch, however, a row of short tracheides passes out 

 with the xylem, thus indicating a break in the wood ring, while in another case of 

 branching, the soft pith has actually grown out of the gap thus formed in the xylem 

 zone (fig. 6, p). Both these examples are in cases of unequal dichotomy. The pith 

 cells average in dimensions '18x "09 x "09 mm. 



From the measurements given of the diameter of the xylem cylinder and that of 

 the pith, it will be seen that the primary wood is in places comparatively small in 

 amount. In a specimen 35 mm. diameter the xylem and pith are together 4*3 mm., 



