SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 609 
The stele, which is clearly marked off from the inner cortex, though no special 
boundary line is present, has a wide clear zone of phloem surrounding the central 
solid strand of xylem. Even in the smallest stems of R. major (fig. 15) the stele 
is relatively large as compared with R. Gwynne-Vaughani. 
The cells of the phloem are thin-walled, and as seen in transverse section (fig. 16) 
fit closely together. The intercellular spaces present in the inner cortex, when this 
is well preserved, cease on passing to the phloem. In longitudinal sections (figs: 17 
and 20) the cells of the phloem are seen to be thin-walled and elongated, resembling 
the same tissue in R. Gwynne-Vaughani. 
The xylem consists of far more numerous tracheides than in most steles of R- 
Gwynne-Vaughani. A distinction, such as was occasionally found in the latter 
plant, is here regularly present between the central and peripheral tracheides. The 
inner tracheides are much smaller than the outer ones, and have thinner walls 
(figs. 22, 16a). Although the xylem appears beautifully clear in many transverse 
sections, no satisfactory indication of the thickening of the tracheide walls is shown 
in any of the longitudinal or oblique sections as yet examined. Everything points 
to the walls having been thickened, but the thickening seems to have readily perished, 
and practically all the material of the plant had undergone decay to this extent at 
least. There is sometimes an appearance of porose thickening, but this often fills up 
the cavity, and from the examination of many specimens we are convinced that this 
appearance is due to an alteration of the original thickening, and that it would not 
be wise at present to attach any weight to it. # 
While, as stated above, no adventitious branches have been met with in R. major, 
the stems of this plant showed equal or dichotomous branching. An example is seen 
in Pl. III, fig. 21, at a, where two steles of the same size are present in the one cortex. 
A number of sporangia of R. major were figured in our previous paper (Part I, 
PI. IX, figs. 62, 64—69), and the description of the sporangium there given is based 
on them, and applies to this species. The figures on this plate show how much larger 
the sporangia of R. major were than those of R. Gwynne-Vaughani. There is some 
range in size of the sporangia of R. major, and still larger examples than any described 
in Part I have since been met with. Thus the sporangium on PI. Ill, fig. 24, was 
nearly 4 mm. in diameter. Its wall was well preserved, and showed the vertically 
extended, thickened epidermal cells (ep.), the zone of thin-walled tissue ( ml .), and 
the “ tapetal ” layer (tap.) around the large cavity filled with spores. t The sporangium 
in PI. Ill, fig. 23, may have been slightly larger, since the wall is broken and over- 
lapped at a ; only the epidermal layer enclosing the spores is preserved. It seems 
clear that the sporangia of R. major attained a length of more than 12 mm., a 
diameter of more than 4 mm., and a thickness of wall of ’3-'4 mm. 
* The decayed steles in Pt. I (PI. X, figs. 76-78) belong to II. major, and the comparison there made with 
Dawson’s figure of Psilophyton must be modified in so far as the outer dark zone in our specimens is derived from 
the outer xylem and not as suggested from the altered phloem. 
t Gf. Pt. I, PI. IX, fig. 67. 
