56 DR. D. H. SCOTT ON A NEW TYPE OF MEDULLOSEJE 
word “mesarch” to such cases. It is very common for the prominent protoxylem- 
groups to run in pairs as shown in this figure. 
Longitudinal sections, where they are well enough preserved, show the small spiral 
elements at the periphery, succeeded towards the interior by denser spiral and scalari- 
form tracheides, and finally the pitted elements characteristic of the bulk of the wood. 
It has not been thought necessary to give a figure of this from the stele, as the structure 
is precisely the same as that shown in Pl. 10. fig. 18 from a large leaf-trace bundle. 
Throughout the plant the structure of the vascular strands of all orders is exarch, 
apart from the trivial irregularities already noted, which need not be regarded as real 
exceptions to the rule. 
We will now pass on to the secondary tissues, which are only represented to a very 
small extent in our specimen. An example is shown in fig. 10 (Pl. 8) from a photograph 
taken from one of the lower sections (No. IV. from the bottom). The thin-walled 
cambial zone, passing over towards the outside into secondary phloem, is well preserved. 
A few secondary tracheides have been derived from it on the inner side, but not more 
than three or four in any radial row. In other parts cambium alone is found, and no 
secondary wood had begun to form. The secondary tracheides are small compared with ` 
those of the primary wood, and do not exceed 120 » in their maximum diameter, which | 
is usually tangential. This character is shared by the secondary wood of Heterangiwm * 
and Megaloxylon t. 
In the lower part of the specimen the formation of secondary wood can be traced all 
round the stele and extends to the meristeles, but, as fig. 7 shows, its amount is 
altogether insignificant as compared with that of the primary xylem. As we follow the 
structure in the upward direction, through the successive transverse sections, we find 
the secondary xylem still less developed, and in some places, though the preservation is 
good enough to show the soft tissues well, even the cambium does not appear to have 
formed. Our specimen thus represents a plant at the stage when secondary thickening 
was just beginning, and the structure, at a more advanced age, may have assumed a very 
different character. On the other hand, it is possible that, as seems to have been the 
case in Helerangium Grievii, the secondary tissues may never have attained any very 
great development. ; 
2. Structure of the Phloem.—We will begin with the phloem of the stele and meri- 
steles, which differs somewhat from that of the smaller vascular bundles. 
The soft tissues surrounding the wood are best preserved in the lower sections of the 
series. In this region a broad band of delicate tissue is often preserved, just outside the 
wood, contrasting sharply with the coarser parenchyma of the cortex (see fig. 7, from 
Section III.). A portion of this tissue is shown, in transverse section, in Pl. 10. fig. 19. 
It is made up, for the most part, of a thin-walled parenchymatous tissue, the cells 
measuring about 80x 50 u, the larger diameter being usually tangential to the adjacent 
wood. Among the parenchyma are numerous small sacs with dark contents, as well as 
* Williamson and Scott, 1895, p. 751, pl. 26. fig. 24, pl. 28, fig. 32. 
t Seward, 1899, p. 160, pl. vii. fig. 8. 
