FROM THE LOWER COAL-MEASURES. 57 
the secretory canals, which are particularly well shown in this tissue. Near the wood, 
especially, smaller elements about 20 « in diameter are met with, usually in groups. 
The longitudinal section (fig. 20) shows that the parenchymatous cells and secretory sacs 
are quite short, while the small elements are greatly elongated, and form, for the most 
part, well-defined strands. The section figured is more nearly tangential than radial, 
and consequently these strands (s./.), which sometimes anastomose, appear to extend 
further out from the wood than is really the case. Presumably they represent the sieve- 
tubes, some of which are of greater diameter than those figured. "The zone in which 
these strands occur clearly constitutes the true phloem, while the more external part of 
the thin-walled tissue may be regarded as pericycle. "There is no defined limit between 
the two. 
It has already been mentioned that secondary phloem was formed from the cambium 
(Pl.8. fig. 10). "This is shown in longitudinal section in Pl. 10. fig. 21, from one of 
Prof. Oliver’s slides. The plane of section is radial, and the elongated elements of the 
phloem are well seen, erossed by medullary rays, which are continuous with the paren- 
chyma of the primary wood. Some of the long elements appear to have transverse 
walls and may represent phloem-parenchyma, while those with tapering ends are 
probably the sieve-tubes. No satisfactory examples of sieve-plates have been found, and 
perhaps the preservation is scarcely good enough to warrant our expecting them. An 
element (/7.) at the edge of the wood, with very delicate areolz on its wall, may be a 
developing tracheide, such as was found in Heterangiwm tilieoides *. 
3. Structure of the Leaf-trace Bundles.—So far we have described the wood and phloem 
as occurring in the stele and meristeles, between which there is no anatomical difference. 
The leaf-trace strands require a separate description. Two types may be distinguished— 
the radially symmetrical, which is generally characteristic of the larger, more internal, 
strands, and the unilateral, which is typical of the smaller strands, and especially of 
those which have passed out towards the periphery of the leaf-base and petioie. The 
distinction applies chiefly to the xylem; in all cases the phloem forms a complete ring, 
though there may be slight differences in its distribution. An example of the radially 
symmetrical type of bundle is shown in Pl. 9. fig. 14, and of the unilateral type 
in fig. 15. The former resembles the meristeles in the fact that there are several 
protoxylem-groups distributed around the outside of the wood, while in the unilateral 
form of bundle the protoxylem is limited to one side of the wood, where it may 
constitute one or two groups. Transitional forms, however, may be met with. In the 
peripheral rank of bundles the protoxylem is regularly directed outwards, while in those 
of the next inner series the orientation is often reversed (fig. 16). | 
An important characteristic of all the bundles, to whatever type they may belong, is 
the presence of xylem-parenchyma among the tracheides (figs. 14 & 15), a point ot 
distinction from the corresponding strands in the genus Medullosa. In this respect, as 
well as in their concentric structure, the bundles of Suéclifia have more in common with 
the stele than is the case in the allied genus. | 
* Williamson and Scott, 1895, p. 761, pl. 29. fig, 37. 
