of Coal-Measure Plants. 163 



contraction of dead tissue, and not an original structure. It is no 

 doubt the case that the large gaps in the tissue of the stem before 

 us have been formed by secondary changes. While the large 

 primary tracheids which constitute so considerable a portion of the 

 stele are shorter than broad, a few of them are elongated, and one 

 prominent feature is their striking irregularity in size and shape ; 

 they occur either singly or more frequently in groups (PI. vii. fig. 9) 

 and in the latter case the individual elements are closely connected 

 by straight or curved pitted walls inclined at various angles. In 

 size these tracheids exceed by about 8 diameters the tracheids of 

 the secondary xylem, some of the larger examples measuring '4 mm. 

 in breadth ; with these unusually large tracheids are occasionally 

 associated others of much smaller size. Text-fig. 4 m illustrates 

 the great variation in size among the short tracheids of the primary 

 stele, or rather of that part of it which may be termed the meta- 

 xylem. 



3. Leaf -traces. 



Attention has already been drawn to the occurrence of a well- 

 defined group of tracheids, It, in the section represented in PI. v. 

 fig. 2, as a darker patch at the periphery of the primary xylem, 

 and in fig. 1, PI. V. as a strand, It, which gradually becomes in- 

 distinguishable from the main mass of xylem in the lower part of 

 the section. In the large section reproduced in fig. 7, PL VI., there 

 is a distinct elliptical group of elements at It 1, which is shown 

 on a larger scale in fig. 5, PL v. This group of slightly crushed 

 tissue consists of tracheids interspersed with parenchyma; in 

 transverse section the two constituents can hardly be distinguished, 

 but in longitudinal view the tracheids appear as long tubular 

 elements separated here and there by rows of short parenchy- 

 matous cells. The tracheids are considerably narrower than those 

 which make up the bulk of the primary xylem, the more central 

 region of which we may speak of as metaxylem, but somewhat 

 wider than those of the secondary xylem. The whole group of 

 xylem of the leaf-trace (It fig. 5) is surrounded by layers of flattened 

 parenchymatous cells, some of which contain a black secreted 

 substance. On the outer edge of the leaf-trace, It, there are several 

 very narrow protoxylem elements situated on that side next the 

 secondary wood of the stem (px fig. 5, PL v.). In a leaf-trace seen 

 in such a position as that of fig. 5, the protoxylem groups are close 

 together ; it is difficult, therefore, to determine the exact number, 

 but there appear to be at least six peripheral strands of narrow 

 elements. In a leaf-trace seen in a lower part of its course, as in 

 figs. 3 and 6, PL v., where the tissues are less compact and more 

 extended laterally, the protoxylem groups, px, are more readily 



