714 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
As regards the number of protoxylem-groups in each bundle, we usually find 
one such group in each strand in the medullary part of its course (see Plate 21, fig. 2), 
except at places where there has been a fusion of strands. 
As a bundle leaves the pith on its outward course, its protoxylem soon becomes 
doubled (see Plate 22, fig. 5 and Plate 23, fig. 9). Where the leaf-trace first divides 
into two bundles, each has a single protoxylem-group (see fig. 6). Further out 
towards the leaf these again double (see Plate 21, fig. 1, 1.1.3, 7.4.4, and 1.2.5), 
In the medullary part of its course the xylem of the bundle contains little or no 
parenchyma, except that adjoining the protoxylem-elements (see figs. 2, 3, and 8). 
Further out, the amount of parenchyma in the xylem becomes greater and it is not 
always limited to its original position. The only other change which need be 
mentioned is that in the outer part of the course of a bundle, scalariform or densely 
spiral trachese become more numerous in comparison with the pitted elements (see 
Plate 23, fig. 13, which is from a petiole). This change begins within the stem. 
As regards the primary phloém of the vascular bundles there is, as might be 
expected, little detail to be given. The large groups, lying in the pericycle imme- 
diately opposite the medullary strands of woods (see Plate 21, fig. 1 and Plate 22, tig. 7, 
ph.) ave in some cases surprisingly well preserved, and must have contained elements 
more resistant than those of the secondary phloém, though the thickness of the cell- 
walls is not sufficient to justify one in speaking of them as hard bast. Secretory sacs 
occur in the primary phloém, as well as in the other soft tissues of the plant. Longi- 
tudinal sections passing through the phloém of leaf-trace bundles, sometimes show the 
structure tolerably well. We find a combination of elongated elements, with oblong 
parenchymatous cells, and may conjecture that the former were the sieve-tubes (see 
fig. 13, ph., from a petiole). 
The conclusions at which we have arrived respecting the structure of the primary 
bundles of Lyginodendron may be stated as follows: 1. The vascular bundles in the 
stem are normally collateral. As they pass out into the leaves, however, their 
structure becomes concentric. 2. The xylem of the bundles in the stem developed 
like that of the foliar bundles in Cycadez. The protoxylem-elements lie in the 
interior of the primary wood, but nearer its outer surface. Thus, the greater part of 
the primary xylem was centripetally developed, while a smaller portion was centri- 
fugal. 3. The primary xylem consisted of spiral, scalariform, and pitted tracheides, 
together with a little parenchyma. 4. The primary phloém consisted of sieve- 
tubes and parenchyma, together with secretory sacs. 
4. The Secondary Tissues. 
With the rarest exceptions all known stems of Lyginodendron already possess 
secondary tissues. The young stem, represented in transverse section in Plate 18, 
