THE ANATOMY OF TODEA. 9215 
undulatory bands stretching a considerable distance round the pith, as seen in transverse 
séction. The tracheal strands consist almost entirely of scalariform tracheids, with here 
and there an included parenchymatous cell; in the strands with mesarch protoxylem 
groups a few parenchymatous elements often accompany the spiral tracheids. An 
occasional tracheid is rarely met with detached from the xylem strands and lying in a 
medullary ray. A few short reticulately thickened tracheids are sometimes found on the 
inner edge of the xylem, as described in some stems of Lepidodendron. The distribution 
of the narrow protoxylem elements is shown in PI. 29. figs. 26, 27, pz, and in text-figs. 
1 & 2 by the dark dots; these usually occupy an endarch position (e. g. Pl. 30. 
fig. 42, px), but some of the xylem groups have a mesarch structure (fig. 26). The 
smaller strands in the xylem ring are occasionally without any protoxylem elements 
(fig. 36 & text-fig. 2), and in the more continuous sinuous bands the protoxylem tracheids 
may be separated by long intervals. The pith consists of parenchymatous tissue in the 
centre of which an irregular group of dark brown sclerous elements occasionally occurs : 
this is more particularly the case in the older parts of the stem. These medullary nests 
of sclerous cells (text-fig. 1, s/") in Todea barbara are not accompanied by an encircling 
endodermal layer, nor have we detected any trace of an internal endodermis in this 
species. The xylem ring is surrounded externally by a band of parenchyma (figs. 16, 26, 
83, vp) which varies in breadth in different regions, and is clearly distinguished by the 
deeply stained cell-contents from the more external phloem-zone. The phloem consists 
internally of large sieve-tubes forming a band of varying thickness, which projects as 
triangular groups into the outer end of the medullary rays (text-figs. 1 & 2; Pl. 28. 
fig. 16, s; Pl. 29. figs. 26, 33, s; Pl. 30. fig. 42, s). The sieve-tubes of Todea agree 
with those of Osmunda * in the possession of numerous sieve-areas on their radial walls. 
Here and there the sieve-tube band is interrupted (fig. 26, and text-fiss. 1 & 2), but the 
phloem as a whole constitutes a continuous zone, the continuity being maintained by the 
more horizontally placed sieve-tube elements (/, fig. 26, &c.) which have been referred 
to in the description of the apical region. The structure of the extra-xylem tissue is 
shown in figs. 16, 33, & 42. The large vertical sieve-tubes, s, are succeeded by the 
more horizontal or tangentially elongated elements with comparatively thick walls 
divided by reticulate thickenings into numerous sieve-areas (/, figs. 26, 33, 40). This 
band of tangentially disposed sieve-tubes is separated from the endodermis by a zone 
composed of from 4-5 layers of cells (pr, figs. 33 & 42), which pass by gradual stages 
into the horizontal sieve-tubes, ¢, and are clearly derived from the same meristematic 
band. The tangentially elongated sieve-tubes pursue a more vertical course opposite the 
xylem which is about to bend outwards as a leaf-trace T. 
As shown in figs. 16 & 33, the cells of this zone, especially those of the layer next the 
horizontal sieve-tubes, £, show traces of thickening bands on their walls; these appear 
to represent in a feebly developed condition the more obvious reticulations of the 
tangentially elongated elements, £. The endodermis, en, usually consists of a single 
row of cells of unequal size, and is not always very pem defined (fig. 16, Pl. 28; 
fig. 33, Pl. 29; fig. 42, Pl. 30). 
* Cf. Zenetti (1895); Janczewski (1880). T Cf. Faull (1901), p. 404. 
SECOND SERIES.—BOTANY, VOL. VL 2N 
