242 MR. A. C. SEWARD AND MISS S. 0. FORD ON 
B. Apex of Main Stem (Todea barbara, T. hymenophylloides, and T. superba). 
A single three-sided cell terminating the apex of the stem has been observed in Todea 
superba (Pl. 98. fig. 23) and T. hymenophylloides. Ina section taken a short distance 
below the apex (Pl. 28. fig. 18) numerous unlignified leaf-traces ( lt) are shown clustered 
round the centre of the stem c, and in the peripheral region broadly triangular leaf- 
bases, with prominent lateral angles formed of the parenchymatous tissue of the basal 
wings *, occur in various stages of separation from the main axis. These leaf-bases 
present an appearance not unlike that of the fleshy leaf-cushions of a Lepidodendron 
twig. At a slightly lower level in the stem a fairly well-defined meristem-cylinder is 
recognized, succeeded internally by strands of large unlignified and nucleated xylem 
tracheids. The outer border of the deeply stained meristematic zone is formed by a 
somewhat irregular layer of cells of unequal size, the walls of which become suberized at 
an early stage (Pi. 28. figs. 14, 15, 19, 21, e»). This layer is the endodermis; we find 
no evidence in support of the view that it has a common origin with the broad band of 
tissue lying internal to it. The outermost xylem tracheids are succeeded by a zone of 
small cells (zp) which persist in the mature stem as a band of parenchyma. Beyond 
this zone a few large cells oceur, which have lost their contents and are easily recognized, 
from their position and structure, as the first representatives of the large sieve-tubes of 
the mature stem (Pl. 28. figs. 14, 15, 19, 21, s; cf. s in PI. 28. fig. 16; PI. 29. figs. 32, 
33; Pl. 30. figs. 42, 44, 46, 47). 
The tissue between the sieve-tubes and the endodermis—the pericyele $— consists of 
severallayers; the cells next the sieve-tubes are longer in a tangential direction and 
contain large nuclei (figs. 14, 15, Ze, 7), and these are undoubtedly the tangentially 
stretched elements of Zenetti (* quergestreckte Zellen") which Faull § describes as 
protophloem. These rectangular cells (/, Pl. 28. figs. 14-17, 19,21; Pl. 29. figs. 26, 33, 
39; Pl. 30. fig. 42) pass gradually into rather shorter elements, while those abutting on 
the endodermis are characterized by still smaller dimensions (Pl. 28. figs. 14, 19, 21). 
A comparison of numerous sections of the stems of Zodea barbara and the two filmy 
species (cf. Pl. 28. figs. 14, 15, 16, 17, 19, 21; Pl. 29. figs. 32, 33; Pl. 30. figs. 42, &c.) 
leads us to the conclusion that the large clear sieve-tubes in the stem are the oldest 
phloem elements—the protophloem ; and the tangentially elongated elements, which at 
a later stage in the growth of the stem assume the characters of sieve-tubes (Pl. 28. 
fig. 16, ¢), are of later origin. This order of development suggests a comparison with the 
centrifugal differentiation of the phloem described in Angiopteris |. Fig. 29, Pl. 29, 
represents a diagrammatic drawing of the transverse section of the young stele of Todea 
superba, part of which is shown on a larger scale in fig. 19. The dark dots (pa) mark 
the position of the protoxylem elements, which in this section always occupy a mesarch 
position close to the outer edge of the tracheal strands. The mesarch character is 
* For an account of the “ wings,” see Bower (1885), pl. 37, &e. 
f The term “peticycle ” may be conveniently used, as suggested by Mr. Boodle, without the implication of a definite 
morphological significance ; Boodle (1900), p. 459. 
1 Zenetti (1895). $ Faull (1901). 
| Shove (1900). Miss Shove’s results as to the internal positi 
ee osition of the prot d 
and extended by Mr. Brebner (1901). j portent Daye retently been contri 
