I 
I 
THE ANATOMY OF TODEA. 243 
particularly striking and in marked contrast to the more usual endarch xylem of Osmunda 
regalis and Todea barbara.  'The conspicuous sieve-tubes are represented by dots, s; 
and the line en marks the position of the endodermis. The abundance of sieve-tubes 
opposite the medullary rays, and external to the concavities on the outer face of the 
xylem groups, is a constant feature in all the species examined. This diagram also 
illustrates stages in the separation of leaf-traces and roots (/¢ and 7) from the xylem of the 
central cylinder. A portion of this section is shown in fig. 21; a lignified protoxylem 
island is seen near the outer edge of the large unlignified tracheids; beyond the zone of 
parenchyma, xp, a band of comparatively large and fully developed sieve-tubes stands 
out clearly; the sieve-tubes contain numerous highly refractive granules, and agree in 
every respect with those of the mature stem. The tangentially elongated cells, £, still 
retain their large nuclei. A similar structure is shown in fig. 15, which represents part 
of a transverse section from the apical region of Todea hymenophylloides; the proto- 
xylem elements are almost exarch in position; the parenchymatous band, cp, is 
followed by a layer of sieve-tubes, s, and beyond these are a few layers of long rectangular 
cells, pr. The endodermis, ez, is rather more distinct in this species than in 7. barbara 
or T. superba, and is characterized by the suberization of the cell-walls occasionally 
extending beyond the limits of a single layer. 
The more important points which we believe to be demonstrated by the sections from 
the apical portion of Todea stems are:—(i) The tangentially elongated elements do not 
represent protophloem, but are of later origin than the large sieve-tubes of the mature 
stem; (ii) the endodermis is not of common origin with the tissues lying internal to it. 
In the tissue between the endodermis and the sieve-tubes ( pr, figs. 14, 15, &e.) there is 
no satisfactory evidence of any distinct line of division separating an outer (pericycle) 
region from an internal undifferentiated phloem portion. At a later stage of growth the 
more internal elements of the zone of cells between the endodermis and the sieve-tubes 
appear as thick-walled elements having their larger axis in a horizontal plane and with 
their tangential walls separated by reticulate thickening bands into numerous sieve-areas 
(Pl. 28. fig. 16, ¢). "Traces of similar bands are occasionally faintly indicated on the 
walls of the more external cells, and even in the mature stem any division of this zone 
into pericycle and external phloem would be purely arbitrary and artificial. 
C. Mature Stem. 
TODEA BARBARA.—A transverse section of a mature stem of Todea barbara presents 
an appearance very similar to that of Osmunda regalis. The stele, which in the plant 
investigated had a diameter of 5 mm., stands out as a clearly marked pentagonal area 
(Pl. 29. fig. 26; text-figs. 1 & 2) limited by an endodermal layer; a band of thin-walled 
parenchyma separates the central cylinder from the peripheral region of the stem, which 
consists of a mass of dark brown sclerenchyma enclosing numerous islands formed of 
outgoing leaves and curving roots cut across in various directions. In the more internal 
region of the cortex the mechanical tissue occurs as crescent-shaped patches on the 
concave face of the leaf-traces (text-fig. 1, si’), but these soon become merged into a 
common dark brown mass formed of the confluent sclerenchymatous envelopes of 
