ADULT ANATOMY OF WELWITSCHIA MIRABILIS, 347 
four groups has been lost, the differentiation of the primary xylem into two groups 
is still present. In some cases a little secondary xylem and even phloem are developed 
in connection with the smaller and outer portion (cf. C, text-fig. 3, E). 
Root.—Still lower down the additional phloem group thus formed ends freely (e, text- 
fig. 3, C, p. 342), but the small mass of xylem extends much further downwards and 
appears and reappears throughout the root *, sometimes dwindling to a single tracheid +. 
Text-figure 5, A-F, shows the changes which take place in this additional mass of 
xylem (c) in a portion of the root-system of the seedling shown in Pl. 34. fig. 2. 
E ule c cC? c---9 a eee 
5 GS A ee gm ^ 
Nd j CZ 
cee 
> ^ * o 
A B C D vo 
Fig. 5, A-F.—Phloem white; secondary xylem diagonally shaded ; primary xylem black; fibres dotted. 
It is to be noticed that at the fusion of the four hypocotyledonary bundles in pairs 
(B-C, text-fig. 5), the small outer masses of primary xylem fuse first, and the larger 
portions with the secondary xylem and phloem fuse later. At D the inner and outer 
groups of the primary xylem are seen to come into contact, and at E to fuse, but lower 
down at F a small group of four or five primary xylem elements is again separated off 
from the main bundle. At C and D a little phloem has been laid down in connection 
With the small outer group of primary xylem. 
Section VIII.—ANATOMY OF THE ADULT HYPOCOTYLEDONARY STEM. 
General. —Concerning the anatomy of the adult hypocotyl, I have little to add to the 
account given by previous authors f. 
The four concentric groups are still visible in very old plants. Newly intercalated 
bundles entering from the leaves are arranged more or less definitely with relation to 
these four centres, often forming a series of concentric rings. It is not, however, easy 
to define the outer limits of the Íeaf-series, as it is at a late stage indistinguishable from 
the series of numerous bundles supplying the ridges and inflorescences. The two series 
together compose Bower's * peripheral series." 
Histology of the Xylem.—In the root and lower regions of the hypocotyl the 
tracheids of the protoxylem are reticulate (Pl. 35. fig. 14), while those of the secondary 
Xy lem have a single or double row of bordered pits (Pl. 35. figs. 13 & 14). The bordered 
pits are not contiguous. | 
In the upper portion of the hypocotyl, and in the ridges and leaves, many of the 
tracheids have both reticulate thickenin g and oblique or horizontal bordered pits § 
(P. 35, fig. 24, ©), 
* Cf. Gingko, Hill & De Fraine, III. 1909. t Ante, p. 337. 
t Hooker, 1863; Bertrand, 1874; De Bary, 1884; Bower, II. 
§ Compare Cupressineæ and some Leguminose. 
