734 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
shown in approximately transverse section in the inner cortex of a stem. Its vascular 
cylinder, which had undergone considerable secondary thickening, is surrounded, 
except on the inner side, by its own cortical zone, which is sharply marked off from 
that of the stem itself. At the same time, owing to the curvature of the organ, the 
connection of its xylem with that of the parent axis is clearly shown. Some of the 
trachex, which are here seen in longitudinal section, are continuous with those of 
the secondary wood, while others extend inwards as far as the primary xylem of the 
stem; the latter, however, is somewhat crushed. 
Photograph 9 is from a tangential section of another stem, and here the appendage 
is shown passing though the outer or “ Dictyoxylon” cortex. On the right a leaf- 
trace bundle on its way to a leaf is shown. Here again the transverse section of the 
appendage is evidently that of a root. The solid vascular cylinder, with its well 
developed secondary wood, is enclosed within a cortex of its own. It will be noticed 
that, in this instance, the appendage is passing out horizontally, which is the more 
usual case. The appendage is slightly compressed laterally, owing to the limitation 
of its growth by the sclerenchymatous strands of cortex through which it had to pass. 
The same phenomenon is observed in the adventitious roots of the Marattiaceze—in 
which, however, the growth is, of course, primary only. 
In photograph 10 another of these organs is shown from the same preparation. It 
is just becoming free from the cortex of the stem, a portion of which is shown to the 
right ; the appendage still retains its compressed form, due to cortical pressure. The 
interesting point is, that it is itself beginning to branch. The two branches shown in 
the section have all the appearance of rootlets. The central cylinder has a primary 
mass of xylem without pith, and possesses abundant secondary wood ; the cortex is 
well preserved, its large-celled outer layers are especially evident. In Plate 19, 
photograph 8a, another of these appendages is shown in longitudinal section (the 
sjem being cut transversely), passing out from the wood, through the whole thickness 
of the cortex. We do not think that any botanist will hesitate, from the evidence 
already brought forward, in regarding these appendicular organs as adventitious roots, 
the name by which we shall henceforth call them. 
Another specimen affords in some points still more conclusive proof of the nature of 
these organs. In this case we have three successive sections of the same specimen. Two 
of these sections are figured (Plate 25, fig. 18 and fig.184). They pass obliquely through 
a stem of Lyginodendron, and at the same time show several of the roots. In fig. 18 
two of the roots shown (rt. 1 and rt. 2) are quite free, and remain so throughout the 
series. Two other roots (rf. 8 and 7t. 4) are shown in transverse section, so far as 
their central cylinders are concerned. These roots, however, are curved, so that the 
section passes tangentially through a portion of the cortex of each, and shows that 
it is continuous with the Lyginodendron stem. The cortex of root 8 can be traced 
right through the outer cortex of the stem. Towards the interior, a porticn of the 
vascular tissue of the root is also shown in longitudinal section A fifth root is seen 
