322 
Sampson . — The Morphology of 
a medullated stele divides, each daughter stele showing a gap on the 
inner surface. 1 The smaller daughter stele, making a sharp bend, sup- 
plies the new tuber. The original gap is not continued over the bend, 
but it appears as the stele of the tuber passes out, giving it a charac- 
teristic U-f° rrrL From the tuber stele several leaf-traces pass out, con- 
structed as those which arise from the main axis, and supplying leaves 
which number among those of the yearly growth. 
The origin of the tuber stele from the stem, its structure, and its 
connexion with leaf-traces, point to one conclusion, namely, that it is 
morphologically a branch. In the following pages it will be seen if these 
features are sufficiently constant in fertile plants to justify this view. 
(b) The Origin, Course , and Form of the Tuber Stele in Fertile Plants. 
In the preceding section of this paper a plant was described, which is 
to a certain extent unique, at least in the material upon which this investi- 
gation was made. Its singularity consists, not so much in any particular 
feature, as in the fact that it combines several features of supreme impor- 
tance in a morphological study. The aim of the present section is an 
examination of these features as they occur in other fertile plants. 
Owing to the diversity in stelar anatomy which fertile plants show, it 
seems best to begin with a brief description of the tuber stele and its relation 
to the main axis in three distinct cases. 
Fig. 5 gives a series of transverse sections of a large fertile plant with 
a single new tuber. Sect, i, through the base of the peduncle, shows the 
stele as an almost continuous ring of xylem surrounding a relatively large 
pith. In sect. 2 this ring possesses a very distinct break at one side, the 
xylem being somewhat horseshoe-shaped, with cortex and medulla con- 
tinuous through the gap. In the cortex facing this gap are three leaf- 
traces, which later become connected by tracheides with one another and 
with the edges of the gap (sect. 3), with the result that a tube of xylem 
is again formed of diameter nearly twice that of the ring seen in a higher 
section. This tubular condition lasts for one or two sections only, as the 
stele divides to form two arcs of xylem, one of which supplies the tuber, 
while the other breaks up to form the root-strands (sects. 4, 5, and 6). 
It is important to note that the arc which passes out into the tuber is that 
connected above with the three leaf- traces already mentioned. 
The origin of the vascular supply to the tuber in this plant resembles, 
in several respects, that of the stunted tuber now to be described. We 
find, as before, at the base of the peduncle a ring of xylem, in which only 
small gaps occur (Fig. 3, sect. 1). Lower down a break appears on one 
side of the stele, giving it a horseshoe form in cross-section (sect. 2). Later 
1 Cf. the gaps occurring at each dichotomy of the axis in members of the Lepidodendreae, 
which possess medullated steles. 
