OF SOME SAPOTACEOUS SEEDLINGS. 191 
widen till the group is pear-shaped (fig. 3). I use the word « group” advisedly, as it 
will be seen that it represents more than one bundle. 
A band of phloem surrounds the rectangular stele; both it and the pericycle are 
interrupted by large-celled parenchyma opposite the protoxylems. 
Corresponding with the zone of greater girth, the transverse section at 48 mm. below 
the node shows a sturdy band of secondary wood about six cells deep. The periphery 
of the pith has also become lignified, leaving a central thin-walled region about a third 
of its whole diameter (figs. 4 & 4a). 
The primary xylem groups have now widened by the addition of metaxylem elements 
on each side of them. 
The secondary thickening dies out about 35 mm. below the node (fig. 5). This 
swollen zone appears to occur generally in this species and is seen elsewhere in the 
order. 
Above this point the interest centres in the changes which take place in the primary 
xylem. The metaxylem forks into two groups, which are placed to right and left of the 
protoxylem and internal to the extremities of the four phloem-bands. The protoxylem 
lies in à broad ray which expands outwards. The metaxylem elements are very scattered 
and often appear isolated and at some distance from the protoxylem (fig. 6). The latter 
is often undifferentiated through several sections (about 30 u), and then again appears 
at higher or lower levels for about 60 u. The spiral tracheids have been stretched apart 
during growth. At each reappearance the protoxylem is a little nearer to the centre 
than below. This goes on till it is seen first to be in a line with the two metaxylem 
groups (fig. 6) and then slightly internal to them, forming the apex of a right angle, 
each of whose containing lines bisects a metaxylem group (figs. 7 & 7a). 
At the node the four phloem-bands break up (fig. 7); the extremities of each of them 
are from this point connected with the metaxylem groups, and form with them a double 
vascular bundle at each angle of the stele. The central portions of each band behave 
differently in each opposite pair. The phloem-bands perpendicular to the cotyledonary 
plane die out at the node in a seedling of this age (fig. 8). The phloem-bands parallel to the 
cotyledonary plane become here internal to the vascular bundles and form two crescents 
(fig. 9), which, higher, unite by their horns and, above the node, form the cylinder of the 
first internode. Internal to the middle of one of the lateral phloem-bands are two xylem 
elements. These can be traced downwards for 6°37 mm. from the node without inter- 
ruption, One reappears at intervals on this side and another lower down on the opposite 
side; but both are lost sight of at about 32 mm. below the node, i. e. before root- 
structure appears. They are doubtless destined to supply the first internode. i 
The transition from stem to root characters takes place here and generally in the 
Material examined either at the node or close below it. 
eanwhile it appears, in passing up through the node, that the protoxylem strand of 
m primary group forks, and each new strand takes up its position at the apex of a 
E vascular bundle (fig. 94). The two bundles are now separated by a wide tin 
ore the cotyledons are completely cut off from the axis these bundles approac 
me another and fuse by their phloem and cambium, while their xylem consists of several 
262 
