the Embryo and Seedling in the Gramineae . 189 
modified as it approaches the first or plumular node. All its elements 
become less differentiated. The cells of the endodermis and conjunctive 
tissue just below the node are little if at all thickened ; the xylem is less 
lignified. In particular all the vessels of large lumen become quite thin- 
walled, and are apt to appear crushed in transverse section. Two phloem 
groups are found in the median trace, so that but for the two sentinel 
vessels (rx.) there would be some difficulty in distinguishing the median 
from the lateral bundles. 
At the node a number of plumular traces combine with the two traces 
from the coleoptile to form the stele of the mesocotyl. In none of these 
traces are any elements of wide lumen present, such as those which extend 
from the root upwards through the mesocotylar stele. All such elements 
appear suddenly just below the node. 
The coleoptile bundles are distinctly double (PL X, Fig. 11). Each 
consists of two phloem groups embraced by a common xylem crescent. 
In places two groups of protoxylem are indicated, corresponding to the two 
phloem groups. At the node the coleoptile bundles run in from opposite 
sides of the axis in a straight line, which is perpendicular to that bisecting 
the midrib of the first leaf. The xylem groups of these bundles meet 
in the centre of the axis, forming a bridge across the node (PI. X, Fig. 13). 
The phloem groups of each bundle separate, and one runs in on either side 
of the xylem. 
In the youngest seedling cut, numerous plumular traces are differentiated 
just above the node. Of these, ten possess at least one lignified xylem 
element. Five lignified traces are on one side of the xylem bridge, and five 
on the other. The stele is not quite symmetrically divided however, as on 
one side the traces are larger than on the other, and more unlignified strands 
are present. The median trace in the larger segment represents the midrib 
of the first leaf; it is commonly the largest trace, and always the best ligni¬ 
fied. Though at one point its xylem just touches the xylem bridge, 
it forms a centre independent of the coleoptile traces. Several unlignified 
strands are inserted on it at the node, and together they form the median 
bundle of the mesocotylar stele (M in PL X, Fig. 10). Of the bundles on 
the same side as the midrib trace, several of the strands, and all four ligni¬ 
fied traces, are inserted on the coleoptile bundles ; and so are all the 
traces—lignified and unlignified—on the other side. 
In the older seedlings the nodal structure is essentially similar, but the 
discrepancy in number and size of the traces on either side of the xylem 
bridge is greater. 
The coleoptile traces form the lateral bundles of the mesocotylar 
stele. The two phloem groups of either lateral bundle are derived from 
those of a single coleoptile trace. The xylem bridge breaks up into two 
groups, each of which-—supplemented by the large vessels derived from the 
