the Embryo and Seedling in the Gramineae . 173 
double (PL IX, Fig. 5). In seedlings of the age drawn in Text-fig. 12, p. 171, 
the two groups of soft bast can be distinguished very clearly by their 
contents. At this period the vessels of the phloem are crammed with 
proteids, which they are conveying from the endosperm to the first node. 
The two groups of soft bast are injected as it were with thick contents, 
which take up most stains readily, and then appear in transverse sections as 
two highly coloured patches within the trace (PI. IX, Fig. 1). They are 
separated from each other by a partial sheath of bast fibres, and by the 
xylem group. In serial sections through the mesocotyl of older seedlings 
the bast vessels of the scutellum trace are nearly empty. The two phloem 
groups, however, are still defined by the greater differentiation of the bast 
fibres which partially sheathe them. The xylem also is better lignified, 
and shows two metaxylem groups with a common cluster of protoxylem 
elements between them (PI. IX, Fig. 2). 
In short, the structure of the scutellum trace is undoubtedly double 
as it approaches the first node. Here it will meet traces from the coleoptile 
and plumule as they enter the stele of the mesocotyl. From this region 
also the plumular or nodal roots will be given off later, and their rudiments 
are present even in the youngest seedlings we have cut. 
The vascular complex at the first node is best attacked from above. 
The stele of the mesocotyl is built up of traces from the plumule and 
coleoptile ; these must therefore be followed downwards until they are 
joined by the scutellum trace. Considerable disturbance is caused at and 
near the node, by the insertion of cauline roots. 
A transverse section taken just above the first node shows twelve traces 
arranged as in Diagram I, Text-fig. 16, p. 174. Seven of the ten plumular 
traces (M ; L v Z 2 , Z 3 ; L\, L' 2 , Z/ 3 ) belong to the first leaf; three (m, / 2 , 
t' 2 ) to the second. They are arranged in two concentric circles. M and m 
are midrib traces, and they face each other on the same diameter. The 
coleoptile bundles P and P f are bisected by a line perpendicular to this 
diameter. 
The midrib trace M from the first leaf passes through the node 
unchanged. No other trace is inserted on it, nor does it contribute vascular 
elements to any cauline root. The midrib trace m from the second leaf 
behaves differently. It divides into two branches, one of which unites with 
the lateral traces / 2 and the other with /' 2 and L\ (Diagram II, Text- 
fig. 16, p. 174). This occurs at the very top of the node, just as the coleoptile 
traces P and P' run into the stele. The gap left by m is clear in several 
consecutive sections below this level. 
The union of L 1 with half of the three principal traces from the second 
leaf gives rise to a lateral plate of internal xylem and external phloem. 
A similar plate is formed on the other side of M by the union of L\ with 
the other second-leaf traces. The steles of the two first-formed nodal roots 
