the Embryo and Seedling in the Gramineae . 191 
We have attempted to explain the vascular symmetry of the Avena 
seedling by supposing that the scutellum trace divides at the plumular node ; 
that one branch runs up one side of the coleoptile, doubles closely on itself 
near the top, and runs down again, thus forming one of the double coleoptile 
bundles. The other branch does the same on the other side. When they 
reach the plumular node again, each downward branch plunges into the 
stele of the mesocotyl. Here they unite to form a compound bundle 
opposite the midrib trace M (PL IX, Fig. 4, also ante, pp. 167-9, and 
Text-figs. 6 and 7, p. 1 66 ). To explain the vascular skeleton of Sorghum in 
the same terms, we must consider the solitary element px v as representing 
the xylem of the upward scutellum trace in Avena , and the two phloem 
groups adjacent to it—within the lateral xylem crescents—as its phloem. 
At the plumular node all these structures are seen to be connected with 
half of each coleoptile trace, just as in Avena ; while the remaining phloem 
group within each lateral crescent belongs to the other half of the corre¬ 
sponding coleoptile trace, and can be traced downwards through the 
plumular node into the stele of the mesocotyl. 
Those who adopt this view will find that the chief difficulty to be 
faced is the existence of an insertion plate at the point of junction of 
the scutellum trace with the stele. For if the scutellum is identified with 
the cotyledon, the level where its trace is inserted on the stele would 
appear at first sight to be undoubtedly the cotyledonary node. But then 
the mesocotyl of Sorghum would represent the first internode of the main 
axis, and this is contrary to our hypothesis. Nor is it consistent with our 
observations, for we have shown that the structure of the upper or plumular 
node of Sorghum is strictly comparable with that of the first or plumular 
node in Avena , where the scutellum trace first unites with the stele. On 
the other hand, the insertion c node ’ in Sorghum resembles with equal 
fidelity the region in Avena where the lower cauline roots are given off. 
We therefore prefer to use the term ‘ insertion plate and we continue 
to regard the node where the coleoptile traces enter the stele as the first 
node of the axis in Sorghum as in Avena . It is strictly comparable in our 
opinion with the node which in a typical Monocotyledon divides the 
hypocotyl from the first internode of the plumule. 
Zea Mays , L. We have already described and figured the ramifications 
of the single bundle in the upper part of the scutellum (Sargant and 
Robertson (’ 05 )). We may therefore begin here with the structure of that 
bundle near the base, just before, it enters the axis. At this level it is oval 
in transverse section, with massive phloem (Text-fig. 21, p. 192). Its xylem 
is compact and well-lignified. The xylem divides into three parts while 
passing into the axis, and the phloem into two. The lateral branches of the 
xylem enter the two cauline roots which are inserted between scutellum and 
