the Embryo and Seedling in the Gramineae . 165 
stalk and axis to become completely united, the sucker will appear sessile 
on the axis at one level, while the sheath is inserted higher up. These are 
precisely the relative positions of scutellum and coleoptile in Arena. As 
far as external structure goes, the mesocotyl may have been derived in some 
such way from a fusion of cotyledonary stalk with hypocotyl. What influence 
would such an ancestry be likely to have on the internal anatomy ? 
In such an ancestor the stalk, before it became fused with the axis, 
would contain at least one bundle; and every bundle present would travel 
upwards within the stalk to the first node, where the sheath is also inserted. 
At the first node each stalk-bundle would enter the axis. After fusion of 
stalk and axis all the stalk-bundles would be enclosed within the same 
compound structure as the stele of the axis. They would to all appearance 
be traces lying side by side with the stele during this part of its course, and 
on reaching the first node they would enter either the sheath or the stele. 
We have compared this imaginary skeleton with that of Arena , or rather 
with that of the Arena type, which we consider the primitive form of Grass 
seedling. 
The Arena skeleton is complicated, but its main features can perhaps 
be followed with the aid of a diagram (Text-fig. 6 , p. 166). The scutellum is 
sessile, and its single massive bundle turns upwards on entering the axis 
without joining the stele. Transverse sections through the mesocotyl show 
the scutellum trace outside the stele, and inverted with respect to it (PI. IX, 
Fig. 4). The inverted trace is clearly double, having two groups of soft bast, 
and in older seedlings two formations of metaxylem also, with a single 
group of protoxylem elements. Here, then, is a trace outside the stele of 
the mesocotyl, and travelling from scutellum to first node; just as in our 
imaginary case a trace or traces travelled from sucker to first node side by 
side with the stele of the hypocotyl. 
Reference to Text-fig. 6 shows that the scutellum trace divides into 
two parts at the first node while still outside the stele. The phloem group 
on either side becomes a phloem group of the double coleoptile trace on 
that side, and half the xylem goes with it. Thus the scutellum trace 
ceases to exist at the first node, but each of its two halves maintains its 
identity within one of the two coleoptile traces. The other half of each 
coleoptile trace is supplied from the stele. 
The peculiar connexion between scutellum trace and coleoptile traces 
can be equally well described from above downwards ; that is, by following 
the coleoptile traces through the first node. 
The structure of the first node is symmetrical about a plane which 
bisects the first leaf, and when prolonged downwards also bisects the scutellum 
trace and the scutellum itself. It is the plane of the paper in Text-fig. 2. 
In Text-figs. 16 and 17, pp. 174 and 175, it is represented by the shorter axis 
of the elliptical diagram, which always bisects both sc., the scutellum trace, 
