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* the Embryo and Seedling in the Gramineae. 
with a fair degree of probability. Only after comparison with the allied 
case of Sorghum , however, which in some respects is much more favourable 
for observation, were we fully convinced that this interpretation was sound. 
The first fixed point is the little group of protoxylem elements, some¬ 
times even reduced to a single annular tracheide with a phloem group 
on either hand, which represents the xylem of the scutellum trace. In the 
diagram (Text-fig. 21) it is marked px x , and the double bundle consisting of 
px 1 and the adjacent phloem strands is called sc., because it answers to 
the upward scutellum trace in A vena. In series which include the 
scutellum insertion, this group (pXj) can of course be determined with 
absolute certainty. But even in an isolated section of the mesocotyl, 
the group px x can be picked out, partly by its position near the periphery 
of the stele, and with still more certainty by reference to a second fixed 
point, the midrib trace from the first leaf (M). In Zea Mays, as in the 
other Grass seedlings examined, the midrib of the first leaf is prolonged 
almost unchanged into a trace which can be followed with certainty 
throughout the mesocotyl. It lies a little nearer the centre of the stele than 
the other traces, and its xylem is better lignified. The group px x is always 
at the further end of the diameter bisecting M. 
The rest of the stele is symmetrical with respect to this diameter. 
The phloem group on either side of px x is followed by another phloem 
group resembling it. Internal to the pair of phloem groups thus lying on 
one side of px x is a wedge-shaped area, over which are scattered xylem 
elements more or less completely lignified. A similar area is found on the 
other side. The two wedges ( x , ;tr in Text-fig. 21) are separated by a lane 
of clear conjunctive tissue, closed at the peripheral end by px x , and between 
the apices of the wedges by another little group of protoxylem elements 
(px 2 ). Thus the diameter m n, which bisects M, also passes through three 
distinct protoxylem groups : px z belonging to M; px 2 common to the two 
areas x, x ; and px x belonging to the scutellum trace. 
The numerous lateral plumular traces lie near the periphery of the 
stele. They occupy the two arcs which are bounded by M on one side, 
and on the other by either extremity of the cotyledonary bundles which 
face it. They are best defined in young seedlings by their phloem groups, 
for only a xylem element here and there is sufficiently well lignified to show 
up clearly in transverse section. Nevertheless, the existence of such 
elements within the larger phloem groups shows conclusively that the 
lateral traces from the first leaf are endarch, like their medium trace M. 
Between the phloem groups of plumular and cotyledonary traces alike 
are large elements, lignified in older seedlings, which clearly correspond to 
the root xylem found in the mesocotyl of Sorghum. They extend in places 
to the periphery of the stele. Since these are by far the most conspicuous 
xylem elements present in the section, and since among them the smaller 
O 
