1 77 
the Embryo and Seedling in the Gramineae. 
Returning to the first node of the younger seedling, the midrib trace m 
from the second leaf has only just divided itself between the embryonic 
root-plates when the coleoptile traces P and P f run in from either side. 
As they do so, traces Z 2 and Z 3 are inserted on P, and Z' 2 , Z' 3 on P\ in 
a way which must be described in detail. 
The double structure of P and P' is clearly indicated, particularly 
in the neighbourhood of the node. Each has two distinct groups of soft 
bast (PL IX, Fig. i). In young seedlings the common group of protoxylem 
only is lignified, but in older ones the metaxylem forms two distinct groups 
(PI. IX, Fig. 2), and the protoxylem is often torn away, leaving a gap. 
As the coleoptile traces enter the stele, the two phloem groups of each 
are separated by the xylem. One phloem group from P unites with that 
of Z 2 , the other with that of Z 3 . Similarly the two phloem groups of P r 
unite with those of L\ and Z' 3 respectively (Diagram II, Text-fig. 16, p. 174). 
The xylem of P divides. One branch turns inwards, that is towards 
the centre of the stele, and carries the xylem of Z 2 with it: the other— 
turning outwards—carries off the xylem of Z 3 . The xylem of P' behaves 
similarly, the inward branch sweeping off with the xylem of Z' 2 , and the 
outward one with that of L\ (PL IX, Fig. 3). 
In the end the ingoing xylem branches unite to form the xylem group 
X., which thereafter occupies the gap opposite M, and becomes part of the 
mesocotylar stele. The corresponding phloem groups accompany the xylem, 
remaining external to it (Diagram IV, Text-fig. 16, p. 174). For some distance 
below the node the two phloem groups are seen in favourable preparations 
to be distinct, but before the transitional region is reached they commonly 
form a single group. Both xylem and phloem can be distinguished in good 
preparations from other tissues of the mesocotyl throughout its length. 
The outgoing branches of xylem from P and P' leave the stele 
altogether, and are followed by the phloem groups (J P + L 3 ) and (J P' + 
Z' 3 ) respectively. United they form a double trace running downwards side 
by side with the central stele (Diagram IV, Text-fig. 16, p. 174, and PL IX, 
Figs. 4 and 5). The two phloem groups remain distinct within the trace 
until it approaches the scutellum. There are two groups of metaxylem, 
divided more or less completely from each other. The common group of 
protoxylem is directed towards the periphery of the mesocotyl (PL IX, 
Figs. 4 and 5). 
The trace thus formed loses its double character as it reaches the 
insertion level of the scutellum, and turns sharply upwards to enter it 
(Text-fig. 2, p. 163). 
This account of the course of the bundles in the first node must be 
understood to give the ground-plan of its structure as observed in seedlings 
of different ages. In such a tangle of traces the junctions are not accom¬ 
plished with mathematical precision. For example, metaxylem elements 
N 
