176 Sargant and Arber.—The Comparative Morphology of 
Thus nineteen plumular traces enter the second node in a seedling 
of this age, as against ten in the younger one. Without following the 
course of the nine new bundles in detail, we may briefly say that they 
appear to fuse with one or other of the root-insertion plates. The midrib 
trace fi divides itself between them. The total effect is to add a massive 
layer of external xylem to both plates. 
Each now consists of two xylem layers with a thin sheet of phloem 
sandwiched in between them. Elements of large lumen are characteristic 
of the xylem. Their thickened walls are very well lignified, and they give 
a distinct character to the root-plates (PI. IX, Fig. 5). 
Three nodal roots are commonly inserted on the two root-plates. The 
latest formed (r 3 in Diagram IV, Text-fig. 17) is also on the highest level. 
It appears behind M. In the seedling from which the diagrams of Text- 
fig. 17 (p. 175) were taken, this root was still rudimentary, but xylem elements 
from both plates are already moving round to the back of M to provide for 
its insertion. In older seedlings the plates will clearly meet behind M, 
almost enclosing it in a deep crescent or horseshoe. 
Two older roots, r 1 and r 2 , are inserted lower down. One is attached 
to either plate, and their position is indicated by arrows in Diagram IV. 
The root-plates are continued down the mesocotyl in a rather simplified 
form. Below the disturbance caused by root-insertions, the xylem layers 
become single again, and appear in transverse section as two lateral chains 
of five to six large thick-walled elements with groups of smaller ones 
among them. PL IX, Fig. 4 is drawn from a section not far below the 
first node, but the stele has almost assumed the appearance which it will 
maintain throughout the mesocotyl. 
The mesocotyl ends with the insertion of three cauline roots, occupying 
the same positions with regard to the stele as the nodal roots at the top of 
the mesocotyl. One is ventral—that is, given off from behind M —and two 
are lateral—external to the two root-plates. Their steles are all inserted 
on the root-plates, which thus serve to connect the two root-systems of the 
seedling. Below this level the mesocotylar stele is prolonged into that 
of the primary root, but the method of transition from stem to root 
structure is almost completely masked by the disturbance caused by root- 
insertions. The root-plates end with the mesocotyl. Some of their larger 
elements, particularly those on the scutellum or dorsal side of the stele, can 
be followed into the stele of the primary root. 
Before leaving the subject of the root-plates, we may observe that they 
are built up at the node from the main traces of the second and third 
leaves, which are thus put into connexion with the nodal roots. But only 
two minor traces from the first leaf enter the root-plates ; the main lateral 
traces, as well as the midrib, are independent of them, and connect the first 
leaf with the primary root and the rest of the lower root-system. 
