186 Sargant and Arber .— The Comparative Morphology of 
frequent on the ventral side, and lower down—where branches are fewer— 
they form a ventral crescent. Still lower, no branches are given off from 
the main bundle, and the xylem elements are collected into a compact 
ventral group. 
The scutellum trace does not run up the mesocotyl side by side with 
the stele as in Avena or Zizania , but enters it at once. Its elements, how¬ 
ever, can be followed within the stele above its insertion. Below that level 
there is a complex formation of xylem and 
phloem resembling that found in Avena in the 
region from which the lower cauline roots are 
given off. The identity of the mesocotylar 
traces is lost in this plexus, which we have 
called the insertion plate. In the oldest 
seedling which we have cut, no cauline roots 
are formed at this level, but they may be 
present in seedlings older still. Below the 
insertion plate, the axis is abruptly transformed 
into the primary root. Above it, the meso¬ 
cotylar stele soon acquires its characteristic 
structure, and preserves it almost unchanged 
up to the plumular node. For convenience 
we must first describe the complete stele 
above the insertion, and return later to the 
entrance of the scutellum trace, though this 
reverses the natural sequence of events. 
The whole stele is surrounded by a very 
well-marked endodermis. The internal wall 
of each cell in this layer is thickened and 
lignified; but the radial walls are little 
thickened, though more or less completely 
j. i-rju. tv. tji/rgnwm i/tM- c> A ' 
gave , Pers. a. Seedling, life-size, lignified (PL X, Fig. io). Within the endo- 
grai» P removedlo'shcw scutelfum!^ dermis are at least two la y ers of thick-walled 
but unlignified cells. 
The vascular tissue of the stele consists of three phloem centres, each 
defined internally by a xylem crescent, and externally by the thick-walled 
peripheral tissues. One of these phloem centres ( M) differs from the other 
two (P, P r ) in containing a single group of soft bast. The lignified xylem 
crescent embracing this phloem group is broken, but clearly indicated. 
Its protoxylem elements ( px z .) are distinct, and nearer the centre of the stele 
than the rest of the crescent. At either extremity are one or two large 
well-lignified elements, but no trace of protoxylem. The whole structure 
forms a typical endarch bundle, which we call the median trace to 
distinguish it from the two lateral traces P and P\ 
