184 Sargant and Arber .— The Comparative Morphology of 
Arena are represented by one or two large pitted vessels on either side 
of the stele. Between them is a group of small well-lignified xylem 
elements common to the two phloem groups. This compound structure, 
two phloem groups opposite to one another and divided by a common 
xylem group, represents the two traces M and x in Arena. 
Lower down in the mesocotyl the stele loses in differentiation. The 
endodermis and pericycle are little thickened, and not lignified at all. The 
central xylem group is only partially lignified. The large lateral vessels 
are comparatively well differentiated and completely lignified throughout 
their course, doubtless because—as in Arena —they connect the cauline 
roots of the insertion region with those of the node. 
The process of transition to a root-structure cannot be followed in 
any of our preparations, nor is the symmetry of the primary root to be 
made out. 
Lolium italicum , A. Br. In the two seedlings examined, the first leaf 
has lately emerged from the coleoptile, but one of them is rather further 
developed than the other. The type is that of Arena , but the seedlings are 
smaller in every dimension; the grain shorter in proportion to its breadth, 
and the axis more slender. A small epiblast is inserted opposite the 
scutellum, and at about the same level. Cauline roots are also formed 
there, and they begin to show externally about the time when the first leaf 
appears, while at the same epoch the nodal roots are still mere rudiments 
within the cortex. 
The bundle of the scutellum is single. Its xylem is massive, and 
gives off a few lateral branches near the apex. Below this, the xylem 
forms a ventral crescent embracing the phloem. At the insertion of the 
scutellum its trace enters the axis, and at once turns sharply upwards, but it 
does not join the stele below the first node. A transverse section through 
any part of the mesocotyl shows the scutellum trace side by side with the 
main stele, and inverted—that is to say, the xylem of the scutellum trace 
is directed away from the centre of the stele. 
At the first node three plumular traces only can be followed with 
certainty into the stele of the mesocotyl. They all belong to the first leaf, 
and represent M, Z 2 , and L\ in Arena. Five more are present, though 
unlignified, above the entrance of the coleoptile traces; they represent L x 
and L\ from the first leaf, and m, / 2 , and l\ from the second. They behave 
exactly like the corresponding traces in Arena, m divides into two branches 
which travel to opposite sides of M\ one uniting with / 2 and L 1 on the 
way, the other with l\ and L' v But all these traces disappear before those 
from the coleoptile run into the stele. In older seedlings they would 
doubtless persist, and then their connexion with the lateral xylem plates of 
the mesocotyl would appear. 
