i87 
the Embryo and Seedling in the Gramineae . 
Each of the lateral traces is separated from the median trace by an 
unthickened element of very large lumen (rx. f rx.) resembling the large 
vessels characteristic of monocotyledonous roots. No such element occurs 
between the lateral traces. They are separated by several layers of 
ordinary conjunctive tissue. In transverse section this appears as a broad 
pathway of clear tissue opposite the protoxylem of the median bundle 
(px z ). Within this pathway, near the periphery, is a single lignified element 
of protoxylem (px 1 .), to which we shall refer later (PI. X, Fig. io). 
The lateral bundles are alike, and each differs from the median bundle 
chiefly in the fact that its metaxylem crescent embraces two groups of soft 
bast instead of one. These groups are partially separated from each other 
by a projection of the peripheral thickened layers. A little group of proto¬ 
xylem elements (px 2 ,)t sometimes reduced to a single lignified tracheide, 
is common to both crescents, and at the extremities of both there are large 
thick-walled vessels. Such vessels are also found at the ends of the median 
crescent M (PL X, Fig. io). 
The characteristic structure just described is found in the mesocotylar 
stele of the five seedlings examined. In all of them the scutellum trace is 
found to be connected with the lateral bundles only. Its scanty xylem 
is represented in the stele by the isolated element of protoxylem (px 1 .) 
already described as occurring between the two lateral crescents. It is not 
always easy to identify this solitary element. When the thickening is 
annular, it can be picked out only in sections which include a ring or part 
of one, and the imperfectly lignified rings do not stain deeply. In three of 
the seedlings examined the whole mesocotyl, from insertion to first node, is 
included in a single series ; and we have satisfied ourselves that the chain of 
single protoxylem elements can be followed from one end of the series to 
the other. In the two remaining seedlings, the mesocotyl was divided 
before microtoming, and accordingly we have two separate series in each : 
one passing through the insertion of the scutellum, and a second—with 
different orientation—through the first or plumular node. The scutellum 
xylem within the mesocotylar stele is represented in both insertion series by 
a single peripheral tracheide, which is either spiral or annular, and is always 
isolated in the conjunctive tissue separating the lateral bundles. In both 
nodal series a similar element is found in the corresponding position, 
and successive elements can be followed upwards to the plumular node. 
The phloem of the scutellum trace is massive. It divides, travelling to 
right and left of its scanty xylem on entering the stele. Below this level all 
traces are confounded in the insertion plate, but it is pretty clear that the 
scutellum trace contributes only a sharp elbow of tissue to this structure. 
Its real course lies up the stele, as already described for its xylem. The 
phloem branch on either side is absorbed in a lateral phloem centre of the 
mesocotyl. During the junction there is no clear division between the two 
