488 Tans ley and Lid ham. — A Study of the 
outer xylem. Just before the eleventh trace departs the internal strand of 
xylem again enters into connexion with the outer, just at the point where 
the dorsally turned wing of the trace is about to separate and appears 
to close the xylem-gap, as in the case of the preceding trace. A trans- 
verse section of the trace close to its point of origin is represented in 
Fig. 22. 
The twelfth trace is considerably bulkier, and a considerable gap 
occurs in the side of the stele, placing the pith of the stele in connexion 
with the cortex. This gap, however, opens before the departure of the 
trace and is not actually made by the latter, the xylem immediately dorsal 
to the gap not departing with the trace, but remaining behind in the stele. 
The xylem of the internal strand also enters into connexion with the 
external xylem on the opposite side to the gap, but does not directly 
contribute to the trace. As the second (ventrally turned) wing of the trace 
is departing, the internal xylem-strand loses its first connexion and makes 
a new one with the external at the point from which the first (dorsally 
turned) wing has gone off, the tracheids again running up into the external 
xylem and taking part in the closure of the xylem-gap (to be distinguished 
from the lateral gap in the stele as a whole, which is quite an exceptional 
phenomenon). 
In the course of the twelfth internode the xylem of the internal strand 
becomes, as usual, separated from the external by phloem, but for a very 
short space. It becomes reconnected again considerably before the depar- 
ture of the thirteenth trace. The stele has not only increased considerably 
in diameter, but also in xylem-elements in proportion to its bulk, so that 
the pith is reduced in one place to three or four cells. The latter rapidly 
increases again, however, at the expense of the internal xylem-strand, 
so that from a crescentic cross-section it acquires a triangular and then 
a circular one, the internal xylem-strand completely disappearing. The 
thirteenth trace now arises from a diametrically opposite position on the 
stele to that at which the internal strand has been absorbed. The trace 
goes off as usual with one wing slightly in advance of the other. It has 
a small pith and is distinctly less bulky than the immediately preceding 
traces. Its departure makes no gap in the xylem of the stele. This trace 
then shows a striking reversion to the state of things existing at the proximal 
end of the plant under description. 
As a preliminary to the origin of the fourteenth trace, a new local 
dilatation arises on the inside of the xylem. The trace departs much like 
the last and the tracheids run from the new internal strand to the point of 
departure of its second wing. The tissues now begin to show the same 
appearance that was noted at the apex of D and the growing point is 
reached. 
The most advanced (F) of the six young plants available (Plate XXXI, 
