370 Chandler . — On the Arrangement of the Vascular Strands 
Lomaria gibba, Labill. 
The diarch xylem plate of the primary root very quickly becomes 
oval and finally circular in section, sheathed externally by a complete layer 
of phloem derived from the tangential extension of the two original phloem 
strands. A characteristic feature of the xylem is the presence of a con- 
siderable amount of parenchyma among the tracheides, and even in the 
earliest stages small parenchyma cells (persisting, however, through only 
two or three sections) appear scattered in various parts of the small xylem 
strand. The pericycle consists of a single layer of small cells crowded with 
contents, and is succeeded by a very definite endodermis. As is so often 
the case, the stem bends somewhat sharply at the transitional region, and 
the changes occurring at this level are frequently difficult to follow. 
Oblique sections, however, clearly show that a few parenchyma cells with 
densely staining contents are differentiated at the centre of the xylem, the 
parenchymatous core often being temporarily connected with the external 
phloem by irregular radial extensions. 
On the stem resuming the vertical position, two or three well-marked 
sieve-tubes appear in the central parenchyma, and immediately the first 
leaf-trace passes out, its structure and mode of exit being essentially 
similar to that described for Doodia aspera. The resulting gap, however, 
was generally found to differ from the corresponding gap of nearly all other 
ferns examined, in being of relatively large size and remaining open through 
some considerable distance. The usual state of affairs is the formation of 
a small, quickly closed gap. 
The gap is repaired in the usual way, viz., by the gradual extension of 
the vascular tissue tangentially, and we have a ring of xylem surrounded by 
phloem and containing a strand of the same tissue, the two phloems having 
been in continuity through the leaf-gap (Figs. 13, 14). 
The second leaf-trace is formed exactly opposite the first and in 
a perfectly similar manner (Fig. 15). Like its predecessor, the trace 
is a very simple concentric strand, consisting of a few tracheides surrounded 
by phloem. The third leaf-trace, however, has a distinctly bilobed strand 
of xylem, a fact hinting at the occurrence of double leaf-traces to supply 
the later leaves. The preparations for the third leaf-trace are also of 
some interest. The xylem of the vascular ring breaks temporarily just 
before its exit, the break being effected by an ‘ encroachment ’ of the 
internal phloem ; the actual gap, however, is occupied merely by paren- 
chyma. As will be seen below, this breaking of the xylem foreshadows the 
preliminary division of the cauline strands which plays so important a part 
in connexion with the formation of the later leaf-traces. 
At a slightly higher level, and before the repair of the last leaf-gap, 
the xylem breaks immediately opposite the latter by an encroachment 
