in the * Seedlings' of Certain Leptosporangiate Ferns , 371 
of the internal phloem exactly as before. In this case, however, the break 
is larger, and occupied not merely by parenchyma, but by sieve-tubes as 
well. The external and internal phloem thus become continuous through 
the break, which is, of course, a preparation for the exit of the next 
leaf-strand. 
The gap in the xylem gradually increases in size, and a root-trace 
joins one of the xylem arcs in the usual way ; its entrance is followed at an 
immediately higher level by the closing of the third leaf-gap (Fig. 18). 
In some cases the break in the xylem was temporarily bridged over by 
a single row of two or three tracheides, but the latter persisted through 
only a few sections. Very soon preparations are made for the actual exit 
of the leaf-trace. A strand of tracheides is nipped off from each of the 
xylem horns, and the two strands, separated by a narrow band of sieve- 
tubes and surrounded by common phloem and endodermal sheaths, pass out 
as the leaf-trace (Figs. 19-31). The single nature of the leaf-traces is 
therefore still maintained. 
In a small proportion of the plants examined, a shallow and indefinite 
‘ground-tissue pocket’ was associated with the exit of the leaf-trace just 
described, an irregular patch of endodermal cells appearing between the 
cauline strand and the outgoing petiolar bundle, just before the latter was 
quite free. The pocket was of so simple a character, compared with similar 
structures fully described elsewhere for other ferns, that no further description 
is necessary. Its occurrence, however, is of interest, since it emphasizes the 
essential uniformity between apparently somewhat different types of tran- 
sitional changes. 
The fourth and following leaves occur in more rapid succession than 
their predecessors, and, during the exit of the fourth leaf-trace, the xylem 
at a point exactly opposite the latter is encroached upon by the central 
phloem and broken into two portions, the break, as before, being occupied 
by phloem (Figs. 20-23). The fourth leaf-gap then closes, and the fifth 
trace is given off, as its predecessor, from the two horns of the broken 
xylem ring ; it is still of the simple type, though the two xylem patches 
are separated by a comparatively wide band of phloem and parenchyma. 
Immediately opposite the leaf-gap, a root-strand joins the cauline system. 
The break formed in the initial stages of the exit of the sixth leaf- 
trace is much wider than in previous cases, though the endodermis merely 
bridges the gap, as shown in Fig. 24. Higher up, however, the xylem 
strands become further separated, and the sieve-tubes of the internal 
phloem, which up to the present have occupied a more or less central 
position in the ‘ pith,’ are now arranged lining the internal surface of the 
xylem. At the same time the phloem-parenchyma cells become larger and 
hence more conspicuous. 
As the two portions of the xylem become more separated, ground- 
