in the ‘ Seedlings' of Certain Leptosporangiate Ferns . 391 
In connexion with the fifth or sixth leaf-trace there is a formation of 
a regular ground-tissue pocket very similar to that already described for 
other ferns. A group of endodermal cells appears between the outgoing 
leaf-trace and the main strand, and at a slightly higher level we have 
a patch of strongly sclerosed cells surrounded by an endodermal sheath. 
At the exit of the leaf-trace the inner endodermis becomes continuous with 
the outer, just as in previous cases. The leaf-gap does not close, but, if 
anything, becomes wider as the next stolon joins the stem ; a root-trace 
enters immediately opposite the gap. After the junction of the stolon 
and root-strands with the cauline system, the latter in transverse section 
appears as an arc of xylem (largely parenchymatous), with a broad and 
shallow gap into which the conspicuous fundamental-tissue dips sharply, 
but to no great depth (Fig. 136). At a higher level the ground-tissue 
is differentiated further into the gap and the vascular tissue appears 
as a reniform strand. The middle portion of the xylem then becomes 
much attenuated, consisting of only one or two rows of tracheides, and, 
with its accompanying phloem, bulges out in the manner indicated in 
the diagrams 137, 138. The fundamental-tissue still further encroaches 
upon the vascular tissue, but gradually loses its highly sclerotic character, 
except that portion actually between the horns of the vascular arc, the 
cells of which are extremely thick-walled. Figs. 1 40-1 42 represent the 
subsequent changes. The attenuated portion of the vascular tissue 
becomes separated from one of the two main masses, and finally the 
whole strand divides into three, the arc-like narrow portion passing 
out as a leaf-trace. The remaining strands then gradually approach 
and finally fuse, though the vascular portions, surrounded by a common 
endodermis, remain separated by pericyclic parenchyma for some little 
distance. 
Just before the union of the two cauline strands, one of the latter, 
viz., that from which the leaf-trace first separated, gives off a portion of its 
vascular tissue which, as a small strand, joins the leaf-trace at a slightly 
higher level. This is the first indication of the multiple leaf-traces occurring 
in mature plants, all previous leaf-strands having been simple. As soon as 
the union of the cauline strands is complete, another stolon is formed in 
the usual way, its formation being followed by a redivision of the vascular 
tissue into two portions. Each of the latter then separates off a small 
strand, one at a much lower level than the other; a third strand is 
then nipped off, so that the leaf-trace has three bundles instead of two 
as before. The three bundles ultimately fuse into a horseshoe-shaped 
strand. 
Subsequent changes consist of a further fractionation of the vascular 
system, and the gradual elaboration of the leaf-traces. 
