Anatomy of Solenostelic Ferns . 71 1 
and above this point it becomes more and more dictyostelic, 
although at first a complete vascular ring is occasionally to 
be met with. The leaf-trace of the first five or six leaves 
consists of a single curved strand. Above this point two 
or three separate strands are given off to each leaf, and at 
about the tenth leaf four such strands are present, two arising 
from each side of the leaf-gap. 
The first indication of internal steles that occur in the mature 
plant is to be found at about the tenth leaf. Just below one 
or both of the two upper (adaxial) traces of this leaf the xylem 
of the stem-stele is seen to project slightly towards within, so 
as to form a small ridge on its internal surface, which is often 
continued as such for some distance down the stem. Some- 
times, however, it separates off completely so as to produce 
a small xylem-strand lying free within the phloem of the 
stele, which either ends blindly below, or eventually fuses up 
again with the main xylem-strand. These free xylem-strands 
are always present at the subsequent leaf-gaps, and although 
still remaining enclosed by the same endodermis, they become 
more and more distinct from the main xylem-strand of the 
stele. Later on they may even separate off from the stele 
altogether in the upper part of their course, only fusing with 
it again at a point lower down. The separation of the small 
xylem-strands from the main stele finally becomes complete 
throughout, and from their starting-point they run as small 
independent vascular strands ending blindly in the central 
ground-tissue, having no further communication with the 
main stele, except sometimes by a small branch near their 
point of origin. 
It seemsj therefore, that the internal vascular strands of 
Alsophila excelsa owe their existence to the same initial- 
phenomena as do those of Dicksonia rubiginosa . That is to 
say, they are probably derived from the elaboration of a local 
thickening of the xylem-ring at the margins of the leaf-gaps 
in the ordinary stelar cylinder. The earlier stages of their 
development also proceed along essentially the same lines, 
although it is to be admitted that there are certain marked 
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