Anatomy of Solenostelic Ferns: /. L ox soma, 83 
interrupted by similar tracts of spongy air-filled tissue ; most 
frequently it occurs as a single continuous line on either side 
of the petiole ; often the line is more or less interrupted, or 
indeed, as in Loxsoma itself, completely broken up, so that 
this tissue is represented by a number of short streaks which 
are not even arranged in the same straight line, although 
they are still aggregated towards the sides of the petiole. 
The very general distribution among the Filicales of this 
tissue, which might be regarded as representing modified 
traces of a mesophyll decurrent along the sides of the petiole, 
is a very significant phenomenon in reference to the view 
advanced by Professor Bower 1 concerning the leaf of the 
Ferns, which he regards as a rachis or phyllopodium , funda- 
mentally winged along its whole length. 
The xylem occupies the centre of the vascular strand of 
the petiole, the outline of which it accurately follows, except 
in the enlarged ends of the flanks of the horseshoe, where 
the xylem-strand is bent on itself, forming a hook which is 
curved inwards and encloses a small bay between itself and 
the rest of the xylem (Figs. 5 and 6 , hk). The phloem 
surrounds the xylem completely but not evenly, being in 
greatest quantity on both sides of the flanks, while on 
the median concave region of the horseshoe it is very 
scanty. 
The protophloem, with its flattened angular elements and 
swollen walls, is well marked, especially on the flanks, but it 
is entirely absent in the median concave region (Fig. 5, ppti). 
The pericycle consists of two or three layers, except in the 
median region, where only one is present ; on the outside 
of the flanks of the horseshoe its cells are considerably larger 
than elsewhere. In examining the petiolar meristele of 
Loxsoma attention is at once attracted to the presence of a 
number of sclerosed fibrous elements situated in close proximity 
to the periphery of the xylem, such elements being of very 
rare occurrence in the vascular strands of Ferns. These 
1 Comp. Morph, of the leaf of the Vase. Crypt. ; Phil. Trans., vol. clxxv, 1884, 
p. 606. 
G 2 
