376 
Stiles .— The Structure of the Aerial Shoots of 
thickenings ; no parenchymatous elements or sclerenchymatous fibres are 
here found among the tracheae. 
Surrounding this xylem-mass is a zone of thin-walled tissue, in which 
are scattered more xylem-elements (Fig. 2). These latter are in the form 
of an irregular broken ring, and are undoubtedly of the same nature as the 
secondary tracheae discovered and described by Boodle 1 in Psilotum 
triquetrum , and which have also been recorded as occurring in that species 
by Miss Ford. Their radial arrangement in P. Jlaccidum is in some cases 
quite as obvious as in the other species, though perhaps in none of my 
preparations were they so numerous as in those figured by Boodle. As 
this author found in P. triquetrum , so in P. Jlaccidum there is no sign of 
a definite cambium. 
Surrounding the zone of secondary xylem-elements is a band of thin- 
walled elements presumably of the nature of phloem, but this tissue is very 
difficult to recognize, especially in transverse sections. There is no well- 
defined pericycle. 
The cells of the inner cortex have their walls much thickened by 
a brown substance (Fig. 3) which is laid down apparently more or less 
irregularly. 2 The cell-walls decrease in thickness as the exterior of the 
stem is approached. The epidermis is one cell wide and has a thick 
cuticle. In the subterranean part of the stem stomata were not found. 
As one passes up the stem it becomes wider in diameter, as does also 
the central stele, which also becomes more completely circular in transverse 
section. The two protoxylem-groups at each end of the band become 
more distinct, while a third protoxylem arises between them on the 
outside of the metaxylem, and eventually the stele becomes triangular in 
outline (Fig. 3). An exactly similar state of affairs is found in P. triquetrum . 3 
In this part of the stem there is a tendency in places to a slight mesarch 
structure, 4 the protoxylem-elements being found not quite at the ends 
of the xylem arms. The secondary tracheides here begin to diminish in 
amount, so that when a point is reached where the stem is triquetrous they 
have almost completely disappeared. When the stem becomes triangular 
three more protoxylem-groups make their appearance between the three 
already present, so that with a triquetrous stem is associated a hexarch 
stele (Fig. 4). Mesarch structure is here sometimes very marked, and in 
some cases is certainly due to the insertion of a leaf-trace (Fig. 5), though 
at other times no leaf-trace is given off. The question of mesarchy will be 
discussed subsequently. 
1 Boodle, L. A. (’ 04 ), p. 505. 
2 Mr. Brooks has pointed out to me that in P. triquetrum this substance is often laid down 
spirally on the longitudinal walls. In all cases examined in P. Jlaccidum the thickening due to this 
substance was reticulate. Sometimes, however, it seemed probable that the first layers of the sub¬ 
stance were laid down spirally. 
3 Boodle, L. A. (’ 04 ), p. 514. 
4 Cf. P. triquetrum , Boodle, L. A., 1 . c. 
