286 Bower . — Studies in the Phytogeny of the Filicales. 
allowance has to be made for the habit. In Gleichenia there is a more 
restricted area of the transverse section, and consequently a greater com- 
pression of the vascular tract. This is evident in the absence of the lateral 
involution between the adaxial and abaxial bays, as well as in the stronger 
curving inwards of the hooks of the horseshoe, which actually meet and 
fuse (Fig. 15, i). Thus the changes which lead to the separation of the pinna- 
traces start from a simpler and more compact state than in Lophosoria. 
The first changes are the separation of the hooks from one another, 
while the loops formed by them are completed (Fig. 15, ii). Each of the 
loops then becomes constricted in the middle, and a C-shaped vascular tract 
is separated off from its distal end (Fig. 15, iii-v). Meanwhile, two pro- 
jections arise from the concave side of the main bundle, opposite to these 
C-shaped tracts (Fig. 15, iii-v), while the latter alter their form by producing 
processes opposite to these (Fig. 15, v). The pairs of processes then fuse, 
and subsequently each bridge of junction splits down its middle into equal 
halves (Fig. 15, vii). The result is the formation of three C-shaped tracts 
of vascular tissue lying in a row, which supply respectively the main rachis 
and the two pinnae. Putting aside the differences of detail, the points of 
similarity with Lophosoria are, (i) that the pinna-trace is intra-marginal in 
its origin, and (ii) that its separation is effected by the splitting down the 
middle of a bridge of junction between the internal faces of the curved 
trace. On the other hand, as points of difference may be noted, (i) the greater 
simplicity of the initial steps in Gleichenia , consequent on the absence of the 
lateral involution, (ii) the difference of behaviour of the margins of the trace, 
consequent on its constriction, and (iii) the equality of size of the traces of 
the rachis and the pinnae. Thus the comparison brings out fundamental 
similarities, while the differences which appear may be attributed to the 
peculiar habit of the Gleicheniaceous leaf, and the extreme contraction of its 
vascular supply. 
It may not be out of place here to remark that notwithstanding the 
well-known contraction of the meristele at the base of the leaf in Gleichenia , 
which is correlated with its continued apical growth and straggling habit, 
the meristele may open out above into a widely curved-horseshoe. Sections 
from the upper regions of the leaf of G. flabellata, and especially of G. linearis , 
show this clearly. In them the xylem widens out into a dilated curve of 
one to three layers in thickness, while the protoxylems, with their cavity 
parenchyma, lie on the adaxial face. The structure is in essentials like that 
of Lophosoria ; the chief difference is in the much lower number and less 
prominent identity of the divergents. 
Hairs. 
The only dermal appendages found on the shoot of Lophosoria are 
simple hairs, composed of a single row of cells. They are thin at the base, 
