430 Bower.—Studies in the Phytogeny of the Filie ales, 
on the peripheral and central sides of the xylem, but thins off considerably 
at the foliar gaps. The xylem, which is massive, is composed of tracheides 
with parenchyma scattered through it; the protoxylem is not clearly 
defined when seen in the mature condition. 
The leaf-trace is initiated in the stelar ring, and may be recognized 
early by an indentation of the inner limit of the xylem opposite.one of the 
sclerotic groups in the pith, which, as mentioned above, subtend the leaf- 
trace, and accompany it outwards. As this indentation deepens a group 
of small tracheides (protoxylem) intermingled with parenchyma becomes 
apparent at its apex, while opposite it the outer limit of the xylem becomes 
strongly convex, forming the projecting bay already noted. Subsequently, 
by abstrictions of the stelar ring at some distance right and left of the 
indentation, with incurving of the sheaths, the leaf-trace is separated, and 
passes outwards. Entering the leaf-base, which is deeply keeled so as to be 
triangular in section, the leaf-trace widens out, and loses its acute shape, the 
apex of the V expanding and rounding off; the sclerenchyma meanwhile 
A B C 
Text-fig. i. Transverse sections of the petiole of P. pymophylla. A is taken from a point 
above the basal swelling ; B, from a point about the middle of the swelling, and it traverses two of the 
pneumatophores. C is from a point near to the insertion of the leaf upon the axis. 
spreads out so as to ensheath the whole strand, the periphery of the section 
being also covered by a hard sclerotic band (Text-fig. i , C). A section 
higher up shows the leaf-base further enlarged, and the strand widened out 
still more, especially in its middle part, while the section may often traverse 
one or more of the large pneumatophores, which are seated on the oblique 
abaxial faces (Text-fig. i, B). As the upper limit of the swollen region 
is reached the vascular strand again contracts, and its sclerenchymatous 
sheath disappears ; the adaxial surface of the strand becomes slightly 
crenate ; examination of these crenations shows that they correspond to the 
groups of cavity-parenchyma ( Liickenparenchym ), which indicate the position 
of the protoxylem-groups ; the number of these is about nine, three on each 
of the lateral flanges, while of the other three one is median, and the other 
two are right and left of it at the angles of the sharp curvature of the leaf- 
trace (Text-fig. i, A). This structure is maintained upwards to the point 
where the pinnae are borne. The structure of the leaf-base is found to be 
the same in P . glauca. 
A curious difference of vascular structure is, however, seen in the leaf- 
base of P. semicordata. It was noted by Mettenius ( 1 . c., p. 3, and Figs. 12 
and 13) that the leaf-trace in this species divided into three strands in the 
