Bower.—Studies in the Phytogeny of the Filicales. 431 
enlarged leaf-base, but that these unite again at its upper limit, traversing 
the rest of the petiole as a single strand. The main fact is correct, but 
there seems to be some indefiniteness in the exact point of reconstruction of 
the single strand. Transverse sections at the base of the leaf show a single 
vascular strand of V-shape, but very soon the apex of the V widens out, 
and in place of the single angle there are two, as in the letter W, with a very 
flattened middle angle (Text-fig. 2, d). At each of the lateral angles of the 
W is a protoxylem-group, while the metaxylem forms a continuous 
strap-shaped band with a slightly hooked adaxial curve at either of its 
two margins; then follows a pericycle of several 
layers, and the endodermis. Thus the structure in 
essentials is the same as in P. pycnophylla , but there is 
considerable difference in the proportions, while the 
number of protoxylem-groups is smaller, and the 
vascular strap much narrower. A little higher up 
from the base the xylem becomes constricted at a 
point some distance from its margin, so that a mass 
about equal to a quarter of the whole is separated 
off from the rest; the phloem and sheaths also curve 
inwards, and a marginal strand is thus constricted off; 
the same happens (but not necessarily at the same 
level) at the other margin, so that the condition is 
reached of a larger median strand, which is placed 
abaxially and has two protoxylems in its concavity, 
and two smaller marginal strands slightly nearer the 
adaxial surface of the leaf (Text-fig. 2, b , c). This 
state is maintained for some distance upwards, and 
in one case which was followed out the strands re¬ 
mained separate for some six inches above the 
enlarged base, and were reconstituted to a single 
strand only in the last half-inch of the rachis below 
the lowest pinnae. The reconstitution of the single 
strand repeats in converse the steps of disintegration which took place 
below (Text-fig. 2, a). The result is that in the upper region of the leaf 
the strand of the rachis is similar in all essentials to that of P. pycnophylla , 
but the protoxylem-groups are fewer, the median one being absent, while the 
more pronounced W-shape of the middle region is a curious specific difference, 
which seems to go along with the absence of the median protoxylem. 
In order to verify the position and relations of the protoxylem in the 
axis and bases of the leaves of a well-grown stock of Plagiogyria , serial 
sections have been cut through the apical region, so that the actual position 
of those xylem-elements which are earliest lignified may be directly observed 
(Fig. 7). It is some time after the stelar tube is already defined in the 
Text-fig. 2. Trans 
verse sections of the petiole 
of P. semicordata. a is 
from a point below the 
lowest pinnae, and it 
shows a single strand, b 
is from a point lower 
down, and the strand is 
seen to have divided into 
three, c is in the swollen 
region of the leaf-base, 
and the strands are uniting 
again, t/shows the strands 
fully united as they are 
seen at the base of the 
petiole before insertion 
upon the axis. 
