26 STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 
cycle and the tips are bent sharply inward or ‘“hooked’’ (figs. 68, 69, 88-92). 
The “‘hook’’ is short at the base of the petiole, and much deeper above. 
Around the xylem there is a layer of conjunctive parenchyma. Between 
this and the pericycle lies the phloem. Its outermost layer all round is 
early developed as protophloem like that of the stem. It appears after the 
protoxylems, and develops first on the outer side of each arm of the bundle. 
Soon afterward it appears on the inner side of each arm. These four patches 
spread by addition of elements at each end until they meet and form a 
continuous sheath. The bulk of the phloem consists of sieve-tubes. No 
protoxylem elements (spiral cells) are found at the base of the petiole, but 
they appear shortly above, and are on the inner (upper, ventral) side of 
the xylem. The first to appear are two patches of slender spiral, annular, 
and spiro-annular elements, one in each corner of the flattened angle of the 
V-shaped xylem. Later another group (and finally two) appears on each 
arm beneath the hooked end ofthexylem. Thus there are six protoxylems 
in all (fig. 68). They develop when the petiole is very young. Buta 
very great enlargement of the tissues, both in length and breadth, takes 
place later, by which the protoxylem elements are quite torn to pieces. 
Neighboring parenchyma cells then push in to occupy the space, and the 
remaining lignin rings and spirals are often pushed out of their original 
places (figs. 116, 117). Thus originates the ‘‘cavity-parenchyma’’ of 
Russow (1871) (cf. Gwynne-Vaughan, 1901, p. 87). Its cells retain a. 
fairly regular rectangular shape and are thin-walled. 
The blade is from 30 to 90 cm. long (see table 6) and 7 to 20 cm. wide, 
lanceolate in outline, with the widest part from 5 to 40cm. above the base. 
It is pubescent, very soft in texture, and quickly wilts when plucked or 
frosted. Its delicate yellowish-green color has been mentioned above. It 
is thrice pinnate (fig. 2) with the margins of the ultimate sezments deeply 
crenate (fig. 5). The veins are forked, without anastomoses. The verna- 
tion is of the typical circinate kind out to the divisions of the third degree. 
The crenations develop as the leaf unfolds. 
The number of pinne varies from 30 to 50 pairs, and the maximum 
number of pinnules on a single pinna is 13 to 25 (see table 6). Though 
we speak of pairs of pinnee, they are not exactly opposite. One of the 
lowest pair may be from 1 to 7 mm. below the other. Those of the next 
pair, 2.5 to 8cm. above, are similarly separated from one another. Farther 
up, the pairs of pinne and the individuals of each pair are closer together, 
but the separation of the members of each pair is sufficient to make them 
appear almost alternate. Near the apex of the leaf the rachis is much like 
a midrib, and the divisions of the leaf diminish in size and degree. The 
uppermost ‘‘pinnz’’ are mere crenations of the margin of a winged rachis. 
But morphologically they are pinnz. The larger pinnz, in a similar 
manner, have large, twice-divided pinnules; but these become smaller the 
farther they are from the rachis, until they, too, become mere crenations. 
