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 of the xylem. Thus there are six protoxylems 

 in all (fig. 68). They develop when the petiole is very young. But a 

 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 segments 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 pinnae 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 pinnae, 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 8 cm. above, are similarly separated from one another. Farther 

 up, the pairs of pinnae 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 "pinnae" are mere crenations of the margin of a winged rachis. 

 But morphologically they are pinnae. The larger pinnae, 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. 



