FILICINEAE.—HETEROSPOROUS FILICINEAE. 



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borne on the ventral side of the petiole of ordinary foliage-leaves (Fig. 190), or at the 

 base of the leaves, but always on the petiole ; their stalks may be simple and bear one 

 sporocarp, or be forked and bear several sporocarps, and several usually grow together 

 from the petiole. The stalk runs along the dorsal edge of the capsule (Fig. 200), 

 and gives off lateral veins to the right and left which branch dichotomously and run 

 to the ventral edge. The ripe capsule is symmetrically bilateral and has within it two 

 rows of chambers, each of which reaches from the ventral to the dorsal edge (Fig. 

 196 A, B) ; these chambers open to the air in the young fruit by narrow canals on the 

 ventral side. In each chamber a cushion of placental tissue runs along the outer side, the 

 side towards the wall of the capsule, and bears the macrosporangia along its middle 

 line and the microsporangia on its flanks ; each chamber contains therefore in Mar- 

 silia as in Pilularia a sorus of two kinds of sporangia. When the capsule bursts it 

 is clearly seen (Fig. 200) that the soft inner tissue forms a small closed sac round the 

 sorus, as in Pilularia. 



Fig. 196. Very young sporocarp of .l/a/'j-^'/M e/a/«. ^ median longitudinal section. /J transverse section. C part 

 of a longitudinal section perpendicular to A ; // vascular bundles, j^ s the sori, sfc canals of the sori ; ma macro- 

 sporangia, ?«;' microsporangia. After Russow. (See FiG. 200.) 



The mature microsporangia contain sixty-four spores, the macrosporangia only 

 one ripe macrospore. 



The formation of the sporangia begins with the appearance of protuberances on 

 certain superficial cells of the placenta that bears the sorus; then these cells by 

 repeated oblique divisions (otherwise therefore than in the Salviniaceae) form three 

 rows of segments, till at length a convex transverse wall cuts off the three-sided apical 

 cell and converts it into the tetrahedral archesporium (Fig. 197 I-III); from the 

 archesporium by further divisions parallel to the last four is formed the tape turn, 

 which as in the Salviniaceae and the true Ferns surrounds the archesporium. 

 The stalk of the sporangium in the Marsiliaceae is from the first formed of three 

 rows of cells, and by longitudinal divisions it comes to be of several rows. While 

 the body of the young sporangium constantly swells to a larger size, radial divisions 

 appear in the cells of the wall, and radial and tangential in the tapetal cells ; 

 then the archesporium divides by successive bipartitions into sixteen spore-mother- 

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