438 MOSSES AND FERNS chap. 



A). By this time the outer epidermal cells begin to thicken, 

 the first indication of the hard shell found in the ripe sporo- 

 carp. 



The development of the sporangia corresponds most nearly 

 to that of the Schizseacese. The surface cells of the sorus pro- 

 trude as papillae, in which the same divisions arise as in other 

 LeptosporangiatEe. The first division wall is usually strongly 

 oblique, but may be transverse. The formation of the arche- 

 sporium is the same, but the apical growth of the sporangia is 

 checked sooner in the earlier ones, which have consequently a 

 very short stalk. In the later ones, which arise between the 

 others, the stalk is longer. The first sporangia are formed at 

 the base of the sorus, and their development proceeds toward 

 the apex; but later secondary ones may arise at any point" in 

 the sorus. 



The tapetum is well developed, and, as in most homospo- 

 rous Ferns, consists of two layers, in some places of three. 

 The number of sporogenous cells is usually eight, but some or 

 all of these may divide again, so that the whole number ranges 

 from eight to sixteen. The dissolution of the tapetum walls 

 and subsequent division of the spores follow precisely as in 

 Azolla. In stained sections the nucleated protoplasm of the 

 tapetal cells is very evident after the walls have disappeared. 

 At this point the difiference in the two kinds of sporangia be- 

 comes manifest. Those in the lower part of the sorus, i. e., 

 the oldest ones, form the macrosporangia, the upper ones 

 microsporangia. In the latter all the spores mature; in the 

 former, as in Azolla, one spore grows at the expense of the 

 others, and finally fills the sporangium completely. 



It has been generally supposed that no trace of an annulus 

 could be detected in the Marsiliacese. The writer has found, 

 however (Campbell (26)), in Pilularia Americana, traces of 

 a terminal annulus like that of the Schizasaceas. The ripe spo- 

 rangium, moreover, is strongly oblique like that of Schizcea. 



As the sporocarp ripens the outer cells become excessively 

 hard, especially the first layer of hypodermal cells (Fig. 256), 

 whose walls become so thick as to almost obliterate the cell 

 cavity. The second hypodermal layer is also thickened, but 

 not so strongly. At maturity the sporocarp of P. Americana 

 forms a globular body about 3 mm. in diameter, covered with 

 hairs, and attached to a long peduncle which bends downward 



