VII PTERIDOPHYTA—FILICINE^—OPHIOGLOSSACE^ 2S7 



rows of cells may be seen extending from the mass of arche- 

 sporial cells to the periphery. In these rows the vertical walls 

 have been more numerous than in the adjacent ones, so that 

 the number of cells in these rows is greater. It is between 

 these rows of cells that the cleft is formed by which the ripe 

 sporangium opens. The outer cells of the sporogenous tissue 

 do not develop into spores, but constitute the "tapetum" (Fig. 

 140, B, t), which serves to nourish ihe developing spores. 



After the full number of cells is reached in the archesporium, 

 their walls become partially disorganized, and the cells round 

 off and separate, exactly as in the sporogonium of a Bryophyte, 

 and each cell is, potentially at least, a spore mother cell. 

 Bower (16) states that only a part of the cells produce spores, 

 and that the rest remain sterile and serve with the disorganised 

 tapetal cells to nourish the growing spores. The final division 

 of the spore mother cells into four spores is identical with that 

 of the Bryophytes. 



At maturity the sporangium opens by a cleft, whose position 

 is indicated as we have seen in the younger stages, and as the 

 cells shrink with the drying of the ripe sporangiophore the 

 spores are forced out through this cleft. 



Ophioglossum vulgatum and the other terrestrial forms 

 show some points of difference when compared with 0. pen- 

 dulum. These grow much more slowly, and longitudinal sec- 

 tions of the upper part of the subterranean stem show several 

 leaves in different stages of development. Each leaf rudiment, 

 as in O. pendulum, is covered by a conical sheath, formed at 

 the base of the next older leaf, and these sheaths are open at the 

 top, so that there is direct communication between the outside 

 air and the youngest of these sheaths which encloses, as in the 

 latter species, the youngest leaf rudiment and stem apex (Ros- 

 towzew ( I ) , p. 45 1 ) . In these terrestrial forms, also, the 

 sporangiophore is longer stalked, and the lamina of the leaf 

 more clearly separated from the petiole, which is not continued 

 into it. The lamina is relatively broader and the venation more 

 complex, in some species showing also free endings to the ulti- 

 mate branches. The sporangia, too, project more strongly 

 and are very evident (Fig. 132). Branching of the roots 

 occurs occasionally, and according to Rostowzew may be either 

 spurious or genuine. In the first place an adventive bud, which 



ordinarily would develop into a stem, develops a single root and 

 1.7 



