PTERIDOPHYTES 



139 



spiral, and every leaf is a sporophyll, either bearing a sporangium or 

 traces of one. In thjs sense the whole sporophyte body is a strobilus. 

 Each leaf is distinctly differentiated into sporangium and foliage regions 

 (fig. 317). The foliage portion of the leaf resembles a narrow grass 

 blade, and contains four longitudinal series of air chambers. At the 

 base of this blade, on the adaxial side, the ligule appears, socketed in a 

 small pitlike depression. Below the 

 ligule the sporangium region occurs, 

 the sporangium developing in a 

 large deep chamber more or less 

 shut off from the outside by a 

 curtain of tissue (velum). This 

 single large sporangium on the 

 adaxial surface of the sporophyll is 

 a very important character relating 

 Isoetes to the other Lycopodiales. 



Sporangia. The sporangium re- 

 sembles also that of the Lycopodiales 

 in arising from a transverse row of 

 initial cells, in this case three or four 

 in number. The method of develop- 

 ment is as usual in eusporangiates, 

 beginning with a periclinal division 



that differentiates the outer wall 



. FIGS. 316, 317. Sporangia of Isoetes: 



Cells trom the inner sporogenous 3l6 , cross section of lower region of leaf, 



Cells. The wall becomes about showing a microsporangium, with its 



four-layered, the innermost layer trabeculae (sterile plates) and numerous 



. . . microspores; 317, longitudinal section of 



entering into the organization of lower region of leaf> showing a mega- 



the tapetal jacket. A large mass sporangium, with its trabeculae and rela- 



Of sporogenous tissue is developed, tivel y few megaspores; also the ligule 



' (above) and the velum extending over the 

 and Up to 15,000-25,000 cells aU Sp0 rangial chamber. 



sporangia are alike. At this stage 



the differences that result in heterospory begin to appear. In those 

 sporangia that are to become microsporangia some of the sporoge- 

 nous tissue forms plates of sterile cells (trabeculae) extending across 

 the sporangium, and all the other cells function in spore formation, 

 producing in a single sporangium 150,000-300,000 microspores (fig. 

 316). In those sporangia that are to become megasporangia, the 

 trabeculae are more massive, and most of the thousands of sporogenous 



317 



