PTERIDOPHYTES 



139 



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

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

 Each leaf is distincdy 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 pitiike 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 ^^ith a periclinal division 

 that differentiates the outer wall 

 cells from the inner sporogenous 

 cells. The wall becomes about 

 four-layered, the innermost layer 

 entering into the organization of 

 the tapetal jacket. A large mass 

 of sporogenous tissue is developed, 

 and up to 15,000-25,000 cells all 

 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 



Figs. 316, 317. — Sporangia of 7jocfcj. • 

 316, cross section of lower region of leaf, 

 showing ^ microsporangium, with its 

 trabeculae (sterile plates) and numerous 

 microspores; 317, longitudinal section of 

 lower region of leaf, showing a mega- 

 sporangium, with its trabeculae and rela- 

 tively few megaspores; also the ligule 

 (above) and the velum extending over the 

 sporangial chamber. 



