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 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 



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 sporangium, with its trabeculae and rela- 

 f .. j i j tivelv few meeaspores; also the ligule 



of sporogenous tissue is developed, ^ and the g ve ^ m ^^ over g the 



and Up to 15,000-25,000 cells all S porangial 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. 



3l6)3I7 ._ Sporang!aof lsxtes! 



31 5, cross section of lower region of leaf, 

 showing a microsporangium, with its 



i ower reg ion of leaf, showing a mega- 



the 

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



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



