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 showing a microsporangium, with its 
four-layered, the innermost layer trabcculae (sterile plates) and numerous 
J f . microspores ; 317, longitudinal section of 
entering into the organization of i ower reg ion of leaf, showing a mega- 
the tapetal jacket. A large mass sporangium, with its trabeculae and rela- 
Of sporogenous tissue is developed, 
r 
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 Isoetes: 
316, cross section of lower region of leaf, 
*** ^w megaspores; also the ligule 
(above) and the velum extending over the 
sporang ial chamber. 
