PTERIDOPHYTES 
175 
the nutritive region from the reproductive region. The small apical cell 
protrudes through the megaspore wall and develops an exposed tissue 
containing archegonia (fig. 406). The nucleus of the large nutritive cell 
(almost the entire body of the megaspore) remains undivided in Salvinia, 
but in Azolla it initiates a series of free nuclear divisions, no cell walls 
FIGS. 403-405. Azolla: 403, massula, with in- 
closed microspores (shaded) andglochidia; 404, de- 
velopment of male gametophyte, the extruded cell 
(antheridium initial) beginning to divide; 405, male 
gametophyte with antheridium complete (wall cells 
enclosing spermatogenous cells). After CAMPBELL. 
FIG. 406. Azolla: female ga- 
metophyte, showing the extruded, 
archegonium-producing tissue and 
the large nutritive cell (iuvested 
by the megaspore wall); around 
the gametophyte are remains of 
the perinium, and above a part 
of the indusium is represented. 
After CAMPBELL. 
being formed. The two chief points of contrast in this developmental 
history, as compared with Selaginella and Isoetes, are (i) the develop- 
ment of a wall across the spore in connection with the first nuclear 
division, forming a diaphragm between the nutritive and reproductive 
regions; and (2) the failure to develop a nutritive tissue. 
Embryo. The development of the embryo differs in no way from 
that of true ferns, except that the first division of the egg is transverse 
to the long axis of the archegonium, a feature characterizing the primitive 
Marattiaceae, but not the modern leptosporangiates. 
