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 (invested 

 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. 



