302 



PLANT MORPHOLOGY 



there is only one antherirliiim and it produces eight sperms (Fig. 257C). 

 The sperms are coiled and multiciliate. 



In the formation of the female gametophyte, the nucleus of the mega- 

 spore divides near the apical end and a small lenticular cell is cut off. 

 The larger cell later undergoes free-nuclear division, but no walls are 

 formed and the cell becomes a food reservoir. The smaller cell gives rise 

 to a tissue that breaks through the heavy megaspore wall and produces 



Fig. 256. Sporangia of Salvinia rotundifolia. A, longitudinal section through a young 

 microsporocarp, XlOO; B, young microsporangium, showing primary sporogenous cell 

 surrounded by tapetum, X600; C, older microsporangium, showing spores embedded in 

 hardened tapetal plasmodium, X160; D, developing megasporangium, showing young 

 functional megaspore surrounded by tapetal nuclei, the nonfunctional megaspores near the 

 wall. X280. 



several archegonia (Fig. 257D). This tissue turns green and becomes 

 rather extensive in Salvinia, but in Azolla is smaller and has little or no 

 chlorophyll. The archegonia resemble those of Marsilea, except that the 

 single neck canal cell is usually binucleate. 



Embryo. In both genera the fertilized egg, by means of two divisions 

 at right angles to each other, gives rise to quadrants, but the first wall is 

 longitudinal in Salvinia and transverse in Azolla. The relation of the 

 four primary organs to one another is the same as in the other leptospo- 

 rangiate ferns. 



Summary. The Hydropteridales are heterosporous and leptosporan- 

 giate. The sporangia are borne in sporocarps representing either a mod- 

 ified leaf segment (Marsileaceae) or a modified indusium (Salviniaceae). 

 Both microsporangia and megasporangia occur in the same sporocarp 

 (Marsileaceae) or in separate sporocarps (Salviniaceae). The sporan- 



