GROUP III. PTERIDOPHYTA : FIL1CINJE ; HYDROPTERIDE^E. 413 



(see p. 407) begins. In the other three genera, two of the epi- 

 basal octants give rise to the first cotyledon, one to the growing- 

 point of the stem, and the fourth to a second cotyledon, so that 

 here there are two cotyledons : the hypobasal octants give rise 

 to foot and root in the usual manner. 



THE G-AMETOPHYTE. As these plants are heterosporous, the 

 gametophyte is represented by distinct male and female indivi- 

 duals : these remain connected with the spores producing them. 



The male individual is developed from a microspore: it consists 

 of a rudimentary prothallium bearing generally a single male 

 organ (antheridium), though in Salvinia there are apparently two 

 antheridia. 



In Salvinia the germination of the microspores takes place with- 

 in the microsporangium ; the inner coat (endospore) of the spore 

 grows out as a longer or shorter tube through the ruptured outer 

 coat (exospore), and eventually makes its way through the wall of 

 the sporangium so that its free end is in the water outside : a trans- 

 verse wall is formed within it which cuts off the apical portion of 

 the tube as the fertile portion of the prothallium. 



In Azolla the microspores germinate within the massula. The 

 exospore of the tetrahedral spore ruptures along the three edges, 

 and the endospore protrudes as a papilla at the apex. A transverse 

 wall is formed across the base of the papilla, which now becomes 

 the mother-cell of the single antheridium. The spermatozoids 

 probably escape from the massula on the deliquescence of its sub- 

 stance. In the Marsileacea3 the male prothallium is formed alto- 

 gether within the microspore : the spore divides into two cells, a 

 smaller basal cell which represents the vegetative portion of the 

 prothallium, and a larger one which is the mother-cell of the 

 antheridium. In all cases the prothallium has no chlorophyll. 



The male organ, or antheridium, is developed from the aiitheridial 

 mother-cell mentioned above. It generally undergoes divisions so 

 as to form a central cell surrounded by a single layer of cells form- 

 ing the wall of the antheridium. But Salvinia is peculiar in that the 

 central cell of each antheridium is not completely surrounded by a 

 parietal layer of cells, but comes to the surface of the antheri- 

 dium. The central cell then undergoes further divisions to form 

 the mother-cells of the spermatozoids of which there are eight in 

 the Salviniaceae (four in each antheridium of Salvinia) and thirty- 

 two in the Marsileaceae. 



The male cells, or spermatozoids, resemble those of the homo- 



