378 PART IV. CLASSIFICATION. 



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 GAMETOPHYTE. 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, represented by a single cell, bearing 

 a single male organ (antheridium). 



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 

 substance. 



In the Marsileaceae the -male prothallium is formed altogether 

 within the microspore ; the spore divides into two cells : a small 

 basal vegetative cell 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 antheri- 

 dial mother-cell mentioned above. It generally undergoes divisions 

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

 forming the wall of the antheridium. But Salvinia is peculiar in 

 that the central cell of each antheridium is not completely sur- 

 rounded by a parietal layer of cells, but comes to the surface of the 

 antheridium. The central cell then undergoes further divisions to 

 form the mother-cells of the spermatozoids, of which there are eight 

 in the Salviniaceae and thirty-two in the Marsileacese. 



The male cells are spermatozoids, which resemble those of the 

 homosporous leptosporangiate Ferns in all essential features of 

 their form and development, as also in their extrusion from the au- 

 theridium. In the Marsileacese, the male prothallium is enclosed 

 within the microspore until the antheridium is mature, when the 



