PTEEIDOPHYTA— HYDROPTEEIDE.E 147 



in almost all other cases, they either have a third coat, derived 

 from the epiplasm of the disorganised tapetal cells, or they 

 are by this epiplasm agglutinated together into one or more 

 spore masses. The conditions differ in the different genera. In 

 Salvinia all the microspores of a sporangium are fastened to- 

 gether ; in AzoUa there are from two to eight of such masses in 

 a sporangium, each being known as a massula. A delicate skin 

 surrounds each massula, and this in some species is furnished 

 with a number of hairs bearing barbed processes, known as 

 glochidia, at their free ends. In the Marsileaceae the microspores 

 are free from each other, but each is coated by its epispore or 

 periniiim, derived from the tapetal epiplasm. 



In Salvinia, Pilularia, and Marsilea the macrospores are 

 similarly invested, but each one is free. The outer layer of the 

 epispore in the last two genera is capable of sw^elling enormously 

 on being wetted, surromiding the apex of the spore with a mu- 

 cilaginous coating. 



In AzoUa the epispore on the lower surface of the macrospore 

 is developed into large spongy masses which serve as floats, 

 enabling it to drift about after partially escaping from the 

 sporangium. The upper surface is firmer and bears filamentous 

 outgrowths. The apex of the spore is generally furnished with 

 a number of delicate filaments extending between the floats. 

 The glochidia of the massulae of microspores generally catch in 

 these filaments, so that the massulae are anchored to the macro - 

 spore. 



The microspores are not set free from the microsporangium 

 in Salvinia, but germinate iji situ. In Azolla the separate 

 massulae escape and float about in the water, those that have 

 glochidia usually becoming entangled in the filaments developed 

 from the perinium of the macrospore. 



The gametophyte in this group shows considerable reduction 

 when compared with that of the ferns. The heterospory, as 

 already pointed out, involves the production of two kinds of 

 gametophyte, one from each kind of spore. We have from the 

 microspore one that bears only male organs, anther idia, and 

 from the macrospore one that bears only female organs, 

 archegonia. In neither case does the gametophyte become 

 entirely free from the spore which gives it origin ; in some 

 cases a good part of it remains enclosed within the spore. 



In Salvinia the microspores germinate by putting out a 

 tubular protrusion of the endospore, which pierces the nnicila- 

 ginous matter in which they are embedded and makes its way 



L 2 



