65 



or male gametes, and the macro sporangia, each bearing one large 

 spore only, which develops into a prothallus on which in its turn 

 develop the female archegonia with their contained egg, or female 

 gamete. 



The microsporangia are grouped in spherical indusial en- 

 velopes that are easily seen on the plants in late summer and 

 autumn. The macrosporangial sorus is small and contains one 

 sporangium only. The two sori are quite distinct and grow to- 

 gether on the apices of the submerged lower lobe of the lowest 

 leaf of a branch. Both make interesting microscopic objects. To 

 mention one feature only out of many. When mature the micro- 

 spores within each sporangium are enclosed in separate masses 

 of metamorphosed protoplasm, the massulae, and attached to 

 these are numerous hooked hairs, the glochidia. In A. filiculoides 

 the stalks of the glochidia are non-septate ; in .4. caroliniana they 

 have 3-5 transverse walls. The massulae are set free by the decay 

 of the sporangial wall and float out. The developing macrospore 

 is similarly released, the projecting filaments of its wall being 

 ready to catch on to the glochidia of a massula. Fertilisation of 

 the egg in the mature archegonium follows on the surface of the 

 water. 



The sporangia of Pilularia, like those of Azolla, are of two 

 kinds, but the resemblance does not go further. Here they are 

 grouped first into sori and these again in groups of four to form 

 with metamorphosed leaf segments the so-called " sporocarps " — 

 globular bodies seen at the stem nodes. Within each sporocarp 

 both kinds of sporangia occur, the micro- and macrosporangia. 

 When ripe the sporocarp bursts at the apex and emits a mucilage 

 carrying the spores already liberated from the two different kinds 

 of sporangia. The germinating spores give rise almost entirely 

 to the sex organs, the purely vegetative parts of the prothalli being 

 represented by a few vestigial cells only. Fertilisation takes 

 place and the embryo of the ordinary plant develops. 



The vegetative structure of Azolla shows many interesting 

 features. The plants float lightly on the surface of the water in 

 dense tufts, with the ends of the shoots curved upwards a little. 

 The leaves are numerous and closely set together, and consist of 

 two nearly equal lobes ; the upper, floating and emergent and the 

 lower submersed. JThe upper lobe bears many unicellular hairs on 

 its upper surface, which keep it from being wetted, and shows a 

 large cavity near the base in which reside as a sort of " room- 

 parasite " the blue-green alga Anaboena. The colour of the freshly 

 grown leaves is bright green, but towards the end of the summer 

 it becomes a brilliant red : the appearance of Jumper's Pond when 

 completely covered by a blood-red sheet of Azolla at the end of the 

 dry summer of 1914 was very striking. The olive green roots are 

 well developed, numerous and fairly long. In the winter the mass 

 of Azolla growing in the pond was flooded over upon the surround- 

 ing marshy border. Here the plants appeared to grow for a while 

 <on the damp surface after the water had retired, but failed to make 



