THE PTERIDOPHYTA : FILICALES, THE FERNS 



NECK ..==^ .NECK 



547 



COVER 



CELL 



CENTRA 

 CELL 



VENTRAL 



CANAL 



CELL 



NECK CANAL 

 CELL 



NECK CANAL 

 CELL 



Fig. 549. — Botrychium virginianum. Development of the archegonium 

 from a superficial cell of the prothallus. (A to D after Campbell, 

 E after Jeffrey. ) 



Hydropterideae : Salviniaceae 



The Salviniaceae are small floating plants which are often referred to as 

 the " Water Ferns." They stand apart from all the other Filicales in being 

 heterosporous, forming microspores which produce male prothalli and 

 megaspores which produce female prothalli, both being enclosed within 

 the spore walls. Both types of spores are found in separate sori, which are 

 each covered by a closed indusium, forming a sporocarp. 



Azolla filiculoides 



Azolla is a very small leafy plant, floating freely on the water surface 

 (Fig. 550). There are only three species, all tropical or sub-tropical. Our 

 type is native to America. Another species, A. caroliniana, is often dis- 

 tinguished from it, but the differences are not constant. About seventy 

 years ago it arrived in southern Europe, from whence it has spread north- 

 wards and is now found in many parts of Britain. It is easily recognizable, 

 covering ponds and ditches, and having a striking crimson colour and a 

 feathery appearance. 



The delicate stems bear alternating leaves in two rows on the upper 

 surface. Each leaf is divided into two unequal parts. The upper lobes are 

 somewhat smaller and overlap closely along the stem. They have a well- 

 marked palisade tissue on their upper (abaxial) surfaces, but have stomata 

 on both surfaces (Fig. 551). An aperture on the lower (adaxial) surface 

 opens into a slime-filled cavity in the leaf tissue (Fig. 552), in which lives, 

 possibly symbiotically, a Blue-green Alga, Atiabaena azoUae, which is credited 

 with the power of Nitrogen fixation. These upper lobes contain chlorophyll 



