The water ferns are so-called because of their aquatic or semi-aquatic habitats. There are two families the free-floating mosquito ferns, Azolla and Salvinia, and the rhizomatous clover ferns, Marsilea, Regnellidium, and Pilularia. The two pair of pinnae (leaflets) sported by Marsilea look a bit like clover, but the venation is all wrong, so they used to use them in the manufacture of good luck charms where they were embedded in plastic as a fob in lieu of getting lucky enough to actually find a four-leafed clover. Regnellidium has just one pair of pinnae, and Pilularia has no pairs having a pinnae-less frond (a pun-loving friend used to call this the "pauper" fern). These ferns grow in shallow water their rhizome rooted along the bottom. The leafier genera have emergent leaves, while the leafless genus can grow completely submerged. Azolla is used in aqua-culture, primarily rice paddies, because it has a symbiotic interaction with the cyanobacterium Anabaena, a nitrogen-fixer. They do lots of interesting things in addition. They form seed-like fertile leaves for surviving droughts. They are the only heterosporous ferns, large spores forming female gametophytes, and small spores forming male gametophytes, modifications for rapidly getting through the haploid phase of their life cycle. And they look pretty cool although not very fern-like in the traditional sense. The image shows one node of Salvinia with a whorl of three leaves, two floating leaves, and a root-like disected submerged leaf, which is also the fertile frond (round sori are visible).
in The Greenhouse