Azolla can be placed in its own family, Azollaceae, but is often included with Salvinia in the family Salviniaceae. Salviniaceae and Marsileaceae (Marsilea, Pilularia and Regnellidium) form a monophyletic group of water ferns, which are all heterosporous, ie. produce two kinds of spores, large female megaspores and small male microspores.
Many fossil species of Azolla are known from the Upper Cretaceous. Today there are only 6 or 7 species in this genus.
Azolla species are difficult to identify because high magnification is required to see the distinguishing characters clearly. In addition, the plants are often sterile and thus lack most of the features essential for identification. This has led to many misidentifications, confused taxonomy and thus uncertainty over species distributions.
The small plants of Azolla filiculoides are prostrate, with branching thread-like horizontal stems generally up to 5cm long.
Numerous unbranched roots up to 5cm long hang below the plant, arising from the underside of the stem at branching points.
The tiny leaves (no more than 1.5 x 2.5mm in size) are alternately arranged in 2 rows on the upper side of the stem and overlap like roof tiles.
Each leaf is divided into 2 lobes:
Spores are produced inside sporocarps. The sporocarps are borne in pairs (sexes the same or different) on the first leaf of each branch, the lower lobe forming the sporocarp and the upper lobe forming a false indusium over the sporocarp:
Azolla filiculoides 2 large microsporocarps with 1 small megasporocarp with dark indusium cap. Photo credit H Taylor.
Azolla caroliniana and A. mexicana are very similar to A. filiculoides. Although A. filiculoides is the common species in western Europe, both the others have been recorded, though some of the identifications are questionable. All three species are native in the Americas.
They differ principally in:
Rich in nutrients.
Barbed hairs on massulae.
Having 2 kinds of spores.
Tissue that protects the sporangia.
Mass of microspores and associated tissue (term is also applied to female ‘floats’ in the megaspore apparatus).
Descended from a single common ancestor.
Minute projection on the surface of a leaf.
Structure that contains the sporangia.
An organism in a close and long-term association with another species.