Azolla Lam.
Descripción
Heterosporous, minute, free-floating or on mud; stems slender, branching, with unbranched roots and two rows of alternate, imbricate leaves; leaves bilobed, with an upper green lobe and colorless lower lobe; cavities in the leaves contain a nitrogenfixing cyanobacterium, Anabaena azollae; sporangia in nut-like unisexual sporocarps; microsporangiocarps globose with numerous microsporangia; megasporocarps each with single megasporangium containing a single megaspore, the microspores in 4 clumps (massulae) per sporangium, each massula with anchorshaped glochidia, the megaspore apparatus with 3 or 9 flotation lobes; x=22.A
Forma de vida
Hidrófita libreA
Distribución
México (Country) native and not endemicB
Categoría IUCN
No incluidaC
Categoría NOM-059-SEMARNAT-2010
No incluidaD
Discusión taxonómica
Azolla is a genus of aquatic, leptosporangiate, heterosporous ferns, and comprises perhaps six species, two widely distributed in the New World, ranging from the United States and Canada to Argentina. Four species are often cited in neotropical floras, e.g., Flora Mesoamerica (Moran in Davidse et al., 1995) and Peru (Tryon & Stolze, 1994). All these belong to sect. Azolla, characterized by the megasporocarps having three floats and the roots lacking root hairs; section Rhizosperma (Meyen) Mett. is apparently known natively only in the Old World and comprises two additional species (Saunders & Fowler, 1992). Especially within sect. Azolla, species boundaries are unclear, and for Mexico, we choose to recognize only two of the four commonly accepted species, regarding the other two as synonymous (at least as to the types) with the two we do treat.
Identification of species is based largely on microscopic characters: whether the glochidia on the microsporangia are septate or not (Svenson, 1944); megaspore surface ornamentation; zonation of the perine (Perkins et al., 1985); and number of cells in epidermal trichomes (Pieterse et al., 1977). Because the majority of specimens are sterile, reproductive character states often cannot be evaluated, and so identification is uncertain. Even if fertile material (indicated by “f” in the citation of specimens below) is available, species identification may not be possible, even with a dissecting microscope (40 times magnification); a compound microscope and even the scanning electron microscope may be necessary.
The duckweed genera Spirodela, Lemna, Wolffia, and Wolffiella (Lemnaceae) often occur mixed with Azolla, sometimes with 5 or 6 species on one sheet. Additional records for Azolla can often be found by checking herbarium specimens of these other small aquatics. All these genera have great proclivity for distribution by waterfowl and by humans.
Various species of Azolla have been used as a green manure, especially in rice paddies. As the plants dry and decay, they release nitrogen from the nitrogen-fixing cyanobacterial colonies of Anabaena azollae, contained in the leaf cavities of the Azolla.
Azolla is related to Salvinia, but the origins of both genera and their ties with the other ferns are remote and not easily discernible. Marsilea and allies have recently been shown to be the sister group to Salvinia and Azolla (Hasebe et al., 1995; Pryer, 1993; Pryer et al., 1995; Rothwell & Stockey, 1994) based on both paleontological and molecular evidence. The spores of extant Azolla appear to be relatively unchanged from the earliest known fossils of Azolla from the Lower Cretaceous.
A
Identification of species is based largely on microscopic characters: whether the glochidia on the microsporangia are septate or not (Svenson, 1944); megaspore surface ornamentation; zonation of the perine (Perkins et al., 1985); and number of cells in epidermal trichomes (Pieterse et al., 1977). Because the majority of specimens are sterile, reproductive character states often cannot be evaluated, and so identification is uncertain. Even if fertile material (indicated by “f” in the citation of specimens below) is available, species identification may not be possible, even with a dissecting microscope (40 times magnification); a compound microscope and even the scanning electron microscope may be necessary.
The duckweed genera Spirodela, Lemna, Wolffia, and Wolffiella (Lemnaceae) often occur mixed with Azolla, sometimes with 5 or 6 species on one sheet. Additional records for Azolla can often be found by checking herbarium specimens of these other small aquatics. All these genera have great proclivity for distribution by waterfowl and by humans.
Various species of Azolla have been used as a green manure, especially in rice paddies. As the plants dry and decay, they release nitrogen from the nitrogen-fixing cyanobacterial colonies of Anabaena azollae, contained in the leaf cavities of the Azolla.
Azolla is related to Salvinia, but the origins of both genera and their ties with the other ferns are remote and not easily discernible. Marsilea and allies have recently been shown to be the sister group to Salvinia and Azolla (Hasebe et al., 1995; Pryer, 1993; Pryer et al., 1995; Rothwell & Stockey, 1994) based on both paleontological and molecular evidence. The spores of extant Azolla appear to be relatively unchanged from the earliest known fossils of Azolla from the Lower Cretaceous.
A