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Published In: Encyclopédie Méthodique, Botanique 1(1): 343. 1783. (1 Dec 1783) (Encycl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

Project Name Data (Last Modified On 7/9/2009)
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Project Data     (Last Modified On 8/5/2009)


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1. Azolla Lam. (mosquito fern)

(Svenson, 1944)

Plants heterosporous, usually less than 2 cm long. Stems elongate, branching, sometimes minutely hairy, bearing short, unbranched roots. Leaves 0.4–1.5 mm long, alternate, 2-lobed; the floating lobe the smaller, green to dark red (later in season), with chlorophyll, lacking venation, nearly glabrous to densely and minutely papillose on the upper surface; the submerged lobe the larger, clear to translucent, lacking chlorophyll. Sori (sporocarps) 0.2–1.0 mm long (those with several microsporangia 0.7–1.0 mm, those with a single megasporangium 0.2–0.3 mm), attached at the base of the submerged leaf lobe, usually in groups of 2–4, the indusia forming ball-like structures enclosing the sporangia. Megasporangia each containing 1 megaspore; microsporangia each containing 64 or fewer microspores Megaspores 0.2–0.4 mm long, consisting of a globose basal portion and a triangular cap consisting of 3 (9 elsewhere) saclike floats covered by the remains of the indusium. Microspores 10–27mm in diameter, trilete, globose, imbedded in amorphous structures known as massulae. Massulae 3–4 per sporangium, covered with hairlike structures with 0–6 crosswalls and triangular, barbed tips known as glochidia. Gametophytes endosporic within the megaspore walls. About 7 species. Worldwide.

The name mosquito fern refers to the belief that populations can grow so densely on the water surface that mosquitoes are unable to breed.

The reproductive structures of this small fern genus are extraordinarily complex, as indicated by the specialized terminology above. The floats help to disperse megaspores in the water, and the glochidia apparently become entangled with hairs on the megaspores, insuring that microspores remain close to the megaspores prior to germination. However, fertile material is infrequently encountered in nature, and reproduction is mostly asexual, by fragmentation of plants and subsequent spread to new sites by waterfowl. Although the term sporocarp has been applied by some authors for the highly modified sori of these plants, they are not homologous to the sporocarps of the Marsileaceae, which are modified leaves.

Azolla plants have chambers in the floating leaf lobes that contain a symbiotic, filamentous cyanobacterium, Anabaena azollae Strasb. These blue-green bacterial filaments are capable of fixing atmospheric nitrogen into a nitrate form that can be utilized by other plants as a fertilizer. The culture of some Azolla species as a green manure in rice paddies has a long history in the Orient, and the genus is currently of tremendous and worldwide economic importance in agriculture.

Although the taxonomy of the Old World section Rhizospermae (2 Old World species with 9 floats per megaspore) is relatively well understood, the about 5 species of section Azolla (with 3 floats per megaspore) is still in turmoil. Many recent authors have emphasized differences in megaspore morphology, but these are very rarely produced in some species. Some authors have questioned whether any microspore or vegetative features will consistently separate the two species listed below, which makes species determinations relatively uncertain. The characters in the following key seem to separate the small number of Missouri collections available in the herbaria.


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1 Plants usually less than 1 cm long; floating leaf-lobes 0.4–0.5 mm long; glochidia with an average of 0 or 1 crosswall 1 Azolla caroliniana
+ Plants often more than 1 cm long; floating leaf-lobes 0.6–0.8 mm long; glochidia with an average of 2–4 crosswalls 2 Azolla mexicana
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