Lemnaceae

Common duckweed Lemna minor, copyright Mokkie.

Belongs within: Araceae.

The Lemnaceae, duckweeds, are a cosmopolitan group of small to minute floating aquatic plants. The body of the plant is reduced to a few flattened leaf-like fronds or undivided thallus (Hillman 1961).

The thalli that are green
Published 6 July 2010

The Araceae is the family of plants including such widely-grown species such as calla lilies and taro. Among other things, the family is famous for including some of the largest floral structures in the world. What is perhaps less widely appreciated is that it also includes some of the smallest and also some of the strangest flowering plants of all*.

*The very strangest flowering plants are the Podostemoideae, but I’ll save them for another day.

Wolffia arrhiza, one of the world’s smallest flowering plants. Each separate dot is an individual plant. Photo by Christian Fischer.

Duckweeds are the minute plant that can be found growing in large numbers on many still bodies of water. The main body of the plant is a flattened, often oval thallus (not a leaf but rather a highly reduced and fused leaf and stem). In three of the recognised genera of duckweeds, Lemna, Spirodela and Landoltia (the ‘Lemneae’), one or more short roots emerge from one end of the thallus (the proximal end). In the other two genera, Wolffia and Wolffiella (the Wolffieae), the thallus lack roots. The vascular system is greatly reduced in ‘Lemneae’ and almost entirely absent in Wolffieae. Much of the thallus is occupied by gas-filled spaces that keep it buoyant (Hillman 1961). The proximal end of the thallus also bears pockets on the underside from which daughter thalli grow vegetatively or in which the flower develops. The ‘Lemneae’ possess two such pockets, one on each side, while the Wolffieae possess only a single pocket. The entire plant is generally less than five millimetres (the size reached by Landoltia punctata) while Wolffia individuals are less than half a millimetre long when mature. Phylogenetic analysis indicates that the ‘Lemneae’ are paraphyletic; Lemna, which is smaller than Spirodela and Landoltia with only a single rooth per thallus, is sister to the even more reduced Wolffieae (Les et al. 2002).

Spirodela (large thalli), Lemna (smaller thalli) and Wolffia (minute thalli). Photo by G. D. Carr.

The flowers of duckweed are correspondingly tiny and many species produce them only rarely with vegetative reproduction being the primary means of multiplication (Hillman 1961). Daughter thalli may begin producing their own daughters before separating from the parent, leading to the production of small colonies. Individuals of the double-pocketed ‘Lemneae’ demonstrate ‘handedness’ in their growth; in a new thallus grown from seed, either the right- or left-hand pocket may be the ‘plus’ pocket from which the first daughter thallus grows, but all successive vegetatively produced thalli will grow their own first daughter thallus on the same side as their parent did. If a flower is produced (and each individual thallus will only ever flower once) then it will always grow on the other side in the ‘minus’ pocket. It is a bit of an open (and somewhat academic) question whether duckweeds produce a single hermaphroditic flower or separate male and female flowers, as there are no petals or sepals, but the important detail is that a thallus produces a single pistil and one to three stamens (usually two in ‘Lemneae’, only ever one in Wolffieae; Wolffieae also only possess a single ovule while ‘Lemneae’ may possess up to six) which project above the surface of the thallus. The pistil matures before the stamens but may remain receptive until after the stamens open so at least some duckweeds are capable of self-pollination. The exact mode of pollen transport is uncertain: pollen may be carried by wind or water but transport by small insects has also been proposed. Seeds are capable of surviving periods of drying out; many species of duckweed are also capable of producing a dormant form called a turion, a modified, thicker thallus that lacks the air spaces of a normal thallus and possesses a dense load of starch grains instead.

Thick growth of Landoltia punctata. Photo by A. Murray.

Though most authors have regarded them as a separate family, the Lemnaceae, a relationship between duckweeds and Araceae has been popular since the 1800s. The main connecting link has been through comparison with the floating aroid Pistia stratiotes which resembles duckweed both in its general lifestyle and in the production of its flowers in basal pockets. Some authors have even proposed including Pistia in the Lemnaceae and the Palaeocene fossil plant Limnobiophyllum (with reduced floating rosettes) has been suggested as a morphological link between the two (Stockey et al. 1997). Molecular studies, while supporting a nested position for duckweeds within Araceae (which is why I refer to them as subfamily Lemnoideae), have not supported a direct relationship between duckweeds and Pistia (Rothwell et al. 2004); however, duckweeds show much greater branch lengths than other Araceae.

Systematics of Lemnaceae

Characters (from Healy & Edgar 1980): Very small gregarious aquatic perennials, usually less than 5 mm long, floating on, or just under, surface of still or slowly moving water, often together forming a complete cover; consisting of more or less flattened leafless green platelets, with or without roots, each new platelet arising from a bud in a horizontal pouch on an older platelet. Inflorescence rarely seen, produced in a similar pouch, of 1–2–(3) male flowers maturing successively, and 1 female flower, the whole enclosed within or associated with a membranous spathe. Male flower a single stamen; filament projecting well beyond spathe; pollen sacs 1–2, rounded, splitting transversely; pollen globular. Female flower of rounded ovary and short erect style; carpel single, ovules 1–6, basal. Fruit flattened, sometimes more or less winged. Seeds relatively large, smooth or ribbed.

<==Lemnaceae
| i. s.: Lemnospermum pistiforme Nikitin 1976CBH93
|--Wolffieae [Wolffioideae]BG96
| |--WolffiellaH61
| | |--W. floridanaH61
| | |--W. lingulataH93
| | `--W. oblongaH93
| `--Wolffia Horkel ex Schleiden 1839ME70
| |--W. arrhiza (Linnaeus) Wimm. 1857 [=Lemna arrhiza Linnaeus 1771]ME70
| |--W. australiana (Bentham) den Hartog & van der Plas 1972HE80 (see below for synonymy)
| |--W. borealisH93
| |--W. brasiliensisH93
| |--W. columbianaH93
| |--W. globosa [incl. W. cylindracea]H93
| |--W. microscopicaH61
| `--W. punctataV72
`--Lemneae [Lemnoideae]H61
|--Spirodela Schleiden 1839H61, ME70
| |--S. polyrhizaHE80 [=Lemna polyrhizaHE80; incl. L. majorH61]
| `--S. punctata (Meyer) Thompson 1897HE80 (see below for synonymy)
`--Lemna Linnaeus 1753ME70
|--L. aequinoctialis [incl. L. paucicostata]H93
|--L. dispermaME70
| |--L. d. var. dispermaME70
| `--L. d. var. fallaxME70
|--L. gibbaH93 [=Telmatosphace gibbaC55]
|--L. minor Linnaeus 1753ME70 [incl. L. perpusillaV72]
|--L. minuscula [incl. L. minima, L. minuta]H93
|--L. paucicostataH61
|--L. perpusillaPP07
|--L. trisulcaME70
|--L. turioniferaH93
`--L. valdiviana [incl. L. cyclostasa]H93

Spirodela punctata (Meyer) Thompson 1897HE80 [=Lemna punctata Meyer 1818HE80; incl. L. melanorrhizaB78, L. oligorrhiza Kurz 1866HE80, Spirodela oligorrhiza (Kurz) Hegelm. 1868ME70, L. pleiorrhizaB78]

Wolffia australiana (Bentham) den Hartog & van der Plas 1972HE80 [=W. arrhiza var. australiana Bentham 1878HE80; incl. W. micheliiB78]

*Type species of generic name indicated

References

[B78] Bentham, G. 1878. Flora Australiensis: A description of the plants of the Australian Territory vol. 7. Roxburghiaceae to Filices. L. Reeve & Co.: London.

[C55] Candolle, A. de. 1855. Géographie Botanique Raisonée: Ou exposition des faits principaux et des lois concernant la distribution géographique des plantes de l’époque actuelle vol. 2. Librairie de Victor Masson: Paris.

[CBH93] Collinson, M. E., M. C. Boulter & P. L. Holmes. 1993. Magnoliophyta (‘Angiospermae’). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 809–841. Chapman & Hall: London.

[HE80] Healy, A. J., & E. Edgar. 1980. Flora of New Zealand vol. 3. Adventive cyperaceous, petalous and spathaceous monocotyledons. P. D. Hasselberg, Government Printer: Wellington (New Zealand).

[H93] Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley (California).

[H61] Hillman, W. S. 1961. The Lemnaceae, or duckweeds: a review of the descriptive and experimental literature. Botanical Review 27 (2): 221–287.

Les, D. H., D. J. Crawford, E. Landolt, J. D. Gabel & R. T. Kimball. 2002. Phylogeny and systematics of Lemnaceae, the duckweed family. Systematic Botany 27 (2): 221–240.

[ME70] Moore, L. B., & E. Edgar. 1970. Flora of New Zealand vol. 2. Indigenous Tracheophyta: Monocotyledones except Gramineae. A. R. Shearer, Government Printer: Wellington (New Zealand).

[PP07] Pandey, R. P., & P. M. Padhye. 2007. Studies on phytodiversity of Arid Machia Safari Park-Kailana in Jodhpur (Rajasthan). Bulletin of the Botanical Survey of India 49: 15–78.

Rothwell, G. W., M. R. Van Atta, H. E. Ballard Jr & R. A. Stockey. 2004. Molecular phylogenetic relationships among Lemnaceae and Araceae using the chloroplast trnL–trnF intergenic spacer. Molecular Phylogenetics and Evolution 30 (2): 378–385.

Stockey, R. A., G. L. Hoffman & G. W. Rothwell. 1997. The fossil monocot Limnobiophyllum scutatum: resolving the phylogeny of Lemnaceae. American Journal of Botany 84 (3): 355–368.

[V72] Voss, E. G. 1972. Michigan Flora. Part I. Gymnosperms and Monocots. Cranbrook Institute of Science and University of Michigan Herbarium.

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