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In this article we will discuss about:- 1. Characters of Lemnaceae 2. Distribution of Lemnaceae 3. Economic Importance 4. Affinities.
Characters of Lemnaceae:
Floating or submerged perennial herbs without roots or with unbranched rhizoids; leafless, monoecious; flower unisexual, naked; staminate flower with one or 2 stamens; pistillate flower with unicarpellary, unilocular; fruit a utricle.
A. Vegetative characters:
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Habit:
Floating or sub-merged perennial herbs; minute, free floating fresh water plants.
Root:
Plants without roots or the roots reduced to unbranched rhizoids.
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The vegetative plant body of Lemna is almost undifferentiated thallus like, it consists of dorsiventral thallus like flat green structure, on the ventral of which adventitious roots come out and comprise apical and basal tapering part, at the ventral portion of the basal part there are two pockets, from the axil of such pocket either branch shoots or inflorescence springs; so the pockets may be interpreted as leaves.
The vegetative body of Lemna according to Hegelmaier is a flat undifferentiated stem without any leaf but with two pockets. It is very distinct that the so called two pockets resemble scale leaves from the axil of which sympodial branches come out, so the entire floating flat body is a metamorphosed shoot like cladode.
Engler considers that the entire floating body is made up of both stem and leaf which are connected firmly in longitudinal direction; the apical part represents leaf, the basal part stem or axis; a longitudinal vein containing conducting vessel runs from base giving off two lateral branches towards the apical part.
Shoot:
Reduced flat dorsiventral shoot.
Plant body reduced to a small or minute oval, oblong flat or globose thallus, often purplish beneath.
Leaf:
Leafless.
Reproducing asexually by buds. The plant body overwintering in temperate regions by production of buds that sink below the surface into subtrate.
B. Floral characters:
Inflorescence:
A central female flower and two male flowers enclosed by a small membranous, spathe like bract.
Flower:
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Unisexual, monoecious perianthless.
Staminate flower:
Androecium:
Stamens 1-2, anthers 1 to 2 celled; filaments absent or if present filiform or fusifom.
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Pistillate Flower:
Gynoecium:
Carpel 1; ovary uniocular, sessile, ovules 1-7, basal, erect, anatropous; style and stigma 1, simple, stigma funnel shaped.
Fruit:
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Utricle.
Seed:
Seed with two seed coats, the fleshy outer and delicate inner; endosperm thin surrounding an embryo with one large cotyledon.
Pollination – by the agency of water, wind or insects.
Floral formulae:
Distribution of Lemnaceae:
The family comprises of 6 genera and 43 species. All the plants are indigenous throughout most of the country. The members are represented in fresh-water habitats throughout much of the world.
Economic Importance of Lemnaceae:
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The direct economic uses of family are unknown. They are serious weeds of still water and are important as a source of food for fish. The members impart foul odour to water.
Affinities of Lemnaceae:
Bessey placed Lemnaceae together with Aracaeae and Cyclanthaceae in Arales which according to him had evolved from Alismatales i.e., Helobieae, so Bessey supported helobiae origin of Lemnaceae. Renele put the family in the Spadiciflorae.
Most of the phylogenists agreed that the family represents a degenerate offshoot of the Aracae with origins probably from Pistia or ancestral stocks of close affinity.
Brooks (1940) on the basis of vegetative and floral anatomy and cytotogical studies, concluded that the Lemnaceae have been derived from the Araceae, and that the Lemnaceae have a very reduced structure and show a reduction series from Spirodela through Lemna to Wolffia.
The work by Mc Clure and Alston (1968) on the Lemnaceae represents an effort to supplement available taxonomic criteria by the use of comparative chemical data concerning the flavonoids. If Spirodela can be assumed as primitive in the family and Wolffia the most advanced, then there is a general tendency for the flavonoid chemistry to be more simplified in the supposedly derived genera.
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Common plants of the family:
1. Lemna paucicostata.
2. Spirodela polyrhiza.
3. Wolffia arrhiza.
They are all found in ponds, jheels, ditches as aquatic floating plants.
