Duckweed biology 

The representatives of the family Lemnaceae are the smallest flowering plants.  They are a distinct group of monocotyledonous plants.  Not in without reason they are called the most mysterious of the flowering plants on the Earth.  Because of their small sizes they sometimes remain elusive from casual observation.  The size of plants does not exceed 1 cm.  Common duckweeds (Lemna), greater duckweed (Spirodela), and water meal (Wolffia) are frequently encountered by a number in one place.

Lemnaceae - aquatic, either freely floating or submerged (for example, ivy leaf duckweed - Lemna trisulca), larger partly long-standing (a structly tropical duckweed, Lemna aequinoctialis, from Africa, is considered as the annual plant.  Lemnaceae - cosmopolitan plants (especially lesser duckweed,  Lemna minor), they extend throughout the entire world, with exception of hot dry deserts and cold polar regions.  About half of the 30 species of this family dwell in the tropics and the subtropics, the rest - in the temperate zone.  The vegetative body of Lemnaceae according to the form resembles the tiny floating sheet or the thallus of the lowest plants; thus for long time they considered to be algae.

No unanimous opinion has been agreed for a word to designate the body of these plants.  Until now, since it has been called a fillokladiyem, a vayey, a shield, a plate, a frond, a listets, a stem or a sheet.  In Russia, botanists more frequently is used the term of listets, in foreign countries - fronds.

The fronds of Lemnaceae grow either singly or connected in small groups or the networks by the short or elongated legs, formed by the narrow part of the frond (transparent thread) (V.V.Petrov, 1991).
 

Green fronds of Lemna gibba and  transparent thread  (waist of listetsa of white color).

Type of the right pocket of Lemna gibba.

 The shapes of fronds are most diverse:  reniform, rounded, elliptical, lancet, linear, globular or oval.  The vascular system in Lemnaceae is practically absent, with exception of Spirodela, in the roots of which there are tracheids.  Roots are absent or are weakly developed (Spirodela) and do not reach down to soil.  They are simple, with a root cap, on one or by beams emerging from the abdominal surface of the fronds and fulfilling mainly functions of anchors, which prevent the movement of plants in the water.  The middle part of the frods of Lemna and Spirodela is split by two lateral pockets, in which are embedded the vegetative buds, from which arise daughter fronds during vegetative multiplication.  Sometimes in one of the pockets a flower bud develops.  In the representatives of other species there is one pocket, and it serves exclusively for the vegetative multiplication, whereas the flower bud is highly simplified:  to 1-2, rarely 3 (in Spirodela) one male and one female flower.  A perianth is absent (I.E.Ivanova, 1971).  Male flowers consist of 1, rarely 2 stamens with size/dimension near 1 mm.  The measurement of a stamen is a little more than 1 mm.  Bilocular anthers are fastened to the sealed threads and are supplied with conical pointed svyaznikom.

The ovary is egg-shaped, is monothecal, with 1-6 germs; post short and thin/fine, and the stigma is extended in the form of shallow hopper or cup with the uneven edges.

  Flowering  in the family of Lemnaceae is plotted not in what known for the flowering plants rhythms, and it was observed so rarely which specially was recorded.  In Finland - are noted 33 cases of flowering from 1685-1947, in Poland - only 2 times with 1679-1959, in America the flowering of Spirodela in the last 200 years has been observed not more than 20 times.

In Armenia, according to the communication of A.L. Takhtadzhyana, in 1932 in the lakes of the Loriyakay highland plain they observed a mass flowering of duckweed. 

For the middle strip of Russia within the period from 1814 through 1978 in published reports appeared only about 25 findings of duckweed flowering.  In the ponds along the Volga river it was possible to observe the yearly flowering of duckweed from 1972 through 1975 (A.L.Takhtadzhan, 1982).  In the Krasnodar edge during June 1995 in Krasnodare was noted an intensive flowering of duckweed in Karasun lake and in the canals of the region of Thapsyg Reservoir, during May 2000  (L.V. Tsatsenko, 2000). 

It is possible that the flowering of Lemnaceae is not always noted by botanists because of the microscopic dimensions of the flowers.
 

Flowering duckweed.

Duckweeds flower in the summer period, usually during June - August, sometimes at the end of May or during September.  At the beginning of flowering more or less long chains separate into smaller groups, consisting of 4-5 fronds in Lemna gibba and of 2-4 fronds in Lemna trisulca.  Before beginning flowering, morphological conversions of fronds occur gradually.  The air chambers increase in Lemna minor, fronds prepared for flowering increase in the size, and  acquire dark green pigmentation.  In Lemna trisulca vegetative fronds are thinned, become pale, and its "top" is bent downward.  In Lemna gibba flowering fronds take on an olive- purple color (A.F.Pantyukh, 1986).

From the moment of the initiation of flowering fronds to the end of the process of flowering pass about 20 days.  For all Lemnaceae it is characteristic protogonia, i.e.the stigma of the female flower  ripens first, then stamen of one of the male flowers, and after this the stamen of the second flower.  The process of the flowering of Lemnaceae can be represented in the form of the following diagram:
 

From the pocket of flowering fronds first appears the stamen, which raises itself above the water.  In the mouth of stigma a large watery droplet of nectar then appears that apparently is used for attracting small insects.

With a touch, the droplet is pulled inside the snout together with any pollen fallen on it.  If dusting did not occur, the drop appears again in  several hours.  The stigma remains fertile over the course of twenty-four hours.  Staurigamia (cross pollination) occurs during this period.

The stamens of the second male flower are advanced from the pocket of flowering frond 24 h after the appearance of the first.  The anthers of the stamen of the first flower are closed until the moment of the propagation of pollen, and stamen itself descends into the water and it becomes transparent.  The stigma is destroyed at this moment and the pollen of the second flower can pollinate only adjacent flowering fronds.

The stamens of the first flower function about  60 h., the second -  90 h.

In the beginning in the mature anthers is revealed "entomophilous" brilliant yellow pollen, in the form of dense, moist clusters.  These clusters are torn off and fall on the surface of the green carpet of water plants, on which small insects frequently warp (water bulb scale mites, aphis).  Dusting/pollination occurs with the aid of  insects .

For 12-24 h after the anthers open, the pollen acquires "anemophilic" appearance - it grows dim, it becomes faded and easily friable.  At this time it is transferred by airstreams over large distances.  Thus at first Lemnaceae are the insect-pollinated plants, then wind-pollinated.

If  cross-pollination did not occur, then self-fertilization can occur.

 Fruits of duckweeds are well visible with the naked eye, according to the sizes/dimensions they are just larger than a poppy seed.

  Propagation of plants.  Fruits of the family of Lemnaceae are transferred to the large distances on the feet of the swimming and marshy birds.  They can also be transported by water.  The predominates mode of reproduction in Lemnaceae, however, is colonization by vegetative individuals, since fruits are formed rarely.  Adult plants spread via slowly flowing water, by frogs and fish, to the bodies of which they stick, but predominantly by birds.

Many wild wild ducks eat Lemna and Spirodela.  They can stick to the paws of birds and are thuy transferred over large distances.