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Pistia Benth. & Hook. f. Gen. Pl. iii. 964 APIOSPERMUM, Klotzsch Benth. & Hook. f. Gen. Pl. iii. 964 APIOSPERMUM, Klotzsch, in Abh. Akad. Berl.(1853) 351. KODDA-PAIL, Adans. Fam. ii. 75 (1763). LIMNONESIS, Klotzsch, in Abh.Akad. Berl. (1853) 352. ZALA, Lour. Fl. Cochinch. 405 (1790).
Only species: Pistia stratiotes Linn. Sp. Pl. 963.
Synonyms: P. aegyptiaca Schleid. in Otto & Dietr. Allg. Gartenz. vi. (1838); P. aethiopica Fenzl, ex Klotzsch, in Abh. Akad. Berl. (1853) 354; P. africana Presl, Epim. Bot. 240; P. amazonica Presl, Epim. Bot. 240; P. brasiliensis Klotzsch, in Abh. Akad. Berl. (1853) 356; P. commutata Schleid. in Otto & Dietr. Allg. Gartenz. vi. (1838); P. crispata Blume, Rumphia, i. 78; P. Cumingii Klotzsch, in Abh. Akad. Berl. (1853) 354; P. Gardneri Klotzsch, in Abh. Akad. Berl. (1853) 356; P. Horkeliana Miq. in Linnaea, xviii. (1844) 81; P. Leprieuri Blume, Rumphia, i. 79; P. linguaeformis Blume, Rumphia, i. 79; P. minor Blume, Rumphia, i. 78; P. natalensis Klotzsch, in Abh. Akad. Berl. (1853) 354; P. obcordata Schleid. in Otto & Dietr. Allg. Gartenz. vi. (1838); P. occidentalis Blume, Rumphia, i. 79; P. Schleideniana Klotzsch, in Abb. Akad. Berl. (1853) 356; P. spathulata Michx. Fl. Bor. Am. ii. 162; P. texensis Klotzsch, in Abh. Akad. Berl. (1853) 356; P. Turpini C. Koch, in Bot. Zeit. x. (1852) 577; P. Weigeltiana Presl, Epim. Bot. 240.
The first descriptions of Pistia, the water lettuce, are by the ancient Egyptians and by the Greek philosphers Dioscorides and Theophrastus (Stoddard 1989).
The free floating habit and the peculiarities of the inflorescence led to the formation of the monogeneric subfamily Pistioideae (Engler 1920b, 1920c, Bogner & Nicolson 1991) or Pistia is set close to the subfamily Aroideae and Colocasioideae, often as a tribe Pistieae (Bentham & Hooker 1883, Buscaloni & Lanza 1928, Engler & Krause 1920, Grayum 1990, Hay & Mabberley 1991, Buzgó 1994, Mayo et al. 1995).
For a long time Pistia has been understood as an intermediate evolution stage between Araceae and Lemnaceae due to its habitat, habit and inflorescence reduction, while today it is not seen as the sister clade of Lemnaceae any longer (Engler 1889, Arber, 1919, Engler 1920a, 1920b, Lawalrée 1945, Maheshwari 1954, 1956, 1958, Haccius 1966, Zennie 1977, Landolt 1980a, 1980b, Grayum 1984, Landolt 1986, Landolt & Kandeler 1987, Les et al. 1994, Kvacek 1995, Crawford et al. 1996).
Due to their habitat and to their reduced habit, they were often understood as a link between Araceae and Lemnaceae.
Pistia occurs all around the globe in tropical and subtropical zones. Sporadically, it can escape from cultivation in temperate regions. Low temperature mostly inhibits seed production (Dray & Center 1989a, 1989b, Pieterse et al. 1981).
Paleobotanical records have been described from late Cretacious sediments, Paleocene and in Oligocene and Miocene, from Europe, West Siberia, East Asia and North America (Stoddard 1989, Kvacek 1995).
Far distribution makes the geographical origin of the genus uncertain. Fossil record and wide distribution imply an ancient origin and an early distribution.
Probably, clones from geographically different origins differ morphologically. Pollination or clonal studies were not done, yet, and Pieterse (1978) showed that phytohormons and environmental conditions have an influence onto the shape of leaf and flower. Therefore, Pistia is treated as a monotypic genus and no subspecies are established, so far.
In the mature seed, the plumule and cotyledon are preformed. When exposed to temperatures over 20 degree Celsius (Pieterse et al. 1981) and water, germination starts. The seedling pushes open the operculum which is a tissue of tannin containing cells formed by the former micropylar integuments (Fig. 1). It unfolds the thin, non-hairy cotyledon which enables it to float in the cohesion field of the water surface, after the plantlet has risen to air contact. Soon, the first foliar leaf exits the seed opening and unfolds (Fig. 2). It is stout, but already hairy, and is formed by a loose aerenchyme. Later, the first foliar leaf takes over the floating function.
Young plants form few round leaves, laying flat or with a slightly convex upper side on the water surface as a flat rosette.
The shoot grows first to a short monopodial section, and soon changes to a purely sympodial system with short modules. These modules follow the sequence K-L-I (K = cataphyll, L = foliar leaf, I = inflorescence; Engler 1920b, 1920c). The cataphyll supports the renewal shoot (Buzgó 1994).
Later, the number of leaves increases. New leaves are hold upright by the older ones on the water surface. They sink down as the older leaves disintegrate and give way to the younger leaves.
The plants multiply vegetatively by stolons. The stolons are formed in the axil of the the foliar leaves. The little rosette at the end of the stolon is detached soon from the mother plant since the stolon base breaks off easily.
The mature leaves are flat, obovate-cuneate or spatulate. The hairy epidermis is water repellent and the abundant aerenchyme leads to a spongy, swimming tissue. While the upper surface is smooth, pale to blue-green (Fig. 3), the surface below is densely hairy and lighter in color and almost white. Further the lower surface forms longitudinal ribs according to the vascular bundles (Fig. 3, 4).
Each leaf bears a membranous stipule. The stipule of the foliar leaf is dimerous at its formation, it stays small and sits basal to the inflorescence. The prophyll, in contrast, has no lamina, but consists of a membranous sheath only (Buzgó 1994).
Some plants (S-E-Asia) form a tight rosette by bending the leaves upright (Fig. 3, 4). These leaves are often crinkled or even plicate on their terminal line. The rosette reaches a diameter of maximally 20 cm (pers. observations). The leaves tend to be bluish.
Other clones (Africa, America) form large, open rosettes with straight, expanded leaves. They reach 40 cm diameter and tend to be brighter, grassy green tone.
The first roots appear early after the first foliar leaf expands. They are always adventive. Young roots emerge on the higher shoot portions. Therefore, of that, they expand outside of the bulk of older roots. The older roots are in the center of the root bunch and disintegrate there.
The formation of lateral roots takes place immediately after the tip of the main root. This leads to a regular, conical silhouette of the root. This pattern is typical for free floating aquatic plants (Clowes 1985). However, contrary to the literature, Pistia forms roots with retarded lateral roots as well, according to its ability to root in the ground or in drifting debris.
The tiny inflorescence is inconspicuous. It is terminal to the monopodial shoot module and displaced from the apical position by the expansion of the continuing renewal shoot. Further the peduncle moves actively by bending (Fig. 6, Buzgó 1994). At first the inflorescence is upright. After anthesis, the peduncle bends actively and pulls the developing fruit under the water surface (as in Lagenandra, Peltandra and other aquatic plants). Finally the fruit is enveloped by the older roots (Fig. 6, Dray & Center 1989a, 1989b).
Between the male and the female part of the spadix, the spathe has a lateral constriction. Basally, the spathe is congenitally fused to the spadix axis (Fig. 8). In the zone of the constriction the fusion of spathe and spadix changes to a postgenital one and finally ends completely (Buzgó 1994). On the outer side the spathe is hairy and thus water repellent, inside it is pale green and naked. The spathe is congenitally fused to a tube. At a higher level the fusion is postgenital and breaks open by senescence (Figs 6, 7, 8, 9).
At anthesis, the spathe below the constriction is opens in the morning hours and exposes the wet stigma (Fig. 5, 14 ). The male organs are still enclosed, then. Some hours later, the spathe opens completely and exposes the male part (Buzgó 1994). The male part of the spadix disintegrates, while the spathe persists longer, protecting the young fruit.
The spadix bears a single female flower and a horse-shoe shaped, sterile scale atop of it (below the spathe constriction). Above the loosening zone of the spathe-spadix fusion, the spadix rises to a vertical position in relation to the spathe (Fig. 8). It bears a sterile, circular structure, surrounding the spadix like a collar (Fig. 12, Engler 1920b, 1920c, Buzgó 1994). Above a small naked stalk, the spadix bears a whorl of 4 to 9 male flowers. It ends in a little, naked processus above of the male flowers, similar to the aroid appendix ((Fig. 12), Buzgó 1994).
The female flower looks like a bottle, half sunken into the spathe (Figs 8, 9). The papillate stigma is raised up by an elongated style, unusual for members of the Aroideae (but found in Ambrosinia). The monolocular ovary is swollen and filled by mucilage, responsible for the transmission of pollen tubes (Fig. 10, Buzgó 1994) and protection of the seeds (Fig. 6, Dray & Center 1989a, 1989b). The placenta is basal and surrounded by trichomes, probably responsible for the mucilage in the ovary (French 1987). The orthotropous ovules are densely packed, they develop and are pollinated in centripetalous sequence (Buzgó 1994).
The mature seeds still possess both integuments. The inner one collapses while the outer swells to several cell layers. Both integuments, especially the outer contain massive amounts of tannin in the zone of the micropyle, leading to an operculum (Engler 1920b, 1920c). The embryo sac is replaced by abundant starch that fills most of the seed content. The globular embryo is located under the micropyle with its shoot apex towards the seed base.
The male flowers are naked, diandrous and sessile to shortly stalked by the fusion of the two filaments (Fig. 8, 9). The two anthers are arranged vertically (Fig. 9, 12), parallel to the spadix axis. The flowers open apically with four stomia (one per theca). Pollen is shed as yellow, dry, coherent thread or mass, finally falling down to the spathe. Pollen grains are striate (Fig. 13) and contain starch (Grayum 1984).
The two sterile structures (scale and collar) are green and have a smooth, shiny surface (Figs 7, 8, 11, 12). Both secrete little droplets of liquid by stomata on their periphery (Fig. 14, Buzgó 1994). The tissue of both organs consists of densely packed, isodiametric cells and contains no intercellular spaces. The inception of the collar is earlier than that of the scale, and they differ in their position. But generally, both organs can bee understood as homologous and as transformed from sterile, fused flowers (Engler 1920b, 1920c, Buzgó 1994). Such sterile flowers are found in Aroideae as well.
Dispersal of Pistia generally takes place by vegetative transport of young rosettes, either by water currents or by animals. Different from all member of Araceae, this plant forms no vegetative persisting organs, but dies down completely in dry or cold seasons.
Therefore, the seeds form the only persisting organs as a seed bank (Pieterse et al. 1981, Dray & Center 1989a, 1989b). The fruit sinks into the water, and there it is covered old roots and remains of leaves. Thus, it probably is not dispersed by birds as mentioned in the literature (Pieterse et al. 1981, Dray & Center 1989a, 1989b, Stoddard 1989)
Pollinators are unknown, but viable seeds are produced even in greenhouses in Zurich (where Pistia is definitely not native). On one hand, pollination could be so generalized, that any small animal may do the job (such as little dipters, beetles). On the other hand, self-pollination or apomixis can not be excluded.
Pistia is offered by plant sellers and aquarium shops. In indoor aquariums, Pistia tends to die slowly. In greenhouses it may do well during summer, but may show difficulties during winter. Outdoors it can grow much better during warm seasons, even in central Europe.
Pistia seems to depend on high quantities of light and to be sensitive to cool temperatures (optimally 25 degree Celsius, Pieterse 1978, Pieterse et al. 1981). It definitely likes high air humidity and nutritious substrates. In contrast to its famous floating habit, it does root in solid ground when given the chance, generally with positive effects.
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