Pistia stratiotes L.

Global description
 
Pistia stratiotes is a floating plant, drifting, without stem, formed of a leaf rosette, 5 to 25 cm in diameter and with short runners giving rise to daughter plants. A large network of fibrous roots dangles underwater. The leaves are in dense rosette, sessile, broadly spatulate, fleshy, and composed of a floating airy tissue. Both sides are hairy, greyish or yellowish green. Flowers are numerous, small and hidden at the base of the leaves, surrounded by a small pale green or white leafy structure, 7 mm long. The fruits are small elongated berries, 6 to 10 mm long, containing several dark seeds.
 
First leaves

First leaves are rounded or obovate, slightly stalked, prostrate at the surface of water, 1 to 2 mm in diameter. The black seed remains attached to the seedling for some time.
 
General habit

It is a floating aquatic plant, drifting, without stem, with spathulate leaves forming a dense rosette,3 to 15 cm in height and 5 to 25 cm in diameter, quickly stoloniferous.
 
Underground system

Many fasciculate fibrous roots, pendulous, measuring up to 50 cm long, with multiple perpendicular rootlets.
 
Stem

The stem is reduced to indistinct tillage plate at the base of the leaves. Lateral stolons are present, extending from the base of the leaves, cylindrical, 1 to 5 mm in diameter, depending on the size of the mother plant, at the end of which thrive the daughter plants. These stolons appear at the stage of 3 to 4 leaves of young plants from germination.

Leaf

The leaves are simple, alternate, sessile, spongy, and with dense spiral rosette. The lamina is spatulate, obovate with a rounded to truncate apex and a wedged base, 6 to 12 cm long and 3 to 6 cm wide, with 5 to 7 (13) sub-parallel veins, especially salient to the underside. The upper surface is green and tomentose while the underside is paler with denser tomentum.
 
Inflorescence

The inflorescence is small, hidden by the leaves, carried by a very short peduncle, about 4 mm long. It is formed by a whitish spathe,5 to 25 mm long, persistent, smooth inside and tomentose on the outside, forming two cavities.
 
Flower

The flower consists of a whitish spathe, 5 to 25 mm long, persistent, glabrous inside and tomentose outside, forming two cavities. There is a very reduced spadix inside, attached to the base of the spathe, with one female flower in the lower cavity, without perianth and 2 to 8 apical male flowers in the upper cavity, inserted on a whorl axis sub-tended by a shallow cup. The male flowers are reduced by 2 fused stamens. The female flowers are solitary, reduced to an ovoid unilocular ovary, inserted through the axis of the spadix, a loculus with several sessile ovules. The style is short, slightly curved towards the axis, topped with a globular stigma.
 
Fruit

The fruit is an ellipsoid berry, 6 to 10 mm long and 3 to 6 mm wide, thin-walled, containing 4 to 12 seeds.
 
Seed

Ovoid or oblong seed, about 2 mm long and 1 mm wide, with truncated apex and depressed disk-shaped. Integument rough, dark brown.

Origin

Pistia stratiotes has uncertain origin, probably native to South America (Rivers, 2002).