Arrowhead family |
The alismataceae have trimerous flowers
The leaves of some alismataceae are arrow-shaped
The Alismataceae with 12 genera and about 100 species are widespread all over the world. They are latex leading, usually perennial herbs with rhizomes, stolons or tubers and grow in or near water, or in swamps and bogs. The upright or lying stems are round, three- or four-edged and contain an air tissue (aerenchyma), so that the roots, that are often under water, can be supplied with oxygen.
The simple, entire, lanceolate, ovate or sometimes arrow-shaped leaves are usually arranged in basal aggregates and are usually stalked. The stem that is sheathed by the petiole, is often triangular. On aquatic plants the leaves may be immersed or they rise out of the water, sometimes floating leaves are formed. Frequently observed, dissimilar leaves occur on one plant (Heterophylly), whereby submerged and emersed leaves vary in shape.
The inflorescence often is whorled and forms panicles or racemes, sometimes even umbels, rarely single flowers occur. Infrequently, the flowers are locate below the water surface. The false whorls of the flowers are surrounded by analog arranged bracts. The white or pink flowers usually have 3 free petals and sepals, 3 to many stamens and 3 to a great many superior carpels that are not fused. The flowers may be hermaphrodite or unisexual.
There occur bisexual and unisexual plants. Between there are transitions like trioecy = male, female and bisexual flowers on the same plant or subdioecy = bisexual and unisexual flowers on one plant. After pollination, mostly mediated by insects, the female flowers form aggregates of usually 1- to 2-seeded nuts, which are floatable and are spread by the water.
| Floral formula: |
| * K3 C3 A3–∞ G3–∞ superior |
Interesting notes
-
Helanthium was recently separated from the most species-rich genus of alismataceae: Echinodorus. Both genera include popular aquarium plants.
-
The often numerous stamens and carpels, whereby the latter are not fused together, is interpreted as an ancestral character of the Alismataceae.