Floral development of Alisma triviale

The primordia of the floral appendages are initiated in acropetal order. They develop in the same order in which they appear but for the petals, which are retarded in their early growth and mature rapidly shortly before anthesis. While the sepal primordia are dorsiventral from their inception, the primordia of other appendages are of nearly radial symmetry and become more or less dorsiventral in their later stages of development. Each petal primordium together with the primordia of a stamen pair arise on one common petal–stamen (CA) primordium. The many pistil primordia arise on three antesepalous gynoecial bulges and the area between them. Thus, in its development the flower exhibits primarily a tricyclic trimerous plan. The floral apices have a two-layered tunica up to the stage of pistil inception. The initiation of all floral appendages occurs by periclinal divisions in the second layer. The third layer (corpus) may contribute, especially in the case of the petal–stamen primordia and the gynoecial bulges. The development of procambium is acropetal. Each primordium receives a single procambial strand shortly after its initiation. Thus, procambial differentiation occurs as a response to primordial inception and not according to the principle of the conservatism of vascular tissue. Additional procambial strands may differentiate as a response to increase in size. The relationships of Alisma to some ranalian families are discussed. Since the floral pattern of Alisma may be considered as a secondary derivation from a trimerous pattern, it does not appear primitive at all. Other primitive features such as apocarpy and lack of fusion of pistil margins are however retained. Thus, Alisma is a good example for heterobathmy.