Development of the inflorescence and flower of Sagittaria cuneata

Abstract

The reproductive region of Sagittaria cuneata is basically trimerous. This trimery is exhibited in the arrangement of the bracts, sepals, petals, pairs of stamens in the male flower, and pairs of staminodia in the female flower. In the male flower after the inception of three sepal primordia, each of the three petal primordia arises with one pair of stamen primordia on an alternisepalous bulge of the floral apex, i.e., a petal–stamen (CA) primordium. Subsequent stamen primordia are formed in alternation with the six first-formed primordia. If, for convenience sake, the first six primordia are referred to as the first whorl of stamens up to four additional whorls may be produced. Depending on the size of the floral bud, the third and fourth whorls (if present) consist of two to six stamen primordia, whereas the fifth whorl (if present) contains one to five stamen primordia. Finally, primordia of pistillodes are formed in varying numbers. In the female flower the presence of CA primordia could not be as clearly established as in the male flower. However, again each petal primordium is definitely associated with a pair of antepetalous primordia. The latter primordia develop into staminodia. In alternation with the first six staminodia six additional staminodia are formed and then again in alternation many whorls of pistils (carpels). Even in the mature flower the basic trimery is reflected in the triangular shape of the globose and massive gynoecium. From a developmental point of view, the male and female flowers are primarily trimerous. The polyandric androecium and the large pleiomerous gynoecium are superimposed on the primary trimery. It appears quite possible that this developmental modification also reflects a phylogenetic derivation. This means that the pleiomerous gynoecium and androecium are not primitive but rather advanced. There is no indication of a spiral arrangement of stamens and carpels.Whereas the foliage leaves, bracts, and sepals are initiated as dorsiventral primordia, the petals, stamens, staminodia, pistils, and pistillodes arise as more or less hemispherical mounds and become dorsiventral thereafter. The vegetative apices, inflorescence apices and the floral apices have a two-layered tunica over a massive corpus. Foliage leaves, bracts, sepals, petals, stamens, staminodia, carpels, and pistillodes are initiated by periclinal divisions in the second tunica layer. In the case of the stamens and staminodia the corpus may also contribute. Ovules are initiated by periclinal divisions of the second layer of the carpel primordium.