ADVERTISEMENTS:
In this article we will discuss about the phylogenetic organs of flower.
Most Taxonomists agree that angiosperms form a natural group. They are monophyletic that is, their origin is from one ancestor only. But the ancestor is to be identified definitely. The ancestral organs from where the extant floral parts evolved are not known. Wilson (1982) regards that angiosperms evolved from gymnospermous ancestors during the Lower Cretaceous Period at about 135 million years ago.
The oldest fossils of flower are reported from Albian Epoch. The fossils are about 120 million years old. The fossils are casts of conduplicate carpel. The conduplicate carpel resembles a folded leaf lamina with appressed margin.
ADVERTISEMENTS:
Among the living angiosperms the conduplicate carpel is found in Drimys, Winteraceae and Degeneria (Degeneriaceae). It is now known that Winteraceae and Degeneriaceae have many primitive characters. So it is assumed that the ancestor of carpel is similar to that of Drimys and Degeneria.
The conduplicate carpel is an elongated structure without any distinction into stigma, style and ovary. Innumerable ovules occur in longitudinal rows along the ovary wall. Numerous papillae are present along the margins of the folded lamina. The papillose surfaces of the margins close the opening of the carpel and thus form a stigmatic crest. Pollen grains are received at the longitudinal crest.
The stigmatic crest in Drimys membrane extends longitudinally from the tip along one side up to the base of ovary. Pollen tube grows through the papilla that acts as stigmatic surface. During evolution the lower and middle portions of conduplicate carpel stopped producing papillae and the middle region elongated. As a result stigma and style evolved.
The fossil flower, reported by Dilcher, has carpel only; the other floral organs like sepals, petals and stamens are absent. The fossil flowers had scars at the region of other floral parts because they had fallen away. The living gymnosperm has microsporophyll that is leaf like. So it is assumed that the ancestral flower had broad stamen.
ADVERTISEMENTS:
This type of stamen is found in primitive angiosperms, e.g. Drimys, Degeneria, Austrobaileya and Himantandra. In these genera the stamens are flat and slightly thickened. They lack the characteristic filament and anther; instead four long sporogenous tissues are present (Fig. 30.11). In Degeneria the sporangia were deeply sunken. During evolution the sporangia became superficial.
Lastly it is to mention that due to lack of fossil evidences, the evolution of flower is hypothesized on the basis of comparative morphology between gymnosperm and extant angiosperms that have primitive features.
