Based on the position of ovary there are three types of flowers- (1) Hypogynous, (2) Perigynous, and (3) Epigynous.
(1). Hypogynous Flower: The most primitive type with convex shaped Thalamus (torus). The ovary is superior and all other floral parts (calyx, corolla and androecium) arise from the base of the ovary.
(2). Perigynous Flower: An intermediate or transient type between Hypogynous and Epigynous flowers. The thalamus is more or less cup shaped and the ovary is half inferior, located at the centre of the thalamus cup. All other floral parts arise from the rim of the thalamus cup. Sometimes the thalamus cup forms a long tube like structure called hypanthium.
(3). Epigynous Flower: The most advanced type of flower. The ovary is inferior and all other floral parts arise from the above portion of the ovary.
(image source: cc wikipedia)
Ø The origin and evolution of inferior ovary is a well debated question in the phylogenetics of Angiosperms.
Ø Comparative morphological, anatomical and paleo-botanical studies suggest that the inferior ovary has evolved many times among different groups of Angiosperms in different ways and in different times in the remote past.
Ø In order to explain the formation of inferior ovary, two theories have developed by the evolutionary biologists.
Ø The two theories are:
(1). Appendicular Theory
(2). Receptacular (axial) Theory
Ø Both these theories are proposed on close attention to the organization of the course of vascular bundles supply to the ovules in the flower.
Ø The two theories also considered that the hypogynous flower is the most primitive one. Moreover, the epigynous condition evolved from a hypogynous condition through the transient perigynous state.
(1). Appendicular Theory
Ø Proposed by Eames in 1961.
Ø According to this theory, extensive fusion (both connation and adnation) of the outer lower portion floral whorls to one another and to the ovary wall has occurred. This result in the formation of an inferior ovary (epigynous condition).
Anatomical evidence for Appendicular Theory:
Ø This type of flower possesses an ovular vascular supply that originates in acropetal fashion moving up the ovary wall.
Ø The ovules are supplied from the traces branching from an ascending bundle.
Ø The dorsal sides of the carpels are surrounded by and united to a floral tube.
Ø The carpel bases where both dorsal and ventral carpellary bundles diverge are positioned below the ovarian cavity
(2). Receptacular (Axial) Theory:
Ø First proposed by Eichler and Sachs
Ø According to this theory, the carpels have physically ‘sunk’ into the thalamus and this is followed by the fusion of the receptacular tissue to the dorsal carpel wall.
Ø The ovary is thus partially enclosed in the receptacular tissue.
Ø Receptacular type of inferior ovary development is very rare among Angiosperms.
Ø This type of flower is characteristic to the members of the Family Cactaceae and Santalaceae.
Anatomical evidence for Receptacular Theory
Ø Prolongation of major vascular bundles to a level just below the insertion point of sepal, petal and stamens.
Ø A subsequent descending course of this ovular vascular supply toward the base of the carpels.
Ø These ‘descending’ vascular bundles are called as ‘recurrent’ bundles.
Ø The Recurrent bundles have inverted orientation of xylem and phloem.
Ø The presence of residual vascular tissues at the base of the ovary provides further evidence for the receptacular theory.
Difference between Receptacular and Appendicular Theory of Epigynous Flower Development in Angiosperms
Sl. No. Appendicular Theory Receptacular Theory 1 Inferior ovary developed by the fusion of thalamus cup to the ovary wall Ovary physically sunk in the thalamus and the thalamus fused to the ovary wall to form the inferior ovary. 2 Commonly found in many Angiosperm families Rare in occurrence. Characteristic of the family Cactaceae and Santalaceae 3 Entry of vascular bundles to the ovules are direct Entry of vascular bundles to the ovules as a descending recurrent bundle 4 Residual vascular bundles are not observed blow the ovary. Residual vascular bundles are observed at the base of the ovary 5 Origination of ovular vascular bundles is straight throughout. Orientation of ovular vascular tissue is inverted at some places.
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