Hydathode – Water Stomata (Structure and Functions of Hydathodes)
What are Hydathodes?
Hydathodes are specialized pores (openings) particularly present on the leaf margins, which exudes or secretes drops of water. The exudation of water as drops from the tip or margin of the leaves is called guttation. The process of guttation is facilitated by the hydathodes.
Ø They are also called as Water Stomata because they structurally resemble stomataand they facilitate guttation (secretion of droplets of water from the pores of plants).
Ø Hydathodes are commonly found in Angiosperms, especially in grasses.
Ø They are also present in some other plants such as water hyacinth, balsam, roses, Hibiscus and rarely in some non-angiospermic plants.
Receptacular vs Appendicular Theory (A Comparative Approach)
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).