Spores and pollen grains of water plants and their dispersal

Abstract

The group of the aquatic plants, comprising more than 2000 species, forms an interesting biological association of heterogenous elements which often are unrelated. This paper gives an account of the spores in aquatic liverworts, non-fern pteridophytes, water ferns, and angiosperms. These plants show different degrees of aquaticism and they determine the mode of dispersal of their spores and pollen grains. In truly aquatic plants, the exosporium is simple and thin-walled. The spores or pollen grains in them are short-lived and germinate quickly. They are dependent on water for dispersal. On the other hand, in semi-aquatic or facultatively aquatic plants, the pollen grains and spores have a thick exosporium. Their dispersal is either by insects or wind. Some of the water plants seem to have become secondarily adapted to an aquatic habitat. Their transition was from animophily to entomophily or from entomophily to partial hydrophily. They seem to have developed special adaptations for such transitions. The water ferns with their well developed exosporium and modified perisporium have special adaptations and special dispersal mechanisms. These have helped them in their survival from the Early Tertiary to the present times. A large number of aquatic angiosperms belong to the monocotyledons. They seem to have deviated from the great Ranalian plexus at a very early stage in evolution, on account of which their pollen-grain characteristics resemble those of primitive families of the Ranales. Despite their excellent adaptation to the water habitat, reflected in their vegetative characters, they are quite conservative in their pollen-grain characters and their pollinating and dispersing mechanisms.