Explain the Flower in diversity of life?
In order for plants to evolve from an aquatic existence to a terrestrial existence, specific adaptations in their basic modes of reproduction had to occur. The land presents several interesting problems that had to be overcome. First, it should be noted that an aquatic environment provides a nice, wet medium for gametes, or sex cells, to be transported to the female, or to free eggs, and fuse. In aquatic species like algae and seaweed, the sperm are either carried by the water, or they have flagellae (Seen below), which propel them to the eggs, enabling fertilization.
Things are different on land. Without a dependable and constant supply of water to transport the gametes, reproduction becomes more of a hit or miss affair, depending upon the presence of water. The evolution to a land-based method of plant reproduction as we know it came about through various adaptations of the sexual organs and gametes. Embryonic development also had to go through the process of natural selection, resulting in an adaptation known as the "seed habit."
On land, the male gametes of plants are called pollen grains, and the female gametes are called eggs. A pollen grain is considered to be a much reduced male gametophyte (generation that produces haploid, or 1n, gametes). Pollen is transported to the female reproductive structure (the ovule in the ovary of the flower) by wind, or animals such as bees or birds instead of water. In contrast to the aquatic environment, which provides an all-encompassing transportation medium, a terrestrial condition presents several problems which must be overcome.
Thus there are many modifications that reflect plant reproductive adaptations to a land existence. Showy, colorful, fragrant, and nectar-producing flower structures help to attract potential pollinators. Some wind-dispersed pollen have modifications such as light weight and wing-like structures that help them ride the wind currents.
Increased protection of the gametes (sexual reproductive cells) and the developing embryo reflect other important evolutionary trends that allowed the survival of plants on land. Eggs became enclosed within specialized organs called ovules, and sperm were produced within special pollen chambers.
Once fertilized, the zygote, later referred to as the embryo, develops within a structure called a seed, such as the pinto beans seen at the left. The embryo is thus protected from drying out and physical harm by a tough seed coat, and it has a self-contained food supply to carry it through the adverse conditions it would experience on land. Another problem that plants face on land has to do with dispersal of the young. In an aquatic habitat, the young are carried by currents or tides away from the parent to populate other areas, with more available resources.
On land, unless there are strong winds or runoff from rain to carry them away, seeds usually fall straight down from the parent to the ground. In fact, plants reflect a multitude of adaptations that aid in their seeds' dispersal beyond their immediate area, thus enhancing their opportunities for survival. Some plants have seeds with wing-like appendages, some have ovaries that develop into fruits, which are then eaten by animals and then dropped elsewhere, and others like the coconut (seen at the right) have tough resistant husks that allow them to float for thousands of miles across the sea before germinating.