Question

Discuss how adaptations allowed plants to move from an aquatic environment to the variety of habitats they inhabit today. You may wish to start out discussing single celled algae and include the transition from prokaryotes to eukaryotes. At the beginning or end of the essay, it may be helpful to write about plants that you have observed (using correct scientific nomenclature), and consider their traits and habitat. Then explain why those traits are adaptive, and how they arose. Don't forget the role of natural selection in adaptation.

Answer 1

The green algae which were the ancestors of today’s plants are aquatic organisms. For the establishment of plants on land and to overcome the environmental challenges, they had adopted some key innovations which led to their terrestrial development. Plants have an “alteration of generations,” the haploid gamete producing plant (gametophyte) alternates with the diploid spore-producing plant(sporophyte).

Among Algae haploid cells dominate in the cell cycle. But the first successful land plants (the nonvascular plants) of the diploid sporophyte is a larger structure and it offers protection for the egg and developing an embryo. As seed plants evolved, the sporophyte form became more dominant in the life cycle.

Nonvascular plants (Eg: liverworts and hornworts) lack the vascular tissue, which greatly reduced the size of these plants. The evolution of vascular tissue by cell division at the tips of the stem and roots (this sort of growth is called primary growth) led the plants to supply the nutrients and water more efficiently to the upper portions allowing the plants to grow larger and in drier conditions. These adaptations enhance the survival fitness of the land plants, so they are favored by the natural selection.

The earliest vascular plants lacked seeds (ferns and mosses). These seedless vascular plants have free-swimming sperm, which requires the presence of free water for fertilization. Vascular seedless plants have both gametophyte (haploid) and sporophyte (diploid) individuals. The dominance of sporophyte generation in the life cycle of these plants reaches its full force with the advent of seed plants.

Gymnosperms were the first seed plants they produce two kinds of gametophytes. Male gametophytes are called pollen grains and female gametophyte is called ovum. Pollen is transferred to an ovule by wind, insects and other agents, thus fertilization occurs. This wind pollination of gymnosperms is inherently inefficient and led to the evolution of angiosperms.

Angiosperms are the flowering plants, in which the ovule is completely enclosed by sporophyte tissue. Flowers are the reproductive organs of the angiosperms, the attractive colors and shape of the flowers induce insects and other animals to carry the pollen. Parts of the flower develop into the seed and the fruit after fertilization.

Advantages of plant's life on land: They can grow taller because of availability of more Sunlight, and other resources. Less competition for the resources and different routes of seed dispersal by wind, insects, animals, etc.

The disadvantages of plant's life on land: Availability of less water in certain regions, need to evolve new methods of fertilization.