Question

1. Describe the broad evolution of plants in the plant kingdom (in terms of phylogeny, reproduction, alternation of generations, leaf modifications, and vascularity)

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 the 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 n drier conditions.

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.

Plant adaptations: The evolutionary pre-adaptations are those that allow plants to survive on land. Evolutionary pre-adaptations are essential to land plant adaptation & evolution because sudden migration of aquatic plans on to the land does not allow them to survive as both the environments are different. For example, land plants cannot readily take water without roots.

The two non-reproductive pre-adaptations of plants are development of root system and vascular tissue to ensure water supply to the plant.

The reproductive pre-adaptations of plants are alterations of generations (reproduction becomes independent of sporophytes) and pollen production (reproduction can occur over long distances).