Question

Create a list of organisms the represent the evolution of major groups of plants starting from the earliest protist relative: The list should include i. the type of plants (group name), ii. the key adaptation that makes that group unique iii. the advantage that the specific adaptation conveyed to that group of plants

Answer 1

Plants are classified as an kingdom (Plantae Kingdom); they are the multicellular eukaryotes, with nucleus, mitochondria, cytoskeleton, and in most cases chloroplasts (for photosynthetic species).

# PROTISTA IN PLANT EVOLUTION:

Plants are thought to have evolved from different protist groups. Many protists have the ability to photosynthesize; this advantage have been thought to have passed into the eukaryotes.

# ALGAE IN PLANT EVOLUTION:

Algae (often considered as photosynthetic protists) are chlorophyll-containing, autotrophic, aquatic or semi-aquatic, mostly unicellular, non-vascular plants.This include the euglenoids (Euglenophyta), dinoflagellates (Dinophyta), diatoms (Bacillariophyta), green algae (Chlorophyta), brown algae (Phaeophyta), red algae ( Rhodophyta).

Plants are thought to have evolved from Charophyta (a class of freshwater, green algae). The two groups of Charophytes, namely Coleochaetales and Charales, resembles the bryophytes (the earliest land-dwelling plants) by the structure of their chloroplasts, sperm cells, and their way of cell division in mitosis.

# BRYOPHYTE IN PLANT EVOLUTION:

Bryopytes are land-dwelling, non-vascular plants. They have rhizoid (anchoring organs), multicellular sex-organs, lengthy sporophytic and dominating gametophytic phase. Their plant body does not differentiate into roots, stem and leaves. They include Liverworts, Hornworts and Mosses.

Vascular plants are most closely related to mosses, because some of them possess leptoids (food-containing cells) and hydroids (water-containing cells) that justifies that they have resemblance with the xylem and phloem of modern vascular plants.

# PTERIDOPHYTA IN PLANT EVOLUTION:

Aglaophyton is considered as the oldest of the vascular plants, which possessed conducting cells that are similar to hydroids of mosses. This prototracheophytes are considered to be an evolutionary link between bryophytes and true tracheophytes (pteridophyte, gymnosperm and angiosperm).

Pteridophytes are vascular, spore producing, sporophyte dominating, well-adapted to land plants. Their plant body is differentiated into roots, stem and leaves.

The main phylla include- Lycophyta, Sphenophyta and Pterophyta. All of the three groups contain living relatives like- Lycopodiaceae, Equisetum and Ferns. Most of the pteridophytes are extinct now, except one phyllum- Psilophyta, which have two living genera.

# GYMNOSPERM IN PLANT EVOLUTION:

Gymnosperms probably evolved from an extinct phyllum of seedless vascular plants- Progymnosperms. They form a connecting link between Trimerophyta and true gymnosperms.

Gymnosperms are naked seeded, with heterosporous life cycle, siphonogamous fertilization, development of cambium and seed are borne in cones. The evolution of seeds, with hard and resilient seed coat is a key factor of success of this group.

Several gymnosperms are extinct now, only four phylla are alive, namely- Cycads, Gnetophytes, Conifers and one single living species Ginkgo. Of these, Conifers are mostly adapted in the present times, and are abundant.

# ANGIOSPERM IN PLANT EVOLUTION:

They are flowering plants of phyllum Anthophyta. They are the most diverse group of all the plant phylla and produce covered seeds. They posseses reduced gametophytic phase, they show the process of double-fertilization, and their ovary ripens into fruit containg seeds. They have a number of unique features including possession of flowers, highly modified shoots, carrying male and female reproductive structures.

Despite the origin of angiosperms is a matter of mystery, most of the comparative studies suggests that the angiosperms have evolved from the Gnetophytes, a group of gymnosperms containing three living genera- Ephedra, Gnetum and Welwitschia. The process of double fertilization have been shown in both Ephedra and Gnetum. The reproductive structures of all these three genera are similar to the flowering structure of some angiosperms.