Question

2. The evolutionary history of plants is studied through 1) comparative studies of living plants and 2) examination of the fossils of extinct plants. What information has the study of fossils contributed to our understanding of plant evolution?

Answer 1

Plant evolution through study of fossils:

Fossils (meaning “extracted from earth” or “dug up” evidences of geological past and comprises of remnants of organisms or parts of the organism, formed if the organism is buried before onset of decay. The branch related to the study of plant fossils is called Paleobotany.

Since, most plants rapidly decompose, after dying, formation of plant fossils, occur in lesser extent. But, both living and extinct plant fossils have been found. Plant fossils may be obtained as:

a. Macrofossils- wood, leaves, seeds etc.

b. Microfossils- pollens, algal forms, etc.

Fossilized forms:

a. Compactions: found in soft sediments, includes fruits, seeds.

b. Compressions or impressions: sedimentary deposits, like leaves. Carbonization of roots, trunks, etc.

c. Petrifactions: sedimentary depositions with mineral content, like petrified wood.

d. Casts and molds: original plant or parts decays away, leaving the cast. Include, roots, wood.

Evolutions in plant kingdom, are analyzed based on certain criteria, like:

1. Method of use of energy-heterotrophic, anaerobic forms to autotrophic aerobic forms.

2. Cellular and tissue organization- Prokaryotic cells to eukaryotic cells, unicellular to multicellular.

3. Plant tissue differentiation- in terms of branching, root development, seeds, stem and leaves tissue organizations.

4. Size of plants- comparison of advanced plants with their extinct forms, in terms of size.

5. Ecological niche-aquatic forms developed to terrestrial forms.

Evolutionary evidences:

1. Algae: Thallophytes, records from middle Devonian period. Lacks true roots, stems, leaves. Aquatic forms. Includes, phylum of photosynthetic protists: Dinophyta, Chlorophyta, Rhodophyta, Phaeophyta.

Plants evolution is attributed to the early class of freshwater algae called the Charophytes. Two major groups include: Coleochaetales and Charales.

2. Bryophytes: moss, liverworts, have leaf like forms, fossilize poorly, Devonian period (409-354 million years ago. Terrestrial forms, higher level tissue organization. Water required for gametes transfer.

3. Early vascular plants: First fossil, middle Silurian period, 425 million years ago. Oldest, Aglaophyton, link bryophytes and tracheophytes.

4. Tracheophytes: Silurian evidences show three evolutionary transitions- seedless vascular plants, gymnosperms, angiosperms. Have vascular tissue, xylem and phloem.

First transition: Late Devonian (375 million years), seedless vascular plants, living form Psilophyta.

Club mosses, Horse tails, early vascular plants, presence of nodes, internodes, faced early Permian extinction.

Followed by (290 million years), complex seedless plants, like phylum Lycophyta.

Second transition: 200 million years. Early Permian, like Medullosa. Had fern like foliage and seeds. Ginkgo is evidence of living fossils of Permian, early carboniferous period.

Gymnosperms dominated around 200 million years ago, like Cycadeoids. They are seed plants, had large leaves growing from barrel like trunk. In fossils leaves dissociated, leaving scar.

Third transition: Most advanced plants, fossil record of 130 million ago. Evidence indicate, evolution of angiosperms from a group of gymnosperms called Gnetophytes: living genera are, Ephedra, Gnetum, Welwitschia. They exhibited double fertilization.

Some oldest Angiosperms, Magnolids, produced flowers.

Present day Angiosperms, monocot and dicot forms, evolution evidences are found as 70 to 100 million years ago. Exhibited distinguished arrangement of vascular bundles, stem, leaves, ventilations, grouping of petals and flowers.