Question

Discuss plant terrestrialization and the timeline of their evolution

Answer 1

The first known land plants were evolved during Silurian. Those plants resembled ferns rather than mosses. Mosses, liverworts, tracheophytes (vascular plants) were developed from green algae (Chlorophyta). Evolution of an external surface and water-impermeable spores, structural support, vascular tissue to transport water within the plant body, and internalised sex organs- these were the characters that were developed by terrestrial plants.

The first known terrestrial organisms are mid-Ordovician very small plants with spores and spore-bearing structures (sporangia), similar to today’s liverworts. By Mid Silurian, there were small vascular plants, less than 10 cm high that had sporangia at the ends of short, leafless, dichotomously branched stalks and lacked true roots. Aglaophyton major – a very early land plant with no roots or leaves. By the end of Devonian, land plants greatly diversified; there were ferns, club mosses, horsetails, some of which were large trees. e.g., huge lycophyte trees like Lepidodendron (carboniferous forests), Stems of sphenopsids with features similar to today’s horsetails. In the earliest vascular plants, haploid phase of the life cycle (gametophyte) was as complex as diploid phase, which produces haploid spores by meiosis. But in the later plants, gametophyte became reduced to a small, inconspicuous part of life cycle. These plants depended on water for fertilization of ovules by swimming sperms. Spore bearing plants were joined at the end of Devonian by the first seed plants. Archaeosperma = earliest seed plants from Devonian. During Carboniferous, landmasses were aggregated into supercontinent Gondowana in the Southern Hemisphere and several smaller continents in Northern Hemisphere. Extensive swamp forests were dominated by horsetails, club mosses, and ferns. Gymnosperms becoming more and more prevalent; horsetails were declined during Permian. Diversification of seed plants occurred in Late Paleozoic. Some of them had wind-dispersed pollen. The evolution of seed provided the embryo with protection against desiccation, as well as store of nutrients that enabled the young plants to grow rapidly and overcome adverse conditions. None of these plants had flowers. Flora dominated by gymnosperms – cycads, conifers, and their relatives, including Ginkgo, a Triassic genus that survives as “living fossil”. Jurassic period known as “age of cycads”. A cycad gymnosperm Williamsonia sewardiana has been reconstructed from Jurassic rocks in India. Angiosperms or flowering plants first appeared in early Creteceous. Many of the anatomical features of angiosperms – flower-like structure evolved individually in various Jurassic groups of gymnosperms. Most advanced group of insects made their appearance during Mesozoic. By the late Creteceous, most families of living insects, ants, social bees evolved. As different groups of pollinating insects developed, adaptive modification of flowers to suit different pollinators gave rise to floral diversity of modern plants. Most of the modern angiosperm families and insects differentiated by Eocene or earlier. Dominant plants of Savannah that developed in Oligocene were grasses (Poaceae) that underwent adaptive radiation at this time. Many groups of herbaceous plants evolved from woody ancestors, e.g., Asteraceae.