Biology : Semester II


IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14

Biology : The Time of Ancient Life : Part Nine

Life on Land:

One of the most famous Devonian fossil plant localities is the Rhynie Chert from Scotland. This deposit is dated as early Devonian and records in fine detail cells of the earlier land vascular plants as well as other fossils. Fossils from the Rhynie Chert were buried in short-lived freshwater deposits that later were subjected to replacement of organic material with silica, forming a chert deposit that preserved in great detail the internal cells of the fossils. Fungi, including mycorrhizal fungi, also have been recovered from the Rhynie Chert, suggesting the symbiosis of plants and fungi is indeed long and essential. Fossils assignable to insects and crustaceans have also been recovered.

The trimeophytes were large plants of the early-middle Devonian. Some trimerophytes are thought ancestral to the sphenopsids and progymnosperms of the later Devonian. Unlike the rhyniophytes, trimerophytes did not produce equal (dichotomous) branches. Instead they branched in such a way that one branch appeared to be the central stem and the other a side shoot.

The Lycophytes

Plants belonging to the division Lycophyta, have their sporangia organized into strobili (sing.: strobilus). Leaves contain vascular tissue.

Drepanophycus is a middle Devonian lycophyte from the Northern Hemisphere. Its features are very similar to modern lycophytes.

The major fossil groups of lycophytes are the lepidodendrids and sigillarids, often referred to as the arborescent lycopods because they usually were large trees. During the middle Devonian lycophytes retained their herbaceous habit, but also began to grow taller, more than a few meters high at first, and developed the capability for secondary growth to produce wood, allowing the plants to grow still taller. Modern lycophytes lack secondary growth and are entirely herbaceous.

Artist’s reconstruction of a Devonian forest. Lepidodendrid trees are on the right, sphenophytes on the left of the image.

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