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The Earliest Known Land Flora1

Abstract

II. COMPARISON of these four fossil species from Rhynie with other fossils already known from the early Devonian period shows that a very homogeneous flora existed at that time, consisting chiefly of leafless and rootless land-living plants. These and other characters, such as their large, distal, sometimes solitary, and often forked sporangia, stamp these plants as exceptionally primitive. Among living plants the nearest of kin to them are clearly the Psilotaceæ, a family which has long presented a problem in morphology and classification. It comprises two living genera, Psilotum and Tmesipteris. Both genera are rootless. Their imperfect morphological differentiation is shown by the fact that botanists are not yet agreed whether their lateral appendages are to be held as truly foliar or not. Psiloturn is native throughout the tropics, and is represented by two well-marked species. The commonest, P. triquetrum, has upright and shrubby aerial shoots, with radial construction and frequent bifurcations. These spring from leafless underground rhizomes, profusely bifurcated. They are covered with rhizoids, and contain a myccrhizic fungus. On the lower part of the aerial shoots simple spine-like leaves are borne, but towards the distal ends these are replaced by forked spurs, between the prongs of which a synangium, usually with three loculi, is seated. The aerial shoot is traversed by a vascular strand consisting of xylem in the form of a hollow many- rayed star, with sclerotic core, and branch-strands run out to the appendages. The whole is covered by epidermis with stomata, and the cortex provides the photosynthetic tissue. Tmesipteris is represented by only one species, limited to Australasia. It grows usually among the massed roots that cover the stems of tree-ferns, but some times upon the ground. Its general form is like that of Psilotum, but the underground rhizomes are longer and the appendages larger, while only two loculi are usually present in each synangium. Clearly the form and vascular structure of these plants are generally like those of the Rhynie flora.

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BOWER, F. The Earliest Known Land Flora1. Nature 105, 712–714 (1920). https://doi.org/10.1038/105712a0

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