. Devonian floras; a study of the origin of Cormophyta. Paleobotany -- Devonian. VI] PTEROPSIDA 79 is first the Archaeopteris type of leaf. This arose from the meta- morphosis of the branches of the nth. order in a system which was at least bipinnate, if not more compound still. Each alternate branchlet was flattened out either into a sterile wedge- shaped leaf, with a radiating nervation, or metamorphosed into one or more sporangia. Every stage is clearly seen in Devonian species of Archaeopteris (Figs. 28-31, pp. 56-59). Further modifications of the wedge-shaped entire, primitive type of lea
. Devonian floras; a study of the origin of Cormophyta. Paleobotany -- Devonian. VI] PTEROPSIDA 79 is first the Archaeopteris type of leaf. This arose from the meta- morphosis of the branches of the nth. order in a system which was at least bipinnate, if not more compound still. Each alternate branchlet was flattened out either into a sterile wedge- shaped leaf, with a radiating nervation, or metamorphosed into one or more sporangia. Every stage is clearly seen in Devonian species of Archaeopteris (Figs. 28-31, pp. 56-59). Further modifications of the wedge-shaped entire, primitive type of leaf soon set in. As in Psygmophyllum, these leaves tend to become lobed or segmented longitudinally. In A. Archetypus, Schmalh. (Fig. 29, p. 57) and A. Rogersi, Daws., the leaflet is primi- tive, large and undivided. We next pass on to types such as A. hihernica, (Forbes) (Fig. 28, p. 5Q) and A. Roemeriana, (Goepp.) in which the leaf tends to become unsymmetrical and more or less toothed or lobed, and finally we reach such types as A. fimbriata, Natli. (Fig. 30, p. 58) and A. fissilis, Schmalh. (Fig. 31, p. 59) in which it is divided nearly to the base, longi- tudinally, into very narrow segments. A still further elaboration of this type by splitting- would be indistinguishable from Spheno- pteris. Further, other later genera, especially cha- racteristic of the Lower Carboniferous, such as Rhacopteris^ (Figs. 42, and 33, p. 61), Adiantites (Fig. 43, p. 80) and Cardiopteris, have Fig. 42 leaves essentially similar to Archaeopteris and P'^"* with a radiating nervation. Further, here also "^g ' lq^^^ ^^r the tendency to longitudinal splitting is marked at least in the two genera first named. Such types are in fact chiefly distinguished from Archaeopteris by the shape of the leaf and its segments—by some small peculiarity of its symmetry. They are all, however, obviously derived from the Archaeopteris type of leaflet. 1 In this genus the frond also lihoc
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