The origin and morphological evolution of th outer integument of angiosperms.

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Bitegumic, as well as anatropous ovules are the most common in and unique to angiosperms. Comparative morphogenesis of the outer integument was performed in several taxa of Magnoliales and Laurales using scanning electron microscope. It aimed to clarify the origin and morphological evolution of the outer integument in the angiosperms. The outer integuments of Winteraceae, Degeneriaceae, Eupomatiaceae, Atherospermataceae, and Lactoridaceae arose as semi-annular outgrowth interrupted on the concaveside of the funiculus and then becomes hood-shaped. In Magnoliacea, Monimiaceae and Trimeniaceae, however, the funicular outgrowth intervenes in the gap of the hood-shaped outer integument on the concave side of the funiculus. This fact favors one hypothesis that the inner integument was derived from a telomic assembly at the origin of gymnosperms and the outer integument from a leaf-like organ at the origin of angiosperms. The outer integument is compared with the ovuliferous sporophylls of the glossopterid and caytonialean seed ferns.In case of semi-anatropous ovules of Himantandraceae and of Myristicaceae, the funicular outgrowth on the concave side of the funiculus was formed so early that the hood-shaped outer integument was not clearly seen from the surface. The outer integument appears to be initiated as on oblique ring-like outgrowth from the beginning as a result of congenital fusion between the hood-shaped outer integument and the funicular outgrowth. The orthotropous ovule of Zippelia (Piperaceae, paleoherb) was shown to arise as entire cup-shaped outgrowth from the beginning. There is no indication here that the outer integument is originally in hood-shaped. Developmental observation does not show the cup-shaped or oblique ring-shaped outer integument was formed as a result of ontogenetic fusion between the hood-shape outer integument and tha funicular outgrowth on the concave side of the funiculus.