Embryology of Amborella (Amborellaceae): Descriptions and polarity of character states

We present the first study on the embryology of Amborella trichopoda Baill. which, based on recent analyses of multiple genes, was identified as the first branch in the angiosperm evolution. Comparisons with two other basal angiosperms (Nymphaeaceae and Illiciales), as well as with the other angiosperms, showed that most of the embryological features of Amborella are plesiomorphies. They include: the tetrasporangiate anther; the fibrous endothecium; the glandular tapetum composed of two-nucleate cells; two-celled mature pollen; formation of the hypostase; bitegmic and crassinucellate ovules;the Polygonum type embryo sac; ephemeral antipodal cells; a lack of vascular bundles in the integuments; ab intio Cellular type endosperm; non-pachychalzal ovule or seed; albuminous mature seed. Contrary to all previous descriptions, Amborella has hemianatropous, rather than anatropous or orthotropous, ovules. For the determination of the polarity of ovule curvature as well as of some other characters such as the presence or absence of the nucellar cap and the obturator, the formation of the micropyle, and seed coat structure, additional studies on Nymphaeaceae and Illiciales are needed.