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If, on the other hand, the endosperm is the product of an act of fertilization as definite as that giving rise to the embryo itself, we have to recognize that twin-plants are produced within the embryo-sac one, the embryo, which becomes the angiospermous plant, the other, the endosperm, a short-lived, undifferentiated nurse to assist in the nutrition of the former, even as the subsidiary embryos in a pluri-embryonic Gymnosperm may facilitate the nutrition of the dominant one.
Phanerogamic plants in their lowest groups of gymnosperm exogens then begin to appear in the Devonian conifers, gradually followed by cycads. And it is not till Cainozoic times that we have the endogenous grasses and palms and angiospermous exogens.
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