Morphology of Sassafras in Relation to Phylogeny of Angiosperms

Abstract

1. The one-celled ovary of Sassafras is normally borne in an ovulate flower in which there are 6 rudimentary stamens. The ovule is solitary, anatropous, and may form more than one archesporial cell. The nucellus is massive and has two integuments. 2. The embryo sac is of the typical 8-nucleate type, but the antipodals are small and evanescent. In the endosperm many free nuclei are formed before wall formation begins. The cells are small near the embryo but very large in the middle of the sac. The perisperm is conspicuous until a late stage. 3. The first division of the fertilized egg occurs when the free nuclear stage in the endosperm is nearly completed, and may be either longitudinal or transverse. 4. The embryo is massive, with short suspensor and peculiarly auricled cotyledons. It eventually fills the ovary cavity. The seed is without endosperm. 5. A stony layer is developed by the elongation and thickening of the innermost layer of the ovary wall. Transfusion tissue forms in the outer integument, noticeably in the micropylar region. 6. The microspores are formed in most of the sporangia by November. A plasmodium of tapetal origin is conspicuous. The microspores are binucleate at time of shedding. 7. Bisporangiate flowers are not uncommon, and show ovulate development apparently much in advance of the normal fertile flower. 8. While Sassafras is one of the earliest known Angiosperms, its Mesozoic records do not differ much from the corresponding features in the living genus which indicates a much more ancient, unrecorded ancestry.