The rudimentary embryos in mature seeds of eight species of Ilex were in a state of quiescence, or growth arrest, rather than a state of extremely slow growth. Ultrastructure of the quiescent embryo showed that the storage material, lipid and protein bodies, accounted for more than 80% of the cellular volume with a few unidentified, membrane-bound spherical bodies located in a thin layer of cytoplasm about the periphery of the nucleus. Other organelles were either scarce or absent. In vitro culture of excised rudimentary embryos resulted in immediate resumption of embryonic growth. Both glucose and non-reducing sugars (sucrose, raffinose, and stachyose) in the culture medium supported embryonic growth, while the presence of Ilex endosperm tissue in culture inhibited such growth. Since these are the major sugars of Ilex endosperm, the embryo culture experiments suggest that an inhibitor in the endosperm is likely to be responsible for the maintenance of embryo quiescence in the seed. Lipid and protein bodies disappeared in in vitro-activated rudimentary embryos. Starch grains became the principal storage material. Vacuoles, mitochondria, dictyosomes, polysomes, rough endoplasmic reticulum, microtubules, and other membrane-bound spherical bodies were prominent. The ultrastructure of the quiescent rudimentary embryo and the changes accompanying the activation of the embryo closely resemble those of quiescent mature embryos and the initial germination changes in embryos of other plants.