ULTRASTRUCTURAL STUDIES ON THE EMBRYO SAC OF VISCUM-MINIMUM .2. MEGAGAMETOGENESIS

Embryo sac development of Viscum minimum was investigated using light and electron microscopy. Stages described involve uninucleate, binucle ate, four-nucleate, and mature embryo sacs following cellularization. During the early stage of development, prior to mitosis, numerous smal l vacuoles are initiated in the cytoplasm of a uninucleate functional megaspore. The centrally located nucleus undergoes the first mitotic d ivision and results in formation of two identical nuclei sharing a com mon cytoplasm. As the vacuole increases rapidly in size, the two nucle i become separated and move to opposite poles where the second mitotic division takes place. A remarkable elongation of the embryo sac is ob served between the second and third mitotic division. Eventually, the embryo sac reaches its final length, two-thirds of the length of the o vary, at cellularization. Elongation of the embryo sac is closely rela ted to the increase in vacuole size. Factors involved in vacuole forma tion and in the elongation of the embryo sac are discussed along with changes accompanying the transition from a sporophytic to a gametophyt ic pattern of development. Ultrastructural studies on the mature embry o sac, following cellularization, suggest that the egg cell is the lea st active cell in the megagametophyte. On the other hand, the synergid s appear metabolically very active, being rich in plastids, mitochondr ia, dictyosomes, numerous vesicles, polysomes, and reserves. The centr al cell is the largest cell in the embryo sac. In a mature embryo sac the central cell has two adjacent nuclei, suggesting that fusion of th e nuclei is completed following pollination and fertilization. The ant ipodals possess a complete set of organelles, numerous free and aggreg ated ribosomes, and endoplasmic reticulum. It is believed the antipoda ls play a significant nutritive role during the development of the emb ryo sac of V. minimum. Modification of the wall between antipodals and central cell and its role in nutrient transportations are discussed.