Microtubular configurations during the cellularization of coenocytic endosperm in Ranunculus sceleratus L.

Summary Endosperm cellularization in Ranunculus sceleratus was studied in terms of the initiation of cell-wall formation in the coenocytic endosperm. The first endosperm cell walls were in an anticlinal position relative to the cell wall of the embryo sac and originated from the cell plates and not from wall ingrowths from the embryo-sac wall itself. Alveolar endosperm was formed 3 days after pollination. Microtubules were associated with the freely growing wall ends of the anticlinal walls and were observed in various orientations that generally ranged from angles of 45 ° to 90 ° to the plane of the wall. They were absent in the regions where vesicles had already fused. These microtubules may function in maintaining the growth and the direction of growth of the anticlinal wall until cellularization is completed. At the site where three neighbouring alveoli share their freely growing wall ends, remarkable configurations of microtubules were observed: in each alveolus, microtubules ran predominantly parallel to the bisector of the angle formed by the common walls. These microtubules may form a physically stable framework and maintain the direction of growth of the wall edges. It is concluded that the growing edge of the anticlinal endosperm wall and its associated microtubules are a special continuum of the original phragmoplast that gave rise to the anticlinal wall.