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A fine structure study of the phragmoplast and developing cell plate has been made on glutaraldehyde-osmium tetroxide-fixed, dividing, cultured cells of the liquid endosperm of Haemanthus katherinae Baker. The phragmoplast arises between the telophase nuclei, usually in association with a remnant strand of spindle elements, and consists of an accumulation of microtubules oriented at right angles to the plane of the future cell plate. The microtubules, which are 200-240 A in diameter, occur in small clusters spaced at approximately 0.2-0.3 micro intervals along the plate. Short interconnections interpreted as "cross-bridges" have been observed between individual microtubules. Within each cluster there is an electron-opaque zone about 0.3 micro in width which can be attributed in part to an overlap of microtubules from both sides of the plate and in part to a local accumulation of an amorphous electron-opaque material. During development these dense zones become aligned in a plane which itself defines the plane of the plate. Vesicles, commonly observed in long files, are derived from a cytoplasmic matrix rich in elements of the endoplasmic reticulum and sparse in dictyosomes. They aggregate between the clusters of microtubules and eventually coalesce to form the cell plate.

MICROTUBULES AND EARLY STAGES OF CELL-PLATE FORMATION IN THE ENDOSPERM OF HAEMANTHUS KATHERINAE BAKER