The fine structure of the embryogenic callus of Ranunculus sceleratus l

Sections of callus of Ranunculus sceleratus reveal its organization into cellular aggregates, the superficial cells of which are highly cytoplasmic and the inner cells of which are larger, have less-dense cytoplasm and are more highly vacuolated. Expansion and ultimate death of some of the internal cells leads to cell separation and break up of the aggregates. Many of the superficial cells are involved in the initiation of embryoids in the callus and they correspond in structure with the embryogenic cells of the seedling stem epidermis. The embryoids are retarded in their development in presence of 2,4-dichlorophenoxyacetic acid (2,4-D) and embryoid development becomes more rapid and more prolific when cultures are transferred to a medium without 2,4-D. The similarity between the embryogenic cells of the callus cultures and those of the seedling stem epidermis extends to cell size, nuclear size, degree of vacuolation, abundance of ribosomes and mitochondria, presence of amyloplasts and prominence of spherosomes. The various forms of spherosome are described and their possible function discussed. Amyloplasts differentiating into chloroplasts are observed in the more advanced embryoids. There is evidence that embryoids can arise from single cells but it is uncertain whether all are of single-cell origin. The embryogenic cells are in protoplasmic continuity with surrounding cells when they embark upon embryogenesis. Some of the superficial cells, also clearly undergoing active division, are of rather different structure; characteristically their nuclei show a high degree of chromatin condensation and their cytoplasm contains bundles of fibrous material. It is suggested that these cells do not function directly in embryogenesis. The internal cells of the aggregates have a low density of ribosomes and very few ER profiles or normal mitochondria. Extremely elongated mitochondrial structures following the outline of the nucleus are observed in these cells. An unidentified structure is frequently observed in cells in which cytoplasmic disorganization appears to be occurring.