EXPERIMENTAL STUDIES OF CYTOPLASMIC ORGANIZATION IN ZONARIA APICAL CELLS

Rapidly growing thalli of Zonaria have been treated with colchicine (10,000, 5,000, and 1,250 μg/ml) and phenobarbital (10,000 and 5,000 μg/ml). Responses of the meristems to drug treatment have been described at the ultrastructural, cellular, and organismic levels. With colchicine treatment microtubule disruption, nuclear envelope abnormalities, nuclear lobing, and increased nuclear sizes were observed. Apical cells ceased dividing but continued to elongate until cessation of treatment when abnormal cytokineses, irregular cell shapes, and cell deaths were observed. The coordinated growth of the meristem margin was disrupted, regions recovering left a zone line on the thalli, and cell deaths resulted in thallus branching. With phenobarbital treatment ER elaboration, inactive-appearing dictyosomes, pyrenoid-like structures, and deeply cup-shaped mitochondria aggregated with ER were observed. Apical cell elongation ceased and the mitotic cycle appeared to be arrested just after cytokinesis. Recovery from 10,000 μg/ml phenobarbital was low, with subapical cells and occasional apical cells starting new thalli. Recovery from 5,000 μg/ml was good, with occasional cell death dividing the meristem and zone lines resulting from the temporary synchrony. Correlations of responses at the ultrastructural, cellular, and organismic levels are discussed.