GROWTH AND ORGANIZED DEVELOPMENT OF CULTURED CELLS. VI. THE BEHAVIOR OF INDIVIDUAL CELLS ON NUTRIENT AGAR

Abstract

Freely suspended cells of a long continuously cultured strain of carrot were dispersed in nutrient agar medium in Petri dishes. After inoculation, cells in randomly chosen fields were photographed at low magnification. The same cells were photographed at intervals over the next 23 days. The fate of 678 units, 24% of which grew visibly, was determined from the photographic record. Cells and units displayed a marked degree of individuality. In some units, there was a progressive increase in cell number; in others, after a few divisions, growth ceased; in still other units, cells enlarged without dividing. Although the evidence of cell division in free cells is conclusive, its incidence and maintenance increased with the size of the initial unit. In general, growth started after an “induction period” of about 5 days, but some cells remained apparently unchanged considerably longer after inoculation, before they started to grow. Although single cells of all sizes were observed to grow by one means or another, it was generally the shorter ones that continued to grow into viable colonies. Elongated cells predominated in the carrot strains used; these grew into colonies first by septation, with little or no over‐all increase in size, followed by a stage of cell enlargement. Growth by both cell enlargement and division then ensued. During growth by septation within single cells, some derived cells were seen to burst and die. The most frequent divisions were observed during septation when the average cell generation time was 24 hr. Lacking the controls that normally operate in the intact plant body, there was much variation in the forms assumed during growth, a number of which are illustrated. The importance of epigenetic factors, or external controls, that determine the growth of cells in the plant body is stressed.