Abstract
The factor determining the length of immature stage in P. multimicronucleatum and P. caudatum was investigated allowing lines of a clone to grow at different fisson rates induced by temperature or by the amount of food. The onset of maturity was determined by the first detection of mating reactivity. In many clones of both species, the length of immature stage was closely related to the number of fissions rather than the physical time; all lines grown at different fission rates became mature after about the same number of fissions. But in some other clones of both species, it was related to both the number of fissions and the physical time; lines at lower fission rates became mature in more days but in less fissions than lines at higher fission rates. This difference among clones did not correlate to mating-type difference.
The transition from immaturity to full maturity did not occur abruptly. Instead, the percentage of mating reactive cells increased steadily with age in some clones and they increased with fluctuation in others.
