PARAMECIUM BURSARIA: LIFE HISTORY. I. IMMATURITY, MATURITY AND AGE

¹ University of California at Los Angeles

The life history of clones of Paramecium bursaria shows successive periods: (1) a period of sexual immaturity, during which sexual reactions and conjugation do not occur: (2) a transitional period during which weak sexual reactions occur in a few individuals: (3) a period of maturity, in which sexual reactions are strongly marked and the individuals conjugate readily: (4) a period of decline, ending in many (or all?) cases in death.

The length of the period of immaturity and the time of attainment of maturity depend on the cultural conditions. If the animals are kept rapidly multiplying, under the best of nutritive conditions, maturity comes on early; if they are subjected to periods of starvation or other depressing conditions, maturity comes on much later or not at all.

Ex-conjugant clones that are kept vigorously multiplying become mature in most cases at the age of three to five months, though cases have been observed of much earlier maturity, the earliest observed age of maturity being 12 days.

Ex-conjugant clones subjected for some time to depressing conditions become mature (even after restoration to favorable conditions) only at the age of 10 to 14 months. Certain clones have lived for years without becoming mature.

If single ex-conjugant clones are divided into two cultures, one subjected to conditions favorable to rapid multiplication, the other to unfavorable conditions, the two parts show these same differences. The part kept under favorable conditions matures months before the other part.

Subjection to depressing conditions for but short periods (18 days) delays maturity for months.

Thus temporary differential action of diverse environments produces in clones differences which persist through months of vegetative reproduction.

In the period of maturity, clotting and conjugation en masse are commonly produced when cultures of clones of different sex type are mixed. But in certain clones clotting occurs without the completion of conjugation; dense clots occur, but no pairs are formed.

The period of maturity lasts for several years. It is followed by a period of decline. In this period fission becomes slower; abnormalities appear; many individuals die, so that the cultures become scanty and finally die out completely. Clones have been cultivated in the laboratory five to eight years, finally showing degeneration and death.

During the period of decline conjugation may occur up to near the very end. But conjugation of aged stocks results in the death of most or all of the ex-conjugants.

The relation of age to mortality at conjugation presents many features of interest, and gives opportunity for study of the nature and progress of ageing. This matter is to be presented in later contributions.

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