PERIODICITY OF MITOSIS AND CELL DIVISION IN THE EUGLENINEAE

Abstract

The periodicity of mitosis and cell division has been investigated in 15 green and 5 colorless species of the Euglenineae. Green species in biphasic culture under natural light conditions have mitosis confined to the dark period. Mitosis begins 1-2 hours after the onset of darkness, each species having a predictable percentage of cells dividing each night. There is a threshold period at the beginning of the dark period after which mitosis cannot be inhibited by light. The mitotic rhythm is exogenous, being removed by growth in artificial light or darkness (resulting in no mitosis), or in a rich organic medium (resulting in continuous mitosis at a constant rate). Colorless species in biphasic culture under any light conditions have an irregular mitotic periodicity, bursts of mitosis occurring at any time of the clock and alternating with periods in which mitosis is almost absent. It is suggested that the presence or absence of regular or irregular mitotic periodicity is related to the different modes and rates of nutrition of green and colorless species in various conditions of light and darkness, and in various media.