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When seedlings of Pharbitis nil are presented with an inductive dark period at varying times, they show a circadian fluctuation in the number of flower buds initiated. This study determines if this fluctuation is due to the plant's perception, at the time of the inductive dark period, of either a rhythmic, external, environmental stimulus or of an endogenous rhythm. Using experimental designs in which the time of planting, the time of seedling emergence from the soil, and the time at which the presentation of an inductive dark period are varied, this fluctuation in flower bud formation is shown to be due to an endogenous rhythm initiated or synchronized by some event associated with the emergence of the seedlings from the soil. The results are inconsistent with the hypothesis that the plants are responding to rhythmic external stimuli.

Biochemical genetic studies on the production of gibberellins by the fungus Gibberella fujikuroi have identified two genes which control different steps in the biosynthetic pathway. One gene (g1) controls the production of all of the gibberellins: the second gene (g2) controls the production of GA1 and GA3 only. Ascospores are not ordered in the ascus of this fungus. This apparent spore slippage precludes mapping of these genes to their respective centromeres.

When young seedlings of Pharbitis nil are grown under continuous light, except for a single inductive dark period, they flower to a varying degree, depending on when this dark period is given. Plants become sensitive to this induction approximately three days after the seedlings emerge from the soil. The expression of flowering varies in a rhythmic fashion for three or more cycles, when an inductive dark period is given at progressively later times. The time between maximum expression of flowering is 24 hours or somewhat longer. It appears necessary that the inductive dark period be of sufficient duration, to only partially induce the plants to flower for this rhythm to be expressed. Under the conditions employed in this study, this duration is 12 hours. If this rhythm is endogenous, it exists at least from when the plants emerged from the soil since no environmental cues are given after that time, and it raises questions of the interpretations of data from previous studies with this organism.

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