FIG. 1. The effect on the growth of tomato plants of 12, 24, and 48 hour cycles of alternating light and dark periods. the endogenous daily rhythm of plants determines such photoperiodic responses as flowering. He has proposed that during one phase (the photophile) of the endogenous daily rhythm, light promotes the as-similatory activities of the plant, such as those that affect flowering. At the other extreme of the endoge-nous dailyrhythm, there exists a phase (the scoto-phile) in which light has no promoting effect or is even inhibitory. The effect which light-dark periodicity has on the growth of tomato plants might well be explained on the basis of Bunning's hypothesis. Thus plants grown in the 6-6 and the 24-24 hour regimes wouldreceive light during all or some portion of the scotophile phase, if the tomato has an endogenous daily rhythm of 24 hours (fig 2). That the tomato does have such a rhythm is shown by such a phenomenon as diurnal rhythms in bleeding (4). Continuous light should have the same effect as the alternation of light and dark periods in the 6-6 and the 24-24 hour regimes, since any light period in excess of about 16 to 18 hours