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Bunning suggested that circadian rhythms provide the basis for time measurement in the photoperiodic responses of plants (2). A classic example is found in Glycine max. L. Merr. (Biloxi soybean), whose flowering response occurs in a rhythmical form having peaks of flowering that were approximately 24 hr apart (6). Recentlv Moore et al. (5) reported that Xanthiuim pensylvanicum did not exhibit a rhythmic flowering response when the plants were subjected to red light breaks and dark periods of various lengths. Likewise Reid et al. (7) found that red and far-red light perturbations did not result in a rhythmic tvpe of flowering response. The results of Moore et al. (5) and Reid et al. (7) neither contradicted nor supported Bunning's hypothesis (2). However, in this respect Moore et al. (5) have hypothesized that although no clear evidence for a flowering rhythm was found, the results were compatible with a rhythm that damped rapidlv within 1 cycle. It was felt that an investigation of leaf movements might provide information as to the nature of the rhythmic process in Xanthium. From such information a clarification of the photoperiodic response of Xanthiutm might be possible. Plants of Xanthium pensylvanicum (Wallr. ) 2 used in these experiments were grown from seeds obtained in the Chicago area. The cultural conditions and methods were similar to those used by previous investigators in this laboratory (7). In preliminary experiments it was found that the young leaves which are in the rapid expansion stage had the greatest amplitude of movement in both continuous darkness (DD) and continuous light (LL). For this reason young leaves in the rapid expansion stage were selected for data collection. The leaf movements were measured with kvmographs. On the graphs obtained in this manner an upward move-

Xanthium Leaf Movements in Light and Dark