The Contributory Length of Sieve Tubes in Isolated Segments of Willow, and the Effect on it of Low Temperatures

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Abstract
Using isolated stem segments in which a gradient of 32P-phosphate activity had been established, it has been demonstrated that the magnitude of the contributory length is much greater than the 16 cm previously measured by Weatherley et al. (1959). On cooling a portion of a stem segment between an aphid colony and the high-activity end of a segment, a pattern of specific activity changes were observed in collected honeydew which show that the effect of cooling was to increase the magnitude of the contributory length. It is considered that this increase provides cogent evidence in favour of the concept that movement of solutes across the lateral wall of the sieve tube is directly or indirectly dependent upon metabolic energy, whilst the longitudinal movement down the sieve tube is not so dependent.