The mechanism of carbon transport in Zea mays leaves was investigated using carbon 11 which is a short lved (half-lfe 20.4 min) positronemitting isotope. The gamma radiation produced on annihilation allows in vivo or nondestructive measurement of the isotope and the short halflife allows many measurements of trandocation to be made on the same leaf within the same day.Carbon 11 produced by the 10B (d,n)"C nuclear reaction was converted to "CO2, fed to a leaf as a short pulse, and assimilated during photosynthesis. The progress of the radioactive pulse along the leaf in the phloem was monitored in severd positions simultaneously with counters. The counters were Nal crystals with photomaltipliers and the output was amplified, passed to single channel analyzers, and the counts accumulated for 20 seconds every 30 seconds. Corrections were made for the half-life and background radiation by computer, and the results were displayed on a high speed plotter. Information derived from the corrected data induded the speed of translocation, the shape of the radioactive carbon pulse, and the influence of light and distance along the leaf on these parmeters. The plants were kept under controlled environment conditions during all measurements.A speed was derived from the time displacement of the midpoint of the front of the pulse, measured at two positions along the leaf. This was an apparent mean speed of translocation because it averaged a variation in speed with distance, variation in speed between or within sieve tubes, and it averaged the mean speed of all of the particles in the pulse.A wide range of speeds of translocation from 0.25 to 11 cm min-' was observed but most of the variability was due to the variation in light available to the leaf. For example, the speed of translocation was proportional to the light level on either the whole plant or individual leaf. Shading of the leaf established that the light effect was not localized in either the feeding area or in the portion of the leaf on which the measurements were made. It was proposed that the speed was dependent on the proportion of the leaf in the light upstream from the last counter. The speed of translocation was relatively independent of the stage of growth of the plant, age of the leaf, and the time during the diurnal light cyde. Data Carbon translocation is an integral component of the growthand yield-determining processes in higher plants. To characterize long distance C transport, it will be necessary to measure the speed and sugar concentration of the solution and the crosssectional area of the sieve tubes active in translocation. Other data, such as the turgor pressure and the water, ion, and sugar exchange rates of the sieve tubes, are required to establish the mechanism of C transport and explain the cause of the variation in C translocation between species, or between plants, under different environmental conditions.Routine measurement of some of these translocation parameters has not been easy. Through the development of techniques to prod...

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