[Bidirectional translocation in sieve tubes].

The honey dew of aphids feeding on the horizontally fixed stem of a Vicia faba plant was collected on a turning table. When the leaf below the pierced stem was provided with fluorescein, and the leaf above with a (14)C-compound, the honey dew mostly contained both tracers. It could be shown that the attractive force of a single aphid is not strong enough to change the direction of transport. The tracers must have moved bidirectional in the same bundle and joined each other in the sieve tube before it was pierced by the aphid. There exists either a bidirectional movement in the single sieve tube, or the tracers move side by side in a "homodromous loop-path".