A theory of translocation in phloem of Heracleum by contractile protein microfibrillar material

Recent findings based on light microscopy, electron microscopy, electrical gradients, microinjection, and tracer feeding, all on isolated but living phloem strands of Heracleum are here summarized and collated. A new theory of translocation is presented in which sucrose transport occurs in two chief modes. The first is based on microperistaltic movement of contractile lipoprotein which is thought to extend axially through the sieve tube. The second is a mass flow of solution around the contractile microfibrillar material, but chiefly activated by it. The microfibrillar material transports sucrose in pulses at about 400 cm h −1 and this mode of translocation is stopped by cold blocks and desiccation, but not by callose. The mass flow mode is slower and is stopped by callose formation and also by the cessation of activity of the microfibrillar or pulsing mode. A third small surface-layer component of translocation is also indicated, operating at speeds above 1000 cm h −1 .