Nature 245, 161 - 163 (21 September 1973); doi:10.1038/245161a0

Transcellular Strands of Cytoplasm in Sieve Tubes of Squash

ROBERT THAINE & MARGARET E. DE MARIA

The Grassland Research Institute, Hurley, Maidenhead, Berkshire SL6 5LR

THE mechanism of long-distance phloem translocation is the subject of several hypotheses based on different concepts of structure and function in sieve tubes¹. One of us (R. T.)²⁻⁴ has proposed that transcellular strands oreytpplasm, continuous from one sieve element to the next through the sieve pores, provide a basis for the development of the physical force required for movement and pathways for translocation. The hypothetical model of a transcellular strand⁵ consists of closely packed filaments of phloem filament (PF)-protein which form a tubular boundary enclosing cytoplasmic particles and a substructure of one or more tubules parallel with the outer boundary and formed from endoplasmic membranes. The number of tubules in each strand depends on the diameter of the sieve pores filled by a single strand. Should a conformational change⁶ occur in the outer boundary and cause a constriction which travelled as a wave in response to the propagation of electrical impulses of regular periodicity, this peristaltic pumping action could impel solution through the enclosed subtubules.

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