Other Possible Mechanisms

  • D. S. Fensom
Part of the Encyclopedia of Plant Physiology book series (PLANT, volume 1)


In this chapter three assumptions will be made. The first will be that we do not yet know everything about the structural details of living phloem, and that there are therefore conditions of fine structure which exist in the living, functioning state which are yet to be discovered. The second is that the knowledge about the actual movement of translocate, the shape of its front, the profile of its concentration pattern with time and tracer selection or distribution, all within a single sieve tube, are not fully elucidated. The third assumption is that each of the chief mechanisms advanced to account for translocation presents formidable difficulties (Weatherley and Johnson, 1969; MacRobbie, 1971; Fensom, 1972; Canny, 1973): the doubtful evidence of two-way trans-plate streaming at adequate rates in the case of the protoplasmic streaming hypothesis; the theoretical and experimental objections to electroosmosis; the problems of adequate pressure-driven flow in the presence of plasmatic filaments in plate pores and the difficulties over apparent metabolic control of movement in the case of pressure flow. These assumptions suggest that further study of the problem is desirable and further speculation about mechanism is quite proper. Indeed the results obtained on isolated phloem strands and by micro-injection of single sieve tubes (see Chapter 9) are so difficult to explain by any of these three mechanisms alone, that alternatives must be explored if this evidence is to be taken seriously.


Mass Flow Contractile Protein Pressure Flow Sieve Tube Sieve Element 
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© Springer-Verlag Berlin · Heidelberg 1975

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  • D. S. Fensom

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