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  • Print publication year: 2005
  • Online publication date: June 2012

Chapter 12 - The phloem


Perspective: evolution of the phloem

With increase in the size of plants over geologic time, efficient systems for the transport of water and minerals (primary and secondary xylem) as well as for photosynthates, hormones and other substances (primary and secondary phloem) evolved (see Chapter 1). The protoplasts of differentiating conducting cells of the xylem (tracheids and vessel members) were eliminated through autolysis, thus providing at functional maturity open, but non-living, passageways through which water could be pulled upward and out through the leaves by the force of transpiration (see Chapter 11). Evolution in the phloem took a different course. An open, but living, system of interconnected tubes, formed by overlapping sieve cells in gymnosperms (and more primitive vascular plants), and superposed sieve tube members forming sieve tubes in angiosperms evolved. The protoplasts of sieve elements became degraded, losing the nucleus, tonoplast (vacuolar membrane) and all other organelles except some mitochondria and endoplasmic reticulum. In conifers and dicotyledons, distinctive plastids and P-proteins (phloem proteins) evolved and, with the mitochondria and ER, became located peripherally in the cells. Concurrently, plasmodesmata which connected contiguous sieve tube members evolved into open pores, thus forming a symplastic system of essentially unimpeded passageways (Ehlers et al., 2000) through which photosynthate and other molecular materials are transported throughout the plant. Although living, but because of the loss of the nucleus, the sieve elements were no longer able to control their genetic and metabolic activities.

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Further reading
Alfieri, F. J. and Kemp, R. I.. 1968. the seasonal cycle of phloem development in Juniperus californica. Am. J. Bot. 70: 891–896
Ayre, B. G., Blair, J. E., and Turgeon, R.. 2003. Functional and phylogenetic analyses of a conserved regulatory program in the phloem of minor veins. Plant Physiol. 133: 1229–1239
Ayre, B. G., Keller, F., and Turgeon, R.. 2003. Symplastic continuity between companion cells and the translocation stream: long-distance transport is controlled by retention and retrieval mechanisms in the phloem. Plant Physiol. 131: 1518–1528
Behnke, H. D. 1974. Companion cells and transfer cells. In Aronoff, al., eds., Phloem Transport. New York: Plenum Press, pp. 153–175
Behnke, H. D. 1995. Sieve-element plastids, phloem proteins, and the evolution of the Ranunculaceae. Plant Syst. Evol. (Suppl.) 9: 25–37
Behnke, H. D. 2002. Sieve-element plastids and evolution of monocotyledons, with emphasis on Melanthiaceae sensu lato and Aristolochiaceae–Asaroideae, a putative dicotyledon sister group. Bot. Rev. 68: 524–544
Behnke, H. D. and Sjölund, R. D. (eds.) 1990. Sieve Elements: Comparative Structure, Induction and Development. Berlin: Springer-Verlag
Cronshaw, J. and Anderson, R.. Sieve plate pores of Nicotiana. J. Ultrastruct. Res. 27: 134–148
Davis, J. D. and Evert, R. F.. 1970. Seasonal cycle of phloem development in woody vines. Bot. Gaz. 131: 128–138
Deshpande, B. P. 1975. Differentiation of the sieve plate of Cucurbita: a further view. Ann. Bot. 39: 1015–1022
Esau, K. 1965. Vascular Differentiation in Plants. New York: Holt, Rinehart and Winston
Esau, K. 1969. Handbuch der Pflanzenanatomie, vol. 5, part 2, The Phloem. Berlin: Bornträger
Esau, K. 1973. Comparative structure of companion cells and phloem parenchyma cells in Mimosa pudica L. Ann. Bot. 37: 625–632
Esau, K. and Cheadle, V. I.. 1958. Wall thickening in sieve elements. Proc. Natl Acad. Sci. USA 44: 546–553
Esau, K. and Cheadle, V. I. 1959. Size of pores and their contents in sieve elements of dicotyledons. Proc. Natl Acad. Sci. USA 45: 156–162
Eschrich, W. 1975. Sealing systems in phloem. Encyclopedia of Plant Physiology, 2nd Ser. 1: 39–56
Evert, R. F. 1963. Ontogeny and structure of the secondary phloem in Pyrus malus. Am. J. Bot. 50: 8–37
Evert, R. F. 1977. Phloem structure and histochemistry. Annu. Rev. Plant Physiol. 28: 199–222
Evert, R. F. 1982. Sieve-tube structure in relation to function. BioScience 32: 789–795
Giaquinta, R. T. 1983. Phloem loading of sucrose. Annu. Rev. Plant Physiol. 34: 347–387
Gottwald, J. R., Krysan, P. J., Young, J. C., Evert, R. F., and Sussman, M. R.. 2000. Genetic evidence for the in planta role of phloem-specific plasma membrane sucrose transporters. Proc. Natl Acad. Sci. USA 97: 13979–13984
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Weatherley, P. E. 1962. The mechanism of sieve-tube translocation: observation, experiment and theory. Adv. Sci. 18: 571–577