• Apoplast;
  • diffusion;
  • extended bundle sheath system;
  • flow;
  • hydathodes;
  • leaf teeth;
  • leaf veins;
  • leaves;
  • membrane permeability;
  • proton pumps;
  • symplast;
  • transfusion tissue;
  • transpiration stream;
  • vessels;
  • water tracers;
  • xylem sap


  1. Top of page
  3. References

Changes of view on the course of the transpiration stream beyond the veins in leaves are followed from the imbibition theory of Sachs, through the (symplastic) endosmotic theory of Pfeffer (which prevailed almost unquestioned until the late 1930s), to Strugger's experiments with fluorescent dye tracers and the epifluorescence microscope. This latter work persuaded many to return to the apoplastic-(wall)-path viewpoint, which, despite early and late criticisms that were never rebutted, is still widely held. Tracer experiments of the same kind are still frequently published without consideration of the evidence that they do not reveal the paths of water movement. Experiments on rehydration kinetics of leaves have not produced unequivocal evidence for either path. The detailed destinies of the solutes that reach the leaf in the transpiration stream have received little attention.

Consideration of physical principles governing flow and evaporation in a transpiring leaf emphasizes that: (1) Diffusion over interveinal distances at the rates in water will account for substantial solute movement in a few minutes, even in the absence of flow. (2) Diffusion can occur also against opposing now. (3) Volume fluxes in veins are determined by the diameter of the largest leaves examined contain high conductance supply veins which are tapped into by low-conductance distributing veins. (4) Edges and teeth of leaves will be places of especially rapid evaporation, and they often have high-conductance veins leading to them. (5) Solutes in the stream will tend to accumulate at leaf margins.

On the basis of recent work, the view is maintained that the water of the stream enters the symplast through cell membranes very close to tracheary elements. Also, that this occurs locally over a small area of membrane. Many solutes in the stream are left outside in the apoplast. This produces regions of high solute concentration in the apoplast and an enrichment of solutes in the stream as it perfuses the leaf. Solutes that enter the symplast are not so easily tracked. Suggestions about where some of them may go can be gained from a fluorescent probe that identifies particular cells (scavenging cells) as having H+-ATPase porter systems to scrub selected solutes from the stream.

Unpublished case-histories are presented which illustrate many aspects of these processes and principles. These are: (1) Maize leaf veins, where the symplastic water path starts at the parenchyma sheath; (2) Lupin veins, where the symplastic path starts at the bundle sheath and where solutes are concentrated in blind terminations; (3) The edges of maize leaves where flow is enhanced by a large vein (open to the apoplast), and solutes are deposited in the apoplast by evaporation; (4) Poplar leaf teeth, which receive strong flows, and where the epithem cells are scavenging cells; (5) Mimosa leaf marginal hairs, which have scavenging cells at their base; (6) Active hydathodes, whose epithem cells are scavenging cells; (7) Pine needle transfusion tissue, which is a site of both solute enrichment (in the tracheids), and scavenging (in the parenchyma); (8) Estimates are made of diffusion coefficients of a solute both along and at right angles to the major diffusive pathway in wheat leaves. The first is 1000 times the second, but is 1/100 of free diffusion in water.

Five general themes of the behaviour and organization of the transpiration stream are induced from the facts reviewed. These are: (1) The stream is channelled into courses of graded intensities by the interplay of the physical forces with the anatomical features, each course with a distinct contribution to the processing of the stream. (2) Water enters the symplast at precise locations as close as possible to the tracheary elements. (3) As the stream moves through the leaf its solute concentration is enriched many-fold at predictable sites. (4) Solutes excluded from the symplast diffuse from these sources of high concentration in specially formed wall paths, in precise patterns, at rates which can be measured, and which are low compared with diffusion in water. (5) Other solutes permeate the symplast, often over the surfaces of groups of cells which are organized into recognized structural features.


extended bundle sheath


fluorescence intensity


sulphorhodamine G


  1. Top of page
  3. References
  • Altus, D. P. & Canny, M. J. (1982). Loading of assimilates in wheat leaves. I. The specialisation of vein types for separate activities. Australian Journal of Plant Physiology 9, 571581.
  • Altus, D. P. & Canny, M. J. (1985a). Water pathways in wheat leaves. I The division of fluxes between different vein types. Australian Journal of Plant Physiology 12, 173181.
  • Altus, D. P. & Canny, M. J. (1985b). Loading of assimilates in wheat leaves. II. The path from chloroplast to vein. Plant Cell and Environment 8, 275285.
  • Altus, D. P., Canny, M. J. & Blackman, D. R. (1985). Water pathways in wheat leaves. II. Water-conducting capacities and vessel diameters of different vein types, and the behaviour of the integrated network. Australian Journal of Plant Physiology 12, 183199.
  • Arens, K. (1934). Die Kutikuläre Exkretion des Laubblattes. Jahrbuch für wissenschaftliche Botanik 80, 248300.
  • Aston, M. J. & Jones, M. M. (1976). A study of the transpiration surfaces of Avena sterilis L. var. Algerian leaves using monosilicic acid as a tracer for water movement. Planta 130, 121129.
  • Bauer, L. (1953). Zur Frage der Stoffbewegungen in der Pflanze mit besonderer Berücksichtigung der Wanderung von Fluorochromen. Planta 42, 367451.
  • Botha, C. E. J. & Evert, R. F. (1986). Free-space marker studies on the leaves of Saccharum officinarum and Bromus uniolodes. Suid-Afrikaanse Tydskrift vir Plantkunde 52, 335342.
  • Boyer, J. S. (1985). Water transport. Annual Review of Plant Physiology 36, 473516.
  • Burbano, J. L., Pizzolato, T. D., Morey, P. R. & Berlin, J. D. (1976). An application of the Prussian blue technique to a light microscope study of water movement in transpiring leaves of cotton (Gossvpium hirsutum L.). Journal of Experimental Botany 27, 134144.
  • Canny, M. J. (1973). Phloem Translocation. Cambridge, London .
  • Canny, M. J. (1986). Water pathways in wheat leaves. III. The passage of the mestome sheath and the function of the suberised lamellae. Physiologia Plantarum 66, 637647.
  • Canny, M. J. (1987). Locating active proton extrusion pumps in leaves. Plant Cell and Environment 10, 271274.
  • Canny, M. J. (1988a). Water pathways in wheat leaves. IV. The interpretation of images of a fluorescent apoplastic tracer. Australian Journal of Plant Physiology 15, 541555.
  • Canny, M. J. (1988b). Bundle sheath tissues of legume leaves as a site of recovery of solutes from the transpiration stream. Physiologia Plantarum 73, 457464.
  • Canny, M. J. & McCully, M. E. (1986). Locating water-soluble vital stains in plant tissues by freeze-substitution and resin embedding. Journal of Microscopy 142, 6370.
  • Canny, M. J. & McCully, M. E. (1988). The xylem sap of maize roots; its collection, composition and formation. Australian Journal of Plant Physiology 15, 557566.
  • Costigan, S. A., Franceschi, V. R. & Ku, M. S. B. (1987). Allantoinase activity and ureide content of mesophyll and paraveinal mesophyll of soybean leaves. Plant Science 50, 179187.
  • Crowdy, S. H. & Tanton, T. W. (1970). Water pathways in higher plants. I. Free space in wheat leaves. Journal of Experimental Botany 21, 102111.
  • Dickson, R. E. (1979). Xylem translocation of amino acids from roots to shoots in cottonwood plants. Canadian Journal of Forest Research 9, 374378.
  • Dickson, R. E., Vogelmann, T. C. & Larson, P. R. (1985). Glutamine transfer from xylem to phloem and translocation to developing leaves of Populus deltoides. Plant Physiology 77, 412417.
  • Edgington, L. V. & Peterson, C. A. (1977). Systemic fungicides: theory, uptake and translocation. In: Antifungal Compounds, vol. 2 (Ed. by M. R.Siegel & H. D.Sisler), pp. 5189. Marcel Dekker, New York .
  • Engel, H. (1939). Das Verhalten der Blätter bei Benetzung mit Wasser. Jahrbuch für Wissenschaftliche Botanik 88, 816861.
  • Evert, R. F., Mierzwa, R. J. & Simpson, R. (1984). The marginal bundle and distribution of inulin [14C]carboxylic acid in the transpiration stream of the leaf of Zea mays. American Journal of Botany 71 (5,2), 27.
  • Fahn, A. (1979). Secretory Tissues in Plants. Academic Press, London .
  • Franceschi, V. R. & Giaquinta, R. T. (1983a). The paraveinal mesophyll of soybean leaves in relation to assimilate transfer and compartmentation. I. Ultrastructure and histochemistry during vegetative development. Planta 157, 411421.
  • Franceschi, V. R. & Giaquinta, R. T. (1983b). The paraveinal mesophyll of soybean leaves in relation to assimilate transfer and compartmentation. II. Structural, metabolic and compartmental changes during seed filling. Planta 157, 422431.
  • Francheschi, V. R., Wittenbach, V. A. & Gtaquinta, R. T. (1983). Paraveinal mesophyll of soybean leaves in relation to assimilate transfer and compartmentation. III. Immunohistochemical localization of specific glycopeptides in the vacuole after depodding. Plant Physiology 72, 586589.
  • Frey-Wyssling, A. (1937). Über die kutikuläre Rekretion. Verhandlung der schweizersche naturforschende Gesellschaft, pp. 146147.
  • Gaff, D. F., Chambers, T. C. & Markus, K. (1964). Studies of extrafascicular movement of water in the leaf. Australian Journal of Biological Sciences 17, 581586.
  • Galatis, B. (1988). Microtubules and epithem-cell morphogenesis in hydathodes of Pilea cardierei. Planta 176, 287297.
  • Gunning, B. E. S. & Robards, A. W. (1976). Plasmodesmata and symplastic transport. In: Transport and Transfer Processes in Plants (Ed. by I. F.Wardlaw & J. B.Passioura), pp. 1541. Academic Press, New York .
  • Haberlandt, G. (1914). Physiological Plant Anatomy (translated by M.Drummond). Macmillan, London .
  • Huber, B. (1947). Zur Mikrotopographie der Saftströme im Transfusionsgewebe der Kiefernadel. I. Anatomischer Teil. Planta 35, 331351.
  • Huber, B. (1961). Grundzüge der Pflanzenanatomie. Springer, Berlin .
  • Huber, B. & Strugger, S. (1943). Ernst Rouschal. Berichte der deutschen botanischen Gesellschaft 60, 186198.
  • Hülsbruch, M. (1944). Fluoresenzoptische Untersuchungen über den Wasserweg in der Wurzel. Planta 34, 221248.
  • Hülsbruch, M. (1954). Zum extrafazikulären Wasserweg in der Wurzel. Planta 43, 566570.
  • Hülsbruch, M. (1956). Wasserleitung in Parenchymen. In: Handbunch der Pflanzenphysiologie, vol. 3 (Ed. by W.Ruhland), pp. 522540. Springer, Berlin .
  • Jachetta, J. J., Appleby, A. P. & Boersma, L. (1986a). Use of the pressure vessel to measure concentrations of solutes in apoplastic and membrane-filtered sap in sunflower leaves. Plant Physiology 82, 995999.
  • Jachetta, J. J., Appleby, A. P. & Boersma, L. (1986b). Apoplastic and symplastic pathways of atrazine and glyphosate transport in shoots of seedling sunflower. Plant Physiology 82, 10001007.
  • Jeje, A. A. (1985). The flow and dispersion of water in the vascular network of dicotyledonous leaves. Biorheology 22, 285302.
  • Jones, H., Leigh, R. A., Wyn Jones, R. G. & Tomos, A. D. (1988). The integration of whole root and cellular hydraulic conductivities in cereal roots. Planta 174, 17.
  • Jones, H., Tomos, A. D., Leigh, R. A. & Wyn Jones, R. G. (1983). Water relation parameters of epidermal and cortical cells in the primary root of Triticum aestivum L. Planta 158, 230236.
  • Kuo, J., O'Brien, T. P. & Canny, M. J. (1974). Pit-field distribution, plasmodesmatal frequency, and assimilate flux in the mestome sheath cells of wheat leaves. Planta 121, 97118.
  • Lackey, J. A. (1978). Leaflet anatomy of Phaseoleae (Leguminosae: Papilionoideae) and its relation to taxonomy. Botanical Gazette 139, 436446.
  • Kevekordes, K., McCully, M. E. & Canny, M. J. (1988). The occurrence of an extended bundle sheath system (paraveinal mesophyll) in the legumes. Canadian Journal of Botany 66, 94100.
  • McCully, M. E. & Canny, M. J. (1988). Pathways and processes of water and nutrient movement in roots. Plant and Soil 111, 159170.
  • Meidner, H. (1975). Water supply, evaporation, and vapour diffusion in leaves. Journal of Experimental Botany 26, 666673.
  • Meidner, H. & Sherrif, D. W. (1976). Water and Plants. Blackie, Glasgow .
  • Molz, F. J. & Ferrier, J. M. (1982). Mathematical treatment of water movement in plant tissue: a review. Plant Cell and Environment 5, 191206.
  • Münch, E. (1930). Die Stoffbewegungem in der Pflanze. Fischer, Jena .
  • O'Brien, T. P. & Carr, D. J. (1970). A suberized layer in the cell walls of the bundle sheath of grasses. Australian Journal of Biological Sciences 23, 275287.
  • Peterson, C. A., Griffith, M. & Huner, N. P. A. (1985). Permeability of the suberized mestome sheath in winter rye. Plant Physiology 77, 157161.
  • Pfeffer, W. (1877). Osmostische Untersuchungen. Engelmann, Leipzig , (translated 1985, G. R.Kepner & E. J.Stadelmann). Van Nostrand Reinhold, New York .
  • Plymale, E. L. & Wylie, R. B. (1944). The major veins of mesomorphic leaves. American Journal of Botany 31, 99106.
  • Richter, E. & Ehwald, R. (1983). Apoplastic mobility of sucrose in storage parenchyma of sugarbeet. Physiologia Plantarum 58, 263268.
  • Rouschal, E. (1941). Beitrage zum Wasserhaushalt von Gramineen und Cyperacean. I. Die faszikuläre Wasserleitung in den Blättern und ihre Beziehung zur Transpiration. Planta 32, 6687.
  • Rouschal, E. & Strugger, S. (1940). Die fluorescenzoptisch-histochemische Nachweis der kutikulären Rekretion und des Salzweges im Mesophyll. Berichte der deutschen botanischen Gesellschaft 58, 5069.
  • Sachs, J. (1865). Handbuch der Experimentalphysiologie der Pflanzen. Engelmann, Leipzig .
  • Schlafke, E. (1958). Kritische Untersuchungen zur Wanderung von Fluorochromen in Blättern. Planta 50, 388422.
  • Smith, J. A. C. & Nobel, P. S. (1986). Water movement and storage in a desert succulent: anatomy and rehydration kinetics of leaves of Agave deserti. Journal of Experimental Botany 37, 10441053.
  • Steudle, E. & Jeschke, W. D. (1983). Water transport in barley roots. Planta 158, 237248.
  • Stroshine, R. L., Rand, R. H., Cook, J. R., Cutler, J. M. & Chabot, J. F. (1985). An analysis of resistance to water flow through wheat and tall fescue leaves during pressure chamber efflux experiments. Plant Cell and Environment 8, 718.
  • Strugger, S. (1938). Die lumineszenzmikroskopische Analyse des Transpirationsstromes in Parenchym. I. Die Methode und die ersten Beobachtungen. Flora (n.s.) 33, 5668.
  • Strugger, S. (1939a). Die lumineszenzmikroskopische Analyse des Transpirationsstromes in Parenchym. II. Die Eigenschaften des Berberinsulfates und seine Speicherung durch lebende Zellen. Biologische Zentralblatt 59, 274288.
  • Strugger, S. (1939b). Die lumineszenzmikroskopische Analyse des Transpirationsstromes in Parenchym. III. Untersuchungen an Helxine soleirolii Req. Biologische Zentralblatt 59, 409442.
  • Strugger, S. (1940). Studien über den Transpirationsstromm im Blatt von Secale cereale und Triticum vulgare. Zeitschrift für Botanik 35, 97113.
  • Strugger, S. (1943). Der aufsteigende Saftstrom in der Pflanze. Naturwissenschaft 31, 181191.
  • Strugger, S. & Peveling, E. (1961). Die electronenmikroskopische Analyse der extrafaszikulären Komponente der Transpirationsstromes mit Hilfe von Edelmetallsuspensoiden adäquater Dispersität. Berichte der deutschen botanischen Gesellschaft 74, 300304.
  • Tanton, T. W. & Crowdy, S. H. (1972). Water pathways in higher plants. III. The transpiration stream within leaves. Journal of Experimental Botany 23, 619625.
  • Thomson, W. W., Platt, K. A. & Campbell, N. (1973). The use of lathanum to delineate the apoplastic continuum in plants. Cytobios 8, 5762.
  • Tyree, M. T., Cruizat, P., Benis, M., Logullo, M. A. & Salleo, S. (1981). The kinetics of rehydration of detached sunflower leaves from different initial water deficits. Plant Cell and Environment 4, 309317.
  • Tyree, M. T. & Yianoulis, P. (1980). The site of water evaporation from sub-stomatal cavities, liquid path resistances and hydroactive closure. Annals of Botany 46, 175193.
  • Ursprung, A. & Blum, G. (1919). Zur Kenntnis der Saugkraft. III. 4. Hedera helix Abgeschnittenes Blatt. Berichte der deutschen botanischen Gesellschaft 37, 453462.
  • Wang, X.-D. & Canny, M. J. (1985). Loading and translocation of assimilate in the fine veins of sunflower leaves. Plant Cell and Environment 8, 669685.
  • Weatherley, P. E. (1963). The pathway of water movement across the root cortex and leaf mesophyll of transpiring plants. In: Symposium of the British Ecological Society (Ed. by A. J.Rutter), pp. 85100. John Wiley & Sons, New York .
  • Wilson, T. P., Canny, M. J. & Mccully, M. E. (1988). Proton pump activity in bundle sheath tissues of broad-leaved trees in relation to leaf age. Physiologia Plantarum 73, 465470.
  • Wylie, R. B. (1943). The role of the epidermis in foliar organization and its relations to the minor venation. American Journal of Botany 30, 273280.