Sieve tubes

Turner speculated that the two families arose from a common ancestor that likely occupied xeric or halophytic habitats in Gondwana in an area that encompassed what were to become southwestern Africa and southeastern South America. The ancestral types do not still exist, but both families share the unusual, betanidin-based floral chemistry, and the equally unique sieve-tube plastids seen in Caryophyllales but are absent elsewhere in the plant kingdom (Behnke and Turner, 1971). 

Behnke, H.-D. and Turner, B. L. 1971. On specific sieve-tube plastids in CaryophyUales. Taxon 20 731-737. 

FIG. 2 Measurements of intracellular potentials in sieve tubes of maize via severed aphid stylets. Stimulation by ice water (above) and electric shock (below) evoked action potentials which were propagated with a velocity of 3-5cms in a basipetal direction. (From Ref. 36.)... 

FIG. 22 The Hodgkin-Huxley equivalent for an axon (a) and the modified HH circuit for sieve tubes in phloem (b). 

Sieve cells More primitive than sieve tube members and occur in nonflowering plants sieve cells contain clusters of pores, which are narrow and uniform in structure Translocation of sugars and other organic nutrients... 

Phloem sieve cells, sieve tubes translocation of organic nutrients... 

Currier HB, ShihCY. Sieve tubes and callose in Elodea leaves. AmJBot 1968 55 145-152. 

The main components of inner bark are sieve elements, parenchyma cells, and sclerenchymatous cells. Sieve elements perform the function for transportation of liquids and nutrients. More specifically and according to their shape the sieve elements are divided into sieve cells and sieve tubes. The former types are present in gym nosperms, the latter in angiosperms. The sieve elements are arranged in longitudinal cell rows which are connected through sieve areas. The sieve cells are comparatively narrow with tapering ends, whereas the sieve tubes are thicker and cylindrical. After 1 -2 years, or after a longer time in the monocotyledons, the activity of the sieve elements ceases and they are replaced by new elements. 

Path and Sink Features. In the translocation path (e.g., stems and petioles), assimilates and solutes move in mass flow through the cylindrical sieve tubes which have open sieve pores. The ability of a chemical to leak across the membrane from the sieve tube during transit will affect its ability to be transported through the entire pathway. 

In sink regions, there are essentially three in vivo pathways by which sucrose exits the sieve tubes (Figure 4). All three are operating in different types of sinks and all are metabolism depend-... 

Figure 1-3. Idealized longitudinal section through part of a vascular bundle in a stem, illustrating various anatomical aspects of the xylem and the phloem. New cells forming in the xylem initially contain cytoplasm, which is lost as the cells mature and become conducting. Fiber cells, which occur in the xylem, are usually quite tapered and provide structural support. The nucleated companion cells are metabolically involved with the sieve-tube members of the phloem.

Because sucrose has a mass of 0.342 kg mol-1, this flux density corresponds to (300 mol m-2 hour-1)(0.342 kg mol-1) or 100 kg m-2 hour-1. In the current example, the flow is per m2 of sieve-tube lumens the rate of flow per unit area of phloem tissue is less by the ratio of the lumen cross-sectional area to the total phloem cross-sectional area, which is usually 0.2 to 0.5. 

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