Sieve tube structure

Evert R F 1982 Sieve-tube structure in relation to function. BioScience 32 789-795... 

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... 

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.

To consider this question further it is necessary to review present views on the structure of sieve tubes. 

The crux of the problem of the relationship of form to function in sieve tubes relates to the disputed nature of the interconnections between the sieve tube units via the sieve plates and in a recent review it was concluded that at least four or five possible arrangements could not be excluded by the structural evidence (Fig. 7.7 a, b, c, d). These structure models can be considered in relation to hypotheses of the mechanism of solute transport in the sieve tubes. 

Fig. 7.7 opposite). Diagrammatic models of possible structure sieve tubes. 

Higher oligosaccharides or polysaccharides possess unfavorable mass-to-charge ratios, preventing their effective resolution in open tubes. The separation of these carbohydrates is possible with capillary gel electrophoresis (CGE). The analytes are selectively retarded by a sieving network (gel or polymer matrix) due to differences in their sizes and structural conformations. 

Support is one of the important factors affecting the performance of catalyst, which include its type, composition, pore structure, surface properties and mechanical strength etc. The supports used include activated carbon, nanocarbon tube, carbon molecular sieves and carbon fiber, oxide, zeolite molecular sieve, alkali metal exchanged X- and Y-type molecular sieve,carbon covered alumina and boron nitride etc. 

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