scholarly journals Symplastic Continuity between Companion Cells and the Translocation Stream: Long-Distance Transport Is Controlled by Retention and Retrieval Mechanisms in the Phloem

2003 ◽  
Vol 131 (4) ◽  
pp. 1518-1528 ◽  
Author(s):  
Brian G. Ayre ◽  
Felix Keller ◽  
Robert Turgeon
Author(s):  
James Cronshaw

Long distance transport in plants takes place in phloem tissue which has characteristic cells, the sieve elements. At maturity these cells have sieve areas in their end walls with specialized perforations. They are associated with companion cells, parenchyma cells, and in some species, with transfer cells. The protoplast of the functioning sieve element contains a high concentration of sugar, and consequently a high hydrostatic pressure, which makes it extremely difficult to fix mature sieve elements for electron microscopical observation without the formation of surge artifacts. Despite many structural studies which have attempted to prevent surge artifacts, several features of mature sieve elements, such as the distribution of P-protein and the nature of the contents of the sieve area pores, remain controversial.


1972 ◽  
Vol 50 (4) ◽  
pp. 839-846 ◽  
Author(s):  
A. P. Singh ◽  
L. M. Srivastava

The differentiation of sieve elements, companion cells, and vascular parenchyma in leaf bundles of corn is described. The sieve elements have plastids with distinctive crystalline inclusions, lack P-protein, and have nacreous walls in which the predominant orientation of microfibrils seems to be at right angles to the length of the cell. The companion and vascular parenchyma cells have numerous, well-developed mitochondria. These and other results are discussed in relation to long distance transport in the sieve elements.


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