ISSN:
1615-6102
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Summary Several theories of phloem transport are currently being considered in various laboratories on the basis of recent evidence. Proponents of the activated diffusion or protoplasmic theories claim support in the disclosure of fibrils, longitudinally arranged, in the connecting strands passing across sieve plates, and in the close connection between respiratory activity and transport. Those favoring a surface migration theory claim support in the demonstrated systems of sieve element lamellae, along whose surfaces one could imagine solute transport to occur. Proponents of the pressure flow theory point to results of exudation studies, tracer investigations, and to histochemical evidence indicating that sieve elements are relatively inactive, metabolically, as well as to the suggestion that perhaps the connecting strands are more “open” (tubular) than they have been considered to be up to now. Callose formation is stimulated by turgor changes, promoted by foreign chemicals, viruses, and, in the sieve element, by a relatively alkaline pH, a high sucrose concentration, and doubtless by the unique unbalanced character of sieve sap composition. The function of callose in older or wounded elements appears to be a constricting or plugging action, but its function in young mature elements is essentially obscure. Recent evidence augments the view that sieve elements display an extraordinary sensitivity towards experimental manipulation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01252054
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