Thin sections of the oldest uncrushed sieve elements in two vascular bundles have been examined by electron microscopy. They are shown to contain bundles of about 50 P-protein tubules which are no different in appearance from tubules in immature sieve elements. Tubules in these old sieve elements have remained interlinked with ‘side-arms’ throughout the time that the sieve elements were translocating. The accumulation of callose on each sieve element pore, which signifies senescence, may incidentally protect the tubules from changes in pressure whilst they are fixed. The generally held conclusion that P-protein tubules change into narrow striated filaments and disperse from their parietal positions as the sieve element reaches maturity is re-examined.
In the development of the sieve element the central body of the P-protein crystal is shown to be present very early, at a stage when the sieve element and the companion cell are otherwise still difficult to distinguish.
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