A close sheathing of the plastids by endoplasmic reticulum has been observed. This is restricted to the companion cells and developing sieve tubes of the phloem of Acer pseudoplatanus and the resin canal cells and leaf callus cells from Pinus pinea. The sheathing is transitory in callus and sieve tubes but is a permanent feature of the companion and the resin canal cells. Possible functional relationships between the two organelles are discussed.
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Wooding et al. (1965) studied this question.
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