Pinus radiata needle trace studies: Fine structure of immature sieve cells in the primary phloem.

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Abstract
The development of sieve cells in the primary phloem of P. radiata needle traces was followed with the EM. Sieve cell precursors have thin walls, a more or less spherical nucleus containing small evenly scattered masses of condensed chromatin, dense cytoplasm and several small vacuoles. Among the most obvious early changes are increase in cell size, condensation of chromatin, nuclear lobing, changes in the location and orientation of microtubules, modifications in the ER [endoplasmic reticulum], increase in vacuolar size, increase in wall thickness and sieve pore development. Later in the development of sieve cells and prior to their maturation the nucleus with greatly condensed chromatin and disrupted envelope assumes a necrotic appearance, and most of the cytoplasmic structures disappear with the exception of mitochondria, plastids and elements of ER. Callose and P-proteins are not seen at any stage in the development of sieve cells.