Studies of the secondary phloem of 6 species of conifers revealed that mature sieve‐cell protoplasts contain internal strands which are derived ontogenetically from slime bodies of immature cells. These strands, each measuring about 0.3 μ in diameter, traverse the cell and run from cell to cell through sieve‐area pores. Coniferous sieve cells have much in common—both ontogenetically and structurally—with dicotyledonous sieve‐tube members.
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Evert et al. (1965) studied this question.