Plastids in sieve elements and their companion cells : Investigations on monocotyledons, with special reference to Smilax and Tradescantia.

Plastids have been identified in the sieve elements and/or companion cells of 14 monocotyledon species. In contrast to earlier reports, plastids are present in the sieve elements of Smilax and the companion cells of Tradescantia. The development and fine structure of the sieve-element plastids in Smilax do not differ from the type found in all of the 230 angiosperm species we have studied so far contain prominent plastids. The companion cells are easily identified by their specialized plasmatic connections with the sieve elements. The leucoplasts in the companion cells of Tradescantia are identical with those reported for many angiosperms.