Comparative Structure of Companion Cells and Phloem Parenchyma Cells in Mimosa pudica L.

Abstract

The phloem of Mimosa pudica L. furnishes an example of definable diversification of the parenchymatic members of the tissue into companion cells and parenchyma cells. The companion cells have dense protoplasts which contain the typical organelles of plant cells, including chloroplasts and many ribosomes. The sieve elements and companion cells are interconnected by numerous branched plasmodesmata. The companion cells degenerate when the associated sieve elements cease to function. The parenchyma cells have less dense protoplasts than the companion cells. In many parenchyma cells the rough endoplasmic reticulum assumes a tubular form, and bundles of microfilaments are present. The cytoplasmic ribosomes occur in groups apparently held together by fibrils. Chloroplasts, mitochondria (some are exceptionally long), dictyosomes, microbodies, and microtubules are the other cell components. Whether the parenchyma cells are ontogenetically related to the sieve elements or not, they do not degenerate when the sieve element ceases to function.