On the ultrastructure and differentiation of the phloem in sugarcane leaves.

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Abstract
Very young sieve elements are characterized by a thin wall, relatively dense cytoplasm, and plastids with the characteristics of proplastids. Among the most pronounced changes accompanying early and mid phases of sieve element differentiation are acquisition of electron dense, proteinaceous inclusions by the plastids; an increase in the size of plastids and their proteinaceous inclusions; an increase in wall thickness, and development of sieve plates and pore sites. In advanced stages of their differentiation, sieve elements are subjected to the action of a variety of enzymes, many of which function in hydrolyzing various protoplasmic components. At maturity, sieve elements lose nuclei, most of the endoplasmic reticulum, ribosomes, dictyosomes, microtubules and tonoplasts. The companion and parenchymatic cells undergo little changes as compared with their adjoining sieve elements. As judged by their nuclear and cytoplasmic condition, companion cells are metabolically active throughout their life. Plasmodesmata are present among sieve elements, companion cells and parenchymatic cells throughout their development and at the maturity of these elements.