Plastids and mitochondria in the phloem of Cucurbita
Contrary to some statements in the literature, the sieve elements of Cucurbita maxima Duchesne contain plastids. In young cells these organelles resemble mitochondria with regard to size and density of the matrix; but they have fewer internal membranes. During cell maturation, the plastids enlarge and their contents become thin and electron-transparent. They assume a partly degenerated appearance and do not deposit starch. The plastids of the companion cells resemble those of the young sieve elements but develop a considerable number of internal membranes, which may be organized into typical chloroplast grana. Starch is rarely encountered in these plastids. The plastids of the phloem-parenchyma cells are chloroplasts, commonly including starch grains. The mitochondria are similar in the three categories of cell but those of the sieve elements may show a denser matrix in young cells and structural abnormalities in mature cells.