STRUCTURE AND DEVELOPMENT OF SIEVE ELEMENTS IN THE LEAF OF ISOETES MURICATA

Leaf tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. The very young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids and the presence of crystalline and fibrillar proteinaceous material in dilated cisternae of the rough ER. During differentiation, the portions of ER enclosing this proteinaceous substance become smooth surfaced and migrate to the cell wall. Along the way they apparently form multivesicular bodies which then fuse with the plasmalemma, discharging their contents to the outside. At maturity, the sieve element contains an elongate nucleus, which consists of dense chromatin material, and remnants of the nuclear envelope. In addition, the mature sieve element is lined by a plasmalemma and a parietal, anastomosing network of smooth ER. Both plastids and mitochondria are present. P-protein is lacking at all stages of development. Tonoplasts are. not discernible in mature sieve elements. The end walls of mature sieve elements contain either plasmodesmata or sieve pores or both, but only plasmodesmata occur in the lateral walls.