Fine structure of the differentiating sieve elements of Vicia faba

The fine structure of the sieve elements of the primary phloem of the epicotyl of Vicia faba is described. The cytoplasm of the young sieve element contains four distinct forms of "slime" body: amorphous, crystalline, tubular (each tubule measuring about 140 Å in diameter), and short fibrillar (each fibril measuring about 350 Å in diameter). At the very early stages of differentiation, polysome helices are prevalent often in close association with the amorphous but not the other forms of "slime" bodies. At the early stages of development of the sieve element the tubular form of "slime" is closely associated with the endoplasmic reticulum, which suggests their possible origin. The plastids of the sieve element lack a well-developed internal membrane system but contain two characteristic types of inclusion bodies, starch granules and crystalloids identical to those revorted for the secondarv vhloem of the root of Pisum sativum. Mitochondria - remain apparently unchanged throughout sieve element development. Microtubules are present during the early stages of sieve element development but become scarce at later stages. Dictyosomes, coated vesicles, ribosomes, polysomes, and nucleus disappear as the sieve element matures. The fine structure of the sieve plate pore initial is complex. It consists of an outer electron-dense ring ("desmotubule") which encloses a central dark core. The developmental pattern of the sieve plate pore has been traced from the very young to the mature stages.