Sieve-element plastids ofGymnospermae: Their ultrastructure in relation to systematics

The distribution of S-type and P-type plastids in the sieve elements of 30 species from 13 families of theConiferophytina andCycadophytina is recorded, of which 21 species were studied for the first time with respect to their sieve-element plastids. While starch storing S-type plastids are the most commonly occurring type throughout both taxa, all thePinaceae examined (11 species of 7 genera) contain P-type plastids characterized by a peripheral, ring-shaped bundle of protein filaments, an additional protein crystalloid, and several starch grains. Starch grains of sieve-element plastids in theConiferophytina andCycadophytina are commonly club-shaped. Taxonomic implications of these ultrastructural findings on sieve-element plastids are discussed.