Sieve–element characters

Of all the ultrastructural features recognized within sieve elements their specific plastids provide the most successful characters in seed plant systematics. Sieve–element plastids are classified into subtypes and forms according to their protein and starch contents. Presently 26 different forms grouped into six subtypes within the two basic types (S– and P–) may be discerned. Plastid forms containing protein crystals, protein filaments and starch grains are proposed to be primitive, forms missing any one of these contents held to be derived (a synoptical key is given to ease the identification of the different plastid forms). Based on the quantitative distribution of over 1500 investigated species and the suggested evolution of plastid forms a cladistic diagram is prepared to demonstrate interrelationships between forms of sieve–element plastids and the evolution of seed plant taxa. Correlations exist between subtype PII and Monocotyledons, subtype PHI and Centrospermae, subtype PIV and Fabales. Genuine plastid forms characterize Buxaceae (PVIc), Erythroxylaceae and Rhizophoraceae (both PVc) and Cyrillaceae (PVcf). The Magnoliiflorae are distinct by presence in its families of a great number of forms of subtype PI. Phylogenetic correlations for some of these taxa are discussed. – Crystalline P–protein of sieve–elements provides another character to be used for the delimitation of some families (e.g. Fabaceae), while presently ER–complexes or other organelles of sieve elements do not contribute to seed plant systematics.