P-TYPE SIEVE-ELEMENT PLASTIDS AND THE SYSTEMATICS OF THE ARALES (SENSU CRONQUIST 1988) - WITH S-TYPE PLASTIDS IN PISTIA

The sieve-element plastids of 126 species of the Arales were investiga ted by transmission electron microscopy. With the exception of Pistia (with S-type plastids) all contained the monocotyledon specific subtyp e-P2 plastids characterized by cuneate protein crystals. While the spe cies studied from both Acoraceae and Lemnaceae have form-P2c plastids (i.e., with cuneate crystals only), those of the Araceae belong to eit her form P2c (14 species), P2cs (the great majority) or P2cfs (Monster a deliciosa, only, with form-P2cs plastids in the other Monstera. spec ies studied). The form-P2cs plastids of the Araceae are grouped into d ifferent categories according to the quantity and quality of their pro tein and starch contents. The subfamily Lasioideae is redefined to com prise all aroid P2c-taxa and those P2cs-genera that contain only one o f very few starch grains. Only little starch is also recorded in the s ieve-element plastids of Gymnostachys (Gymnostachydoideae), with the o ther plastid data denying a close relationship to Acorus. While equal amounts of starch and protein are generally present in sieve-element p lastids of the subfamilies Pothoideae, Monsteroideae, Colocasioideae, Philodendroideae, and Aroideae, maximum starch content and only very f ew protein crystals are found in form-P2cs plastids of Calla (Calloide ae), Ariopsis (Aroideae), and Remusatia (Colocasioideae?). In the latt er, both morphology and size of sieve-element plastids are close to th ose of Pistia. - In the Araceae the diameters of the sieve-element pla stids exhibit a great size range, but are consistent within a species and within a defined part of the plant body. Comparative data are main ly available for stem and petiole sieve-element plastids. - The accumu lated data are used to suggest an affiliation of the species to subfam ilies and to discuss the phylogeny of the Arales. Forms and sizes of t heir plastids support a separation of both Acoraceae and Lemnaceae fro m the Araceae. The presence of S-type plastids in Pistia does not favo ur direct and close relationships to the form-P2c genus Lemna. - The p revailing form-P2cs plastids might support proposals that place the Ar ales (together with also form-P2cs plastid containing Dioscoreales) in the neighbourhood of basal dicotyledons. Besides Asarum and Saruma (A ristolochiaceae), with monocotyledonous form-P2c plastids, Pistia (wit h dicotyledonous S-type plastids) gives another example for a link bet ween the two angiosperm classes.