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 investigated by transmission electron microscopy. With the exception of Pistia (with S-type plastids) all contained the monocotyledon specific subtype-P2 plastids characterized by cuneate protein crystals. While the species studied from both Acoraceae and Lemnaceae have form-P2c plastids (i.e., with cuneate crystals only), those of the Araceae belong to either form P2c (14 species), P2cs (the great majority) or P2cfs (Monstera deliciosa, only, with form-P2cs plastids in the other Monstera. species studied). The form-P2cs plastids of the Araceae are grouped into different categories according to the quantity and quality of their protein and starch contents. The subfamily Lasioideae is redefined to comprise all aroid P2c-taxa and those P2cs-genera that contain only one of very few starch grains. Only little starch is also recorded in the sieve-element plastids of Gymnostachys (Gymnostachydoideae), with the other plastid data denying a close relationship to Acorus. While equal amounts of starch and protein are generally present in sieve-element plastids of the subfamilies Pothoideae, Monsteroideae, Colocasioideae, Philodendroideae, and Aroideae, maximum starch content and only very few protein crystals are found in form-P2cs plastids of Calla (Calloideae), Ariopsis (Aroideae), and Remusatia (Colocasioideae?). In the latter, both morphology and size of sieve-element plastids are close to those of Pistia. - In the Araceae the diameters of the sieve-element plastids exhibit a great size range, but are consistent within a species and within a defined part of the plant body. Comparative data are mainly available for stem and petiole sieve-element plastids. - The accumulated data are used to suggest an affiliation of the species to subfamilies and to discuss the phylogeny of the Arales. Forms and sizes of their plastids support a separation of both Acoraceae and Lemnaceae from the Araceae. The presence of S-type plastids in Pistia does not favour direct and close relationships to the form-P2c genus Lemna. - The prevailing form-P2cs plastids might support proposals that place the Arales (together with also form-P2cs plastid containing Dioscoreales) in the neighbourhood of basal dicotyledons. Besides Asarum and Saruma (Aristolochiaceae), with monocotyledonous form-P2c plastids, Pistia (with dicotyledonous S-type plastids) gives another example for a link between the two angiosperm classes.