Anatomy of vegetative organs in Allionia (Nyctaginaceae), with emphasis on the vascular system

Allionia is a small genus within the tribe Nyctagineae (Nyctaginaceae) which has a controversial, infrageneric delimitation. Here, we investigated the two known species of Allionia in order to characterize the anatomy of leaves, stems and roots, with further notes on vascular system development. Additionally, the present study aimed to broaden our knowledge of stem vascular diversity and to survey for anatomical features with diagnostic value in distinguishing A. choisyi from A. incarnata. Leaf anatomy of other Nyctagineae taxa was also analysed. Anatomical and ontogenetic observations from the vegetative organs in Allionia revealed no diagnostic features to distinguish the two species. We illustrated the occurrence of Kranz anatomy, which in Nyctaginaceae is only known in Allionia, Boerhavia, and Okenia. The stem primary vascular system was unusual in showing a polycyclic eustele (medullary bundles + continuous concentric procambium). Likewise, mature stems and roots show vascular cambial variants (successive cambia) that arise from the pericycle. The anatomy and histochemistry of multicellular glan-dular trichomes observed in aerial organs were presented. Raphids were seen in all organs. Although no strong xerophytic features were observed in Allionia, several characteristics can be associated with their arid habitats. Our findings on the vascular system of Allionia showed the two species to be much the same and reinforced earlier findings that the stem anatomy of Nyctaginaceae is complex and intriguing.