The polytenic endosperm haustorium of Rhinanthus minor (Scrophulariaceae): functional ultrastructure

The seed of Rhinanthus undergoes an uncommon differentiation in developing both a chalazal and a micropylar endosperm haustorium. The chalazal haustorium is described at the ultrastructural level. It shows two giant nuclei with polytene chromosomes and numerous complex nucleoli. The nuclear surface is highly interdigitated with the cytoplasm and displays many pores. The cell wall of the apical region of the haustorium displays a prominent wall labyrinth. A large number of mitochondria are located in this region, while leucoplasts are mainly found in vicinity of the nuclei. The structural features of the haustorium are discussed in relation to its expected functions, i.e., synthetic activity and transfer of nutritive material from surrounding tissues to the endosperm proper. Key words: endosperm, haustorium, polyteny, Rhinanthus, transfer cell.