Floral structure of Philodendron propinquum (Araceae) and a comparative study of the Philodendron subgenera

Philodendron flowers are numerous, small, unisexual or sterile, without perianth and exclusively pollinated by beetles, representing an Angiosperm-ancient pollination syndrome. Species of the genus are traditionally classified under three subgenera, P. subgenus Philodendron, P. subgenus Meconostigma, and P. subgenus Pteromischum. In recent phylogenetic analyses, two main lineages of Philodendron were recovered, Meconostigma and a Pteromischum + Philodendron lineage. Although a modest advance in the knowledge of the evolutionary history of the genus has been made, little attention has been paid to floral morphology in this group, in particular to structural studies on flowers of P. subg. Pteromischum in light of pollination biology. Our aims were (i) to investigate the flower structure of P. propinquum (P. subg. Pteromischum) through light and scanning electron microscopy and histochemistry; and (ii) to compare relevant floral characters among the three subgenera. Flowers of P. propinquum, which is endemic to the Atlantic Forest, are described for the first time. The pistillate flower presents a shallow, broad compitum, and a large number of ovules per locule; pistillate and staminate flowers present subepidermal druses; sterile units present raphids and starch grains; the papillose epidermal cells from sterile units and staminate flowers present terpene contents and stomata, probably representing diffuse osmophores. Our analysis suggests that the balance between pollen uptake (stylar lobes) and accessibility to the locules (stylar canals and compitum) were decisive to drive gynoecium evolution of the Philodendron genus. The presence and location of druses and raphides in stamen and gynoecium indicate a protective function against florivores.