Phyllonoma

Abstract: Phyllonoma, a small tree genus of four species distributed from Mexico to Peru, has been placed in various families (mainly in Saxifragaceae), but now, based on molecular evidence, is placed in a distinct family Phyllonomaceae in Aquifoliales. To better understand the morphological relationships of the genus and family, I studied its floral morphology, anatomy, and vasculature using P. tenuidens. Most of the external and internal floral characteristics were described more than 120 years ago. Although some of them were confirmed, some were substantially revised, mainly those concerning the gynoecial structure. Flowers are small and basically pentamerous, consisting of five sepals, five petals, five stamens, and a gynoecium composed of two carpels usually in transversal position. Comparisons with other Aquifoliales show that Phyllonomaceae share the inferior ovary, epiphyllous inflorescence and epigynous disc nectary with East-Asian Helwingiaceae (Helwingia only), but clearly differ from Helwingiaceae in having glandular trichomes on the sepal margins and a bicarpellate, unilocular gynoecium bearing many ovules on the parietal placentae. Evidence from floral morphology and structure supports the distinctness of Phyllonomaceae and its sister-group relationship with Helwingiaceae. Its floral characteristics suggest that Phyllonomaceae have evolved by adapting to distinct biological habitats in relation to pollination and seed dispersal.

Abstract: Phyllonoma, the sole genus of Phyllonomaceae (Aquifoliales) consisting of four Central American species, has not been well-characterized morphologically. Following a previous study of flower and inflorescence morphology, I here report the embryology of the genus based on P. tenuidens and compare its characteristics with those of other aquifolialean families, namely, Aquifoliaceae, Cardiopteridaceae, Helwingiaceae, and Stemonuraceae. Comparisons indicate that although Phyllonoma resembles all the other families embryologically, it more closely resembles Aquifoliaceae and Helwingiaceae in lacking a vascular bundle in its integument and bearing ab initio Cellular endosperm. The genus especially resembles Helwingiaceae by possessing a tenuinucellate ovule. This result corroborates molecular and floral morphological evidence, supporting the distinctness of Phyllonoma as a family and its sister-group relationship to East‒Asian Helwingiaceae. However, Phyllonoma is clearly distinguished from Helwingiaceae by seed coat structure. In Phyllonoma, the seeds (dispersed in berries) have a thick seed coat composed of irregularly enlarged, thick-walled exotestal cells, whereas the seeds (dispersed in drupes) have a thin membranous seed coat in Helwingiaceae. Taken together with earlier information on pollination (entomophily in Phyllonoma versus ambophily in Helwingiaceae), embryological evidence shows that distinct evolution has occurred in reproductive characters relating to pollination and seed dispersal in Phyllonoma.

Abstract: Helwingia, a shrub or small tree of four species distributed in East Asia, has been assigned to various families, mainly Cornaceae. However, molecular analyses show that the genus belongs to its own family Helwingiaceae which is sister to Phyllonomaceae (Phyllonoma only) in the order Aquifoliales. On the basis of H. japonica, we investigated the poorly understood floral and embryological characters of Helwingia, and compared their features with those of other Aquifoliales, particularly those of Phyllonomaceae. Results showed that perianth leaves of Helwingia represent sepals, because in plesiomorphic pentamerous flowers, they agreed in position with sepals (not with petals) in pentamerous flowers of Phyllonoma. Overall comparisons based on available information show that, while sharing with Phyllonoma the epiphyllous inflorescence, the inferior ovary, and an epigynous disc nectary as syapomorphies, Helwingia is characterized by loss of petals, obhaplostemony, large recurved stigmas, poorly developed disc nectary, tenuinucellate ovules with a mature female gametophyte filled with densely stained cytoplasm, and a thin mature seed coat. Morphological evidence, like molecular evidence, confirms that Helwingia is sufficiently distinct to be placed in its own family. Morphological and field observations suggest wind and insect pollination in H. japonica, which is the first example of ambophily in Aquifoliales.