Morphological characterisation of silica phytoliths in Neotropical Marantaceae leaves

The Marantaceae family is composed of 550 species of pantropical distribution, with a strong clustering in the Neotropics. Although the family forms a monophyletic group, the circumscription of genera and consequently its phylogenetic relationships are not sufficiently understood. Its species produces silica phytoliths in varied forms, ornamentation and distribution in leaves, and these silica deposits are considered important in vegetable physiology and taxonomy. We have conducted the morphological characterisation of phytoliths and their distribution in foliar blades of Neotropical Marantaceae genera, evaluating the use of such characters to support the morphological delimitation of the genera and the physiological aspects of the family's adaptation to rainforests. Phytoliths of 22 species of 12 genera, namely Calathea, Ctenanthe, Goeppertia, Hylaeanthe, Ischnosiphon, Koernickanthe, Maranta, Monotagma, Myrosma, Saranthe, Stromanthe and Thalia, were analysed using light and scanning electron microscopes. When comparing Neotropical species to those of the old world, one can observe the family's homogeneity by the presence of two morphotypes in the foliar blade, the globular and the irregular, as well as stegmata associated with the fibres. However, two distinguishing features have been identified in these Neotropical species: (1) a new morphotype, the cylindroid, located in the mesophyll, (2) intra- and extracellular amorphous silica deposits associated with the stomata, which play an important role in the restriction of transpiration. In addition, the form, size and distribution of the silica deposits appear conservative in all analysed samples, which makes them useful in the morphological characterisation of the genera and species, and consequently, in the taxonomic circumscription of the Marantaceae.