Tubular sclereids: A newly identified sclerenchyma cell type in tropical seeds.

Hard seed coats are a common adaptation that provides protection and regulates germination in many tropical plant species. One of the most characteristic cell types contributing to seed coat hardness is the sclereid. Although several sclereid types have been described, their anatomical diversity in the seed coats of tropical taxa remains underexplored. In this study, we examined the seed coat anatomy of six tropical plant species collected from southern Thailand using light microscopy, cell scanning electron microscopy and quantitative analyses. We identified a distinct sclereid form that shares partial similarity with previously described elongated sclereid types, including macrosclereids, filiform sclereids, and vermiform sclereids, but is clearly distinguished by differences in cell shape, length and overall morphology. These sclereids exhibit smooth, evenly thickened cell walls, a continuous elongated lumen extending along the entire cell length, circular blunt ends, and a cylindrical morphology. We designate this cell type as tubular sclereids. Tubular sclereids were consistently located in the tegmen layer of Croton persimilis, Entada rheedei, Elateriospermum tapos, Ricinus communis, Hevea brasiliensis, and Jatropha curcas. This finding expands the known diversity of sclerenchyma cell types and suggests a specialized role in seed coat adaptation in tropical environments.