Is Plant Morphology Still Alive? Fundamental Flaws in Genetic and Transcriptomic Support for the Bipartite Interpretation of the Grass Cotyledon

Despite over two centuries of debate, the morphological nature of the grass embryo’s parts remains unresolved. The bipartite interpretation of the grass cotyledon suggests that it is composed of two parts: the scutellum and the coleoptile. According to Wu et al. (2024, 2025), the cotyledon in maize is organized similarly to the vegetative leaf, comprising scutellum and coleoptile, homologous to the leaf blade and sheath, respectively. This view, which Wu et al. (2024, 2025) erroneously associate with the bipartite model of grass cotyledon, is morphologically impossible because within the embryo, the coleoptile is located above the scutellum and therefore cannot be homologous to the latter’s sheath that is suppressed. Within a bipartite context, the coleoptile may only be homologous to the leaf ligule or represent a de novo outgrowth of the scutellum’s reduced sheath, thus lacking homology with mature leaf structures. Molecular evidence from the mesocotyl and the leaf collar region is essential for evaluating the bipartite nature of the grass cotyledon. The genetic results of Wu et al. (2024) do, in fact, support the view that the coleoptile is the next leaf on the embryo’s axis after the scutellum, thus disproving the bipartite view of the cotyledon in Poaceae.