Developmental morphology and cytology of the young Maize embryo (Zea mays L.)

Young embryos of maize were analyzed with light and electron microscopical techniques to determine the sequence of morphological, anatomical and cytological events resulting in the onset and initial development of the embryo axis and the scutellum. The proembryo stage ends at 5 days after pollination (DAP). The radial symmetry of the proembryo changes into a bilateral symmetry, probably caused by the excentric position of the embryo apex within the endosperm. Scutellum formation starts at the apex and posterior side of the embryo proper and is characterized cytologically by different types of cell growth and multiplication in epidermis and mesophyl. From anatomical observations it is concluded that the coleoptile arises as a protuberance of the scutellum. Shoot meristem formation initiates in the protoderm at the anterior side of the embryo and is characterized cytologically by decreasing cell sizes, by vacuolation and by an increase of cytoplasm. The lateral orientation of the shoot meristem is likely caused by the failure of a second cotyledon to develop. Root meristem formation is somewhat retarded. Only vacuolized cells at the base of the embryo proper were detected at 5 DAP but at 7 to 8 DAP the root meristem was identified as a group of dividing cells with few vacuoles and much cytoplasm. The meristem is not located at the exterior of the embryo but at the base of the embryo proper, in direct line with the suspensor. The coleorhiza is formed adjacent to the seminal root. Its exogenous character was not established. The new axis from root to shoot meristem deviates from the axis of the proerabryo as a result of the lateral position of the shoot meristem. Lateral outgrowth of the shoot is, however, restricted by mechanical forces from outside and growth of tissues inside the embryo. In the determination of the location of the meristems next to morphological, also physiological and genetical factors are important. Metabolic gradients and physiological sinks are established already before the zygote divides and expression of polarity is present in all cormophyta.