The structure and prophylactic role of the angiosperm embryo sac and its associated tissues:Zea mays as a model

Summary Various developmental phases can be distinguished in the definition of the archesporium and the early life of the embryo, takingZea mays (maize) as a model within the family Gramineae, and other families where pertinent: (1) the isolation of the megasporocyte and the functional spore derived from it; (2) the maturation of the specialized walls of the embryo sac, and their reinforcement by ensheathments derived from the contiguous nucellar cells during a sequence of phased genetic ablation; (3) the differentiation of the synergids, the associated flange, and the filiform apparatuses; (4) the blocking of the pollen tube pathway by secondary secretions in the micropylar region and the coagulation of the pollen tube cytoplasm within the filiform apparatuses during the process of fertilization; and finally (5) the development of a compound cutinized envelope of four fused layers (six where the outer integument is also involved) after fertilization. For the nascent haploid generation, the period of maximum vulnerability in respect to both pathogen invasion and the transition from diplophase control occurs during these phases. It is concluded that many of the protective features form a prophylactic shield and are key components of the angiosperms in general, which may have contributed to their evolutionary success as a group. Other physiological or biochemical adaptations or barriers may also supplement the mainly structural features described here.