The chalaza (pronounced /kəˈleɪzə/; from Greek χάλαζα "hailstone"; plural chalazas or chalazae) is a structure inside animal eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.
In animals
In animal eggs, the chalaza is composed of one or two spiral bands of tissue that suspend the yolk in the center of the white. It acts similar to an umbilical cord in mammals; the growing embryo receives its nutrients from the yolk. The purpose of the chalaza is to hold the yolk in place.
In plants
In plant ovules, the chalaza is located opposite the micropyle opening of the integuments. It is the tissue where the integuments and nucellus are joined. Nutrients from the plant travel through vascular tissue in the funiculus and outer integument through the chalaza into the nucellus. During the development of the embryo sac inside a flowering plant ovule, the three cells at the chalazal end become the antipodal cells.
Chalazogamy
In most flowering plants, the pollen tube enters the ovule through the micropyle opening in the integuments for fertilization. In chalazogamous fertilization, the pollen tubes penetrate the ovule through the chalaza rather than the micropyle opening. Chalazogamy was first discovered in monoecious plant species of the Casuarinaceae family, but has since then also been observed in others, for example in pistachio and walnut.
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