Nucellus

Nucellus is a part of the ovule of seed plants. It is the largest part of the ovule and is located in the center. The nucellus is surrounded by two protective layers, the integuments and the chalaza. The nucellus is the site of megagametogenesis, the process that leads to the development of the megagametophyte, which will become the embryo sac during ovule development.

Structure and Function

The nucellus is a tissue that is part of the innermost layer of the ovule, being surrounded by the integuments. It is equivalent to the megasporangium in non-seed plants. The nucellus remains connected to the placenta of the ovule via the funiculus.

The primary function of the nucellus is to nourish the developing megagametophyte. After fertilization, it may also contribute to the nourishment of the embryo. In some species, the cells of the nucellus are capable of dividing and can contribute to the formation of the endosperm, a tissue that provides additional nourishment to the developing embryo.

Development

During the development of the ovule, the nucellus undergoes a process called megagametogenesis to form the megagametophyte. This process begins with the differentiation of a single cell of the nucellus, the megaspore mother cell. This cell undergoes meiosis to produce four megaspores, of which usually only one survives.

The surviving megaspore undergoes several rounds of mitosis to produce the megagametophyte. This structure, also known as the embryo sac, contains the egg cell that will be fertilized to form the embryo, as well as several other cell types that have roles in fertilization and embryo development.

Evolution

The presence of a nucellus in the ovule is a characteristic feature of seed plants, and is not found in non-seed plants. The evolution of the nucellus is thought to be related to the transition from a gametophyte-dominant life cycle to a sporophyte-dominant life cycle, which is a key feature of the evolution of seed plants.

See also

• Ovule
• Megagametogenesis
• Integuments
• Chalaza
• Funiculus
• Endosperm