The nucellus is the central core of tissue within a plant ovule, enclosed by integuments, and serves as the site where the female megasporocyte undergoes meiosis and where the female gametophyte (embryo sac) develops.
Understanding the Nucellus in Detail
The nucellus plays a vital role in plant reproduction. Here’s a breakdown of its structure and function:
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Location: The nucellus is located within the ovule, the structure that will eventually develop into a seed after fertilization. It's surrounded by one or two protective layers called integuments, which leave a small opening called the micropyle.
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Composition: It consists of diploid (2n) parenchymatous cells.
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Function: The primary function of the nucellus is to house the megasporocyte (megaspore mother cell). This cell undergoes meiosis to produce four haploid (n) megaspores. Typically, only one of these megaspores survives, while the others degenerate. The surviving megaspore then develops into the female gametophyte, also known as the embryo sac. The embryo sac contains the egg cell, which will be fertilized by a sperm cell from the pollen grain.
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Nutrition: The nucellus also provides nourishment to the developing embryo sac. In some plants, the nucellus persists even after fertilization and provides nutrients to the developing embryo. This persistent nucellus is called the perisperm.
Nucellus and Embryo Sac Development
The development of the embryo sac within the nucellus is a crucial step in plant reproduction:
- Megasporogenesis: The megasporocyte within the nucellus undergoes meiosis, forming four megaspores.
- Megagametogenesis: Typically, one megaspore survives and develops into the female gametophyte (embryo sac) through mitosis. This embryo sac houses the egg cell, synergids, antipodal cells, and the central cell (containing polar nuclei).
Nucellus and Seed Development
After fertilization, the nucellus may or may not persist:
- Exalbuminous seeds: In some seeds, the developing embryo completely consumes the nucellus. These seeds are called exalbuminous seeds (e.g., beans, peas).
- Albuminous seeds: In other seeds, a portion of the nucellus persists as the perisperm, a nutritive tissue. These seeds are called albuminous seeds (e.g., coffee, black pepper).
Significance
The nucellus is essential for sexual reproduction in flowering plants, as it is directly involved in the formation of the female gametophyte and provides initial nourishment for the developing embryo. Its presence or absence in the mature seed also defines seed type, influencing germination and seedling establishment.
