Ovule

Ovules in flower

Ovule morphology anatropous

Ovule-Gymno-Angio-en

Modellreihe von Grundformen der Samenanlagen -Brendel Nr. 164-166-

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Embryosack Polygonum Lilium

Embryosac-en

Ovule refers to the structure in seed plants that develops into a seed after fertilization. It is a key component in the reproductive process of gymnosperms and angiosperms, playing a crucial role in plant sexual reproduction. The ovule is typically found within the ovary of a flower in angiosperms or exposed on the scales of a cone in gymnosperms. This article provides an overview of the structure, function, and development of the ovule in plants.

Structure

The ovule consists of several key parts, each playing a vital role in its development and the eventual formation of seeds. The main parts include the nucellus, surrounded by one or two protective layers called integuments. The only opening in the integuments, the micropyle, allows for the entry of pollen or pollen tubes during the process of fertilization. Inside the nucellus lies the megaspore mother cell, which will undergo meiosis to produce megaspores, one of which will develop into the embryo sac or female gametophyte.

Function

The primary function of the ovule is to develop into a seed following fertilization, which involves the fusion of male and female gametes. The ovule provides the necessary environment and nutrients for the developing embryo sac, facilitating the processes of gamete production, fertilization, and seed development. Once fertilization occurs, the ovule undergoes a series of developmental changes, leading to the formation of a seed. The seed contains the embryonic plant, endosperm (a source of nutrition for the embryo), and is enclosed by a protective seed coat, which develops from the integuments of the ovule.

Development

Ovule development is a complex process that begins with the differentiation of a small group of cells within the ovary into ovule primordia. Through a series of developmental stages, these primordia form the nucellus, integuments, and other structures of the ovule. The megaspore mother cell within the nucellus undergoes meiosis to produce four megaspores, of which usually only one survives and develops into the embryo sac. This embryo sac will contain the egg cell, ready for fertilization.

Types of Ovules

Ovules can vary in their orientation and structure, leading to different classifications, such as:

• Orthotropous ovule: The ovule is straight with the micropyle, nucellus, and funiculus (the stalk attaching the ovule to the ovary) aligned.
• Anatropous ovule: The most common type in angiosperms, where the ovule is inverted, bringing the micropyle close to the point of attachment.
• Campylotropous ovule: The ovule is curved, with the micropyle located near the base.
• Hemitropous ovule: The ovule is half inverted, with the micropyle and the funiculus at right angles.

Pollination and Fertilization

Pollination involves the transfer of pollen from the male structures (anther) to the female part (stigma) of a flower or directly to the ovule in some gymnosperms. Following pollination, a pollen tube grows from the pollen grain, through the micropyle, and into the nucellus to deliver sperm cells to the egg cell within the embryo sac. Fertilization results in the formation of a zygote and eventually a seed, marking the completion of the sexual reproduction cycle in plants.

Conclusion

The ovule is a fundamental structure in the reproduction of seed plants, facilitating the processes of gamete production, fertilization, and seed development. Its complex structure and development are essential for the successful propagation of plant species, making it a critical area of study in botany and plant sciences.