BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.5. Sexual reproduction in flowering plants

21.5.4. Development of the ovule

Each carpel consists of a stigma, a style, and an Ovary. Within the ovary, one or more ovules are formed, each attached to the ovary wall at a region called the Placenta by means of a short stalk known as the funicle; Water and nutrients are supplied to the developing ovule through the funicle.

The main body of the ovule is the nucellus, which is surrounded by two protective layers called integuments. At one end of the ovule, the integuments leave a small pore known as the micropyle.

Within the nucellus near the micropyle, a single spore mother Cell develops, known as the embryo sac mother cell. This diploid cell undergoes Meiosis to form four haploid Cells, of which only one survives to give rise to the embryo sac (Fig. 21.24). The embryo sac enlarges, and its Nucleus undergoes repeated mitotic divisions until eight nuclei are formed, four at each end of the embryo sac. One of these Functions as the female gamete nucleus.

Two of the nuclei migrate to the center of the embryo sac and fuse to form a single diploid nucleus. The remaining six nuclei, three at each end, become enclosed by thin cell walls; usually, only one of these—the female gamete nucleus—takes part in subsequent development, while the others degenerate.

The final Structure of a mature carpel is shown in Fig. 21.25.

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Fig. 21.24. Development of the embryo sac and female gamete.

Fig. 21.25. Cytology/practical/54.html">Longitudinal section of a carpel at the time of Fertilization. Note that the ovule, which develops into the seed after fertilization, contains both diploid parental tissue and haploid embryo sac tissue.