Structure of the ovule Posted Date: 12 Feb 2009     Resource Type: Articles/Knowledge Sharing     Category: Education Author: Hari Member Level: Silver     Rating: Points: 4

Structure of the ovule Each ovule is provided with two coats, an outer and an inner. These coats are known as the integuments. In some ovules, however, there is only one integument. Enclosed within these integument, and forming the bulk of the ovule, there is a special kind of tissue called the nucellus. The nucellus is very rich in reserve food materials. Lying embedded in the nucellus, is the most important part of the ovule. This is a sac-like structure (or cell) known as the embryo sac. The ovule is attached to the placenta by a small stalk called the funicle. The two integuments do not cover the nucellus completely, but leave a minute opening at the top. This is the micropyle and it leads to the nucellus. The integuments and the nucellus are free from each other at all places except at the base where they are completely fused. This place of fusion is known as the chalaza. The place where the funicle enters the ovule is called the hilum. The embryo sac The embryo sac is situated in the nucellus, a little below the micropyle. In the middle of the embryo sac, there is a small, highly specialized, spherical body known as the nucleus. This nucleus divides into two. The two daughter nuclei move away from each other and reach the opposite ends of the embryo sac. Then, each of them divides ywice in succession, so that at each end of the embryo sac there are now four nuclei. One nucleus form each end travels to the middle of the embryo sac. The two nuclei meet and fuse together. The fused nucleus is known as the secondary nucleus. The three nuclei at the micropylar end surround themselves with walls so as to form three small compartments or cells. These constitute the egg apparatus. The middle one is the egg (female gamete). The two side ones are known as the helpers or synergids. Similarly, the three nuclei at the other end of the embryo sac also get organized into three cells called the antipodal cells. This is the structure of the embryo sac at the time of fertilization. (Fig.)

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