The following four generalizations apply to both plants and animals.

1. A gamete is a reproductive cell. A male gamete is usually small with a nucleus and little cytoplasm; it is the gamete which leaves the male organ and moves about, either by its Own power or by external agencies like wind or insects.

2. The female gamete is larger, with a nucleus and more cytoplasm than the male; it sometimes contains food reserves. Often it does not leave the female organ or body in which it is produced until after it is fertilized. The male gamete in a flowering plant is a nucleus in the pollen grain; in most animals it is the sperm. The female gamete in plants is a large egg-cell in the ovule, while in animals it is the ovum.

3. The product of the fusion of male and female gametes is called a zygote.

4. Fertilization is the fusion (Gjoining together) of the nuclei of male and female gametes to form a zygote. After fertilization the zygote undergoes cell division and growth, developing into a new individual or a preliminary form which may be an embryo, a seed, or a larva.

Fertilization in plants

Fertilization follows pollination, but the interval of time be tween the two events varies in different species from sixteen hours to twelve months. The pollen grain absorbs nutriment secreted by the stigma, and the cytoplasm in the grain grows out as a tube. This tube grows down through the style between the cells absorbing a nutritive fluid from them. On reaching the ovary it grows to one of the ovules and enters it through a hole, the micropyle. The tip of the pollen tube breaks open in the ovule, and the male nucleus, which has been passing down the tube, enters the ovule and fuses with the female nucleus there.

Each egg-cell of an ovule can be fertilized only by a male nucleus from a separate pollen grain.

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Result of fertilization

Fruit and seed formation. After fertilization the petals, stamens, style and stigma wither and usually fall off.

The sepals may persist in a dried and shrivelled form. Food made in the leaves reaches the fertilized ovules and the ovary, which grow rapidly. Inside the ovule cell division and growth produce a seed containing a potential plant or embryo. The embryo consists of a miniature root or radicle, a small shoot or plumule, and one or two leaves, the cotyledons, which usually Contain food reserves. The integuments of the ovules become thicker and harder, forming the testas of the seeds, and finally, water is withdrawn from the seeds, making them dry and hard. In this condition they are best able to withstand droght and other adverse conditions.

The ovary wall may become dry and hard, forming a capsule or pod as in Aristolochia and Crotalaria, or it may become succulent and fleshy as in the tomato, pawpaw and mango Fruits. In a strictly biological sense, the term fruit means the fertilized ovary of a flower and in only a small number of cases is such a fruit edible.

There are various ways of classifying fruits; for example, into dehiscent and indehiscent categories. A dehiscent fruit is one which opens to release its seeds, e.g. Thunbergia. Ainndehiscent fruit does not open; it falls from the plant as a whole and may have to decay at least partially before the seeds are able to germinate, e.g. pawpaw and tomato. The dehiscent fruits are usually dry but the indehiscent fruits may be dry, e.g. the sunflower fruit, or succulent. There are two categories of succulent indehiscent fruits, drupes and berries. A drupe is a fleshy fruit containing one seed; the ovary wall forms three layers, the outer *skin" or epicarp, the inner fleshy or fibrous layer or mesocarp and a hard stony layer, the endocarp, which surrounds the seed. The mango and the oil palm fruit are examples of drupes. A berry is a fleshy fruit, usually with more than one seed. The ovary forms an epicarp and a fleshy mesocarp but no hard endocarp. Examples are thee tomato and the guava. A multiple fruit such as the fig or the pineapple is formed from an inflorescence rather than a single flower Parthenocarpy. Strictly, parthenocarpy means the develop- ment of a fruit from an ovary without fertilization. In some cases the stimulus of pollination is needed for fruit develop- ment, in others, e.g. banana, pollination is unnecessary.

Parthenocarpic fruits are often seedless, e.g. certain pineapples, and since it is not always easy to tell if fertilization has occurred, the term parthenocarpic is often applied to all seedless fruits.


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