Sexual and Asexual Reproduction in living organisms

SEXUAL AND ASEXUAL REPRODUCTION 
One or the most important characteristics of living orgamsms is their power of reproducing their kind. Among plants reproduction may take place in a variety of different ways. It is quite common for the simplest ts to multiply by the method of binary fission, in which the parent individual splits into two. In others, a small portion of the plant grows out to form a bud, which eventually enlarges into a new individual like the parent, from which it may be finally separated. Many plants, too, produce small, round cells, often with a thick resistant coat which withstands adverse conditions. These cells are called spores, and under suitable conditions. These cells called spores, and under suitable conition they are detached and develop into new plants.
In all these methods of reproduction only one individual is necessary :or the production of a new generation. Such a type of reproduction is called asexual. More commonly, however, reproduction is preceded by a sexual act, for the purpose of which two individuals or two cells are generally required. The sexual act always includes a fusion of nuclei, which is the essential feature, and generally also the fusion of cells.
In sexual reproduction special cells are set apart in the body for the roduction of new individuals. In all but the lowest types these cells are of two kinds, which we call male and female. They are often produced in the same individual, but if not, we may distinguish male and female organisms according to the type of cell produced. These special cells are the sex cells, or gametes. For reproduction to take place it is necessary for two gametes, one from the male and the other from the female, to come together and fuse completely, forming a fusion cell, or zygote. The zygote is the potential new individual.
The male and female gametes are usually unlike in appearance and behaviour. The female gamete is generally a large cell with much cytoplasm and the nucleus is relatively small. As a result of its size this cell is usually non-motile. Such a body is termed an oosphere.
The male gamete presents a marked contrast to the oosphere. It is comparatively small, and is usually produced in large numbers. It may consist only of nuclear material and at most it has only a small amount of cytoplasm. In very many plants it is provided with hair-like protoplasmic appendages called flagella, by means of which it can propel itself through water. In this way it is able to swim about in search of the female gamete. On coming into contact with an oosphere it has the power of penetrating into the cell, whereupon its nucleus travels through the cytoplasm and fuses with the nucleus of the oosphere. This process is spoken of as fertilization. The resulting fusion cell or zygote may rest for a time, but it eventually starts to grow, dividing again and again, and building up an embryo or immature individual, which subsequently develops into a new adult form.

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