Cell division leading to the development of the vegetative body (soma) of the plant is known as somatic cell division. It includes the division of the nucleus called mitosis (mitose=thread) or karyokinesis (karyon=nut or nucleus) (Kinesis=movement) or indirect nuclear division and the division of cytoplasm is called cytokinesis (kytos=cell). It occurs in growing regions as in the root tip and stem tip.
In Mitosis there are four stages as under:
(1) PROPHASE
The first sign of the prophase is the appearance of a number of separate slender, crooked threads called chromosomes. The chromosomes are spirally coiled. The individual chromosomes are always double with two threads called chromatids. Chromosomes are composed of nucleoproteins. As prophase proceeds the chromosomes relex their coils and thicken somewhat. Their double nature becomes more apparent. As prophase advances a chromosomal substance accumulates in a sheath or matrix round each chromosome and the chromatids become closely coiled in it. Each chromatid divides longitudinally into two. Thus at this stage each chromosome consists of four threads.
In well fixed chromosomes some unstained gaps or constrictions are seen; those are the attachment regions called centrosomes. The nucleoli lose their staining power and disappear completely. The nucleus then rapidly passes into the next stage the metaphase.
(2) METAPHASE
The nuclear membrane and nucleolus disappear and a spindle like body called nuclear spindle is formed. Spindle may be of nuclear origin or more probably of cytoplasmic origin. Commonly in root tips it appears as two opposite polar caps outside the nuclear membrane. The membrane then disappears and the spindle extends into the nuclear area. The chromosomes move to the equatorial plane on the spindle and there clearly apart from one another. At this stage the chromatids come even more closely together. From the centrosome of each pair of chromatids fibre like extensions called tractile fibres are formed towards the opposite poles through the nuclear spindle. The number of chromosomes is normally constant for a particular species of plants and this number is also normally even, expressed as 2n (or 2x) or diploid. Chromosome numbers cover a wide range but 24 seem to be a common figure.
(3) ANAPHASE
In the end of metaphase the centrosomes of each pair of chromatids appear to repel each other. They diverge and move ahead towards the opposite poles along the course of tractile fibres. The chromatids soon become separated from each other. Anaphase covers the shortest period in mitosis.
(4) TELOPHASE
At each pole the chromatids (daughter chromosomes) form a close group. The nuclear spindle disappears and so does the matrix. A nuclear membrane is formed round each group of chromatids. The chromatids duplicate themselves into full chromosomes which recognize themselves within the nuclear membrane. Nucleoli reappear at definite points on certain chromosomes. The nuclear sap reappears and each nucleus increases in size. It passes into metabolic stage or prepares for next division.
Importance: In this complicated process of nuclear division the constituents of chromosomes are equally appropriated to two daughter nuclei and thus they become qualitatively and quantitatively similar to the mother nucleus. Chromosomes are the bearers of hereditary characters and because of even distribution of chromosomal substances particularly DNA, two daughter nuclei possess all characteristics and quantities of the mother nucleus.
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