562 BOTANY OF THE LIVING PLANT 



Mitosis and Meiosis. 



There are two distinct normal methods of nuclear division : (a) mitosis 

 or somatic division and (b) meiosis or reduction division. Mitosis 

 takes place in the cells of the vegetative plant body (somatic cells) by 

 which it increases the number of its cells during growth. Meiosis is the 

 process, comprising two nuclear divisions of special type, which 

 initiates the gametophyte (haploid) stage in the life-cycle of any plant 

 that reproduces sexually. 



Mitosis. 



Mitosis is a complicated process and for convenience of description may be 

 regarded as taking place in a series of stages to which distinctive names have 

 been given. 



During prophase (Fig. 428, b and c) the chromosome reticulum becomes 

 resolved into a definite number of chromonemata ; these are long slender 

 threads, more or less twisted, and double. The threads gradually become 

 shorter and thicker and finally take the form of V- or L-shaped deeply staining 

 rods, the chromosomes. Each chromosome, like the chromonema from which it 

 is derived, is a double structure, being divided lengthwise into two equal 

 halves, the chromatids. 



In the second stage or metaphase (Fig. 428, /), the chromosomes arrange 

 themselves in the equatorial plane — i.e. the plane of division of the cell — with 

 the point of the V or L facing inwards. At the same time delicate protoplasmic 

 threads {spindle fibres) appear in the dividing cell and group themselves into a 

 spindle-shaped figure (Fig. 428, g) placed at right angles to the equatorial 

 plane. The chromatids are of uniform thickness, except for certain con- 

 strictions, the number and position of which are specific for each chromosome ; 

 there is always at least one constriction, the attachment-constriction or 

 centromere, which is believed to control the movements of the chromatid. 



At the third stage or anaphase (Fig. 428, g-i), the halves of each chromo- 

 some (chromatids) separate and pass to the opposite poles of the spindle. It 

 is thought that this separation is initiated by a mutual repulsion between the 

 centromeres. 



The final stage or telophase (Fig. 428,7'-/) is essentially a reversal of prophase. 

 The " daughter chromosomes," as the separated chromatids may now be 

 called, lengthen into slender threads which become entangled together and 

 reconstitute a " daughter nucleus " at each pole of the cell. In the resting 

 stage the chromosomes lengthen and become threadlike, and again split 

 lengthwise in preparation for the next division. 



The nucleoli and the nuclear membrane disappear during the early stages 

 of mitosis and reappear at telophase. The new cell-wall most commonly 

 arises as a thin lamella (" middle lamella " or " primary cell-wall ") within 

 the cell-plate, a specialised layer of the cytoplasm which becomes gradually 

 differentiated during telophase in the equatorial plane. 



The outstanding features of mitosis are three . 



(1) For the cells of each kind of plant (or animal) there is a definite number 

 of chromosomes which normallv remains constant for all the somatic divisions. 



