GROWTH AND CELL DIVISION 



81 



In all of the higher plants a' different kind of cell division, 

 called the reduction division (Fig. 43, 6), occurs in the spore 

 mother cells which give rise to the spores from which the 

 gametes, egg and sperm cells, are ultimately derived. In such a 

 spore mother cell - for example, the mother cell of a pollen grain 

 the chromosomes become associated in pairs in prophase (7) 

 and are thus arranged on the equator of the spindle as double 

 chromosomes in metaphase (2). These paired chromosomes do 

 not split in metaphase as they do in vegetative mitosis, but the 



Egg cell 



cell 



Seedling plant 



FIG. 44. A diagram illustrating the history of the chromosomes in the develop- 

 ment of a plant 



Note the following important facts brought out in the diagram: The chromosomes 



are three in number in each gamete ; they are doubled in the zygote by fertilization ; 



in the succeeding vegetative mitoses the number of chromosomes remains the same 



as in the zygote cell. Consult the text for a discussion of this figure 



two chromosomes of each pair separate as whole chromosomes and 

 migrate to opposite poles of the spindle in anaphase (3) to form 

 the chromosomes of the new daughter nuclei (4*). As a result each 

 daughter nucleus receives one half of the number of chromosomes 

 contained in the original mother cell, and so one half of the num- 

 ber contained in all of the vegetative cells of the plant body. 



This method of cell division is termed reduction division, and 

 it is of the greatest importance in reproduction and in the life his- 

 tory of plants. In the formation of spores in the higher plants 

 the daughter nuclei divide at once, by the ordinary vegetative 

 method without reduction (^), to form four nuclei, which remain 

 associated in groups of four called tetrads (#). Each member 



