Division of Plant Cells 11 



and may be twisted about one another. They may be so close 

 together that the separation is not visible except at the end, and 

 the chromosome may appear as a single structure forked at the 

 ends. 



It is at metaphase that the form of the individual chromo- 

 somes is most easily seen. In some plants, as the onion, all the 

 chromosomes are practically alike in size and shape, but in 

 many other plants this is not so. In plants in which the chromo- 

 somes are not all alike, however, every chromosome is never 

 different from every other one, for there are always two of each 

 type. Thus they are always in pairs, and the two members of 

 any pair are called homologues or homologous chromosomes. 

 The chromosomes occur in pairs because one member of each pair 

 has been received from the male parent and the other member 

 from the female parent, and these two chromosomes are identical 

 as far as visual means can detect. In the onion, for example, 

 where all chromosomes look alike, pairs still are present but are 

 not readily detectable because of the general morphological simi- 

 larity of all the chromosomes. The onion has 16 chromosomes, 

 but, since the chromosomes occur in pairs, it is equally accurate 

 to say that the onion has 8 pairs. This method of designation 

 is frequently used. Similarly, maize has 20 chromosomes or 10 

 pairs, rye has 14 chromosomes or 7 pairs, and cabbage has 18 

 chromosomes or 9 pairs. 



In general, we may say that a plant has n pairs of chromo- 

 somes, or 2n chromosomes, where n is a specific number, such 

 as 10 for maize and 7 for some wheat species. In mitosis in most 

 organisms, all the chromosomes are spread out at random on the 

 equatorial plate. Even though the chromosomes exist in pairs, 

 any one chromosome can ordinarily lie next to any other, and 

 there is absolutely no tendency for the two members of a pair 

 to lie near one another. This is a general rule, although there 

 are some outstanding exceptions. 



Anaphase. After the chromosomes have become arranged on 

 the equator, the two chromatids of each chromosome move apart 

 from one another, each going towards its nearest pole. How this 

 movement is brought about is still a puzzle, but possibly it is 

 initiated by a repulsion of perhaps an electrical nature between 

 the two centromeres. The centromeres are the active force in 

 the separation of the daughter chromosomes, and the arms are 



