480 TEXTBOOK OF BOTANY 



one or in several pairs of chromosomes. Or the chromosomes might con- 

 tinue to adhere in certain parts, and separate and break in other parts, 

 with the result that each chromosome would contain one or more pieces 

 of its synaptic mate. Sometimes when chromosomes of two different 

 species are brought together in the cells of a hybrid by means of cross- 

 fertilization thev fail to mate regularly in reduction division, and many 

 irregularities occur. As shown by the literature of cytology, these sus- 

 pected irregularities actually do occur. Each of them will be considered 

 briefly. 



When the chromosomes fail to undergo reduction division, the result- 

 ing spores, sperms, and eggs not only have the diploid ( 2n ) number of 

 chromosomes but they also have the same chromosome complement, 

 which is identical with that of the cells of the vegetative body of the 

 plant. This is the usual process in dandelion. Since fertihzation does not 

 occur in dandelion, the parthenogenic embryo develops directly from 

 diploid (2?i) eggs. Thus there is no change in chromosome number in 

 the cells during the entire life cycle of the dandelion. If self-fertilization 

 were to occur among these gametes of dandelion, absolutely homozygous 

 plants would be obtained until some mutation occurred in one or more 

 of the chromosomes. 



When fertilization does occur in such plants, a union of diploid ( 2n ) 

 gametes results in tetraploid (4n ) plants. This is the second way in which 

 tetraploid plants or plants with some higher multiple number of chromo- 

 somes may originate. 



The type of irregularity by which the members of one or more pairs 

 of homologous chromosomes fail to separate during reduction division 

 results in a change in chromosome number and complement. If the 

 chromosomes of one pair fail to separate, both of them migrate to the 

 same pole of the spindle. As a result, some of the spores, sperms, and 

 eggs then have n + 1 chromosomes; others have n — 1 chromosomes. If 

 two of the pairs of chromosomes fail to separate, the gametes have n + 

 2 and m — 2 chromosomes. Hence some fertilized eggs lack one or more 

 chromosomes, and others have one or more extra chromosomes with all 

 their genes and potentialities. 



Such changes in chromosome behavior may be the initial steps in the 

 formation of new varieties and species. Within certain genera of plants 

 one may arrange several of the species in a series, such that each species 

 in the series differs from the preceding one and the next one following 

 it bv but one or two chromosomes. A similar situation may be found 



