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TEXTBOOK OF BOTANY 



A 



B 



each subsequent generation. They seldom change mates. Irregularities 

 in their mating usually result in recognizable mutations. In hybrids re- 

 sulting from cross-fertilization of remotely related plants, the chromo- 

 somes brought together in the same cells may never have occurred to- 

 gether before. Sometimes they fail to mate or to mate regularly during 



reduction division, and very unu- 

 sual sorts of progeny may be ob- 

 tained, or the plant may fail to re- 

 produce at all. 



While the same chromosomes 

 usually pair in all microsporocytes 

 and in all megasporocytes of each 

 generation of a plant, their orienta- 

 tion in the cell with respect to the 

 poles of the spindle is a matter of 

 chance. The two possible chances 

 of orientation when only two pairs 

 of unlike chromosomes are present 

 are illustrated in Fig. 210. Evidently 

 all the paternal chromosomes may 

 go to one pole of the cell and all 

 the maternal chromosomes to the 

 other pole; or some maternal and 

 some paternal chromosomes may go 

 to the same pole. 



This chance or random orienta- 

 tion of pairs of unlike chromosomes 

 during reduction division becomes 

 more interesting as the number of 

 pairs of unlike chromosomes that 

 are considered is increased. Thus if 

 the megasporocyte or microsporo- 

 cyte has three pairs of unlike 

 chromosomes, there are four different ways in which the pairs may be- 

 come oriented on the spindle, and consequently eight different kinds of 

 megaspores or microspores with respect to chromosome complement 

 may be formed (Fig. 211). On the other hand, if there are only one pair 



* The term "like chromosomes" is somewhat of a misnomer in that the chromosomes are 

 usually not alike with respect to all genes, but are alike with respect to those genes which 

 are being considered. 



c 



Fig. 210. a diagrammatic representa- 

 tion of the behavior of chromosomes 

 during the formation of microspores 

 from microsporocytes, or of megaspores 

 from megasporocytes in a plant having 

 two pairs of unlike chromosomes. 



The sperms formed in the pollen 

 from this plant have the same chromo- 

 some complement as the microspores 

 from which the pollen grains develop. 

 Likewise, the eggs have the same 

 chromosome complement as the mega- 

 spores from which the embryo sacs 

 develop. 



