CHAPTER XXXIX 

 MUTATIONS 



We have been emphasizing the individuaHty, stabiUty, and regularity of 

 chromosomes and the smaller hereditary units of matter in the proto- 

 plasm, particularly the genes. It is the stability and orderly behavior of 

 these hereditary units of matter that maintain the constancy of the dif- 

 ferent kinds of plants during their lifetime, and during succeeding 

 generations. This constancy in organization and processes is, however, 

 only relative. Changes in both chromosomes and genes are known to 

 occur, and departures from the orderly splitting, pairing, and migration 

 of chromosomes have been observed many times. When these changes 

 in the hereditary units of matter in the cell are relatively stable, they are 

 initial steps in the evolution of new kinds of plants. Their eflFects may 

 become evident in the appearance of new characters or in the modifica- 

 tion of preexisting ones, and in the consequent formation of new varieties 

 of plants. 



We have already learned that all that is inherited is within the proto- 

 plasm of the sperm and the egg, and hence in the resultant fertihzed 

 egg. The hereditary units of matter in these cells, and the physiological 

 processes they condition are the precursors of all the characters we see 

 in the growing plant. The inherent differences in plants we see about us, 

 therefore, are dependent upon the composition and arrangement of the 

 microscopic and submicroscopic units of matter of which protoplasm is 

 composed. Likewise the heritable changes that occur in the visible char- 

 acters, through time, must be dependent upon certain alterations in these 

 small heritable units of matter. In them may be found the origin of all 

 heritable differences and of all the initial steps of evolution. If we wish 

 to understand the gross phenomena of living organisms we must first try 

 to understand some of the microscopic and submicroscopic structures 

 and processes of which these gross features are the consequences. 



A change in a single gene may result in a very striking change in the 

 development of some visible character. In such cases the relation of the 

 character to the gene is easily detected by the study of Mendelian ratios 



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