8 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



For the purposes of the selection that has to be maintamed in order 

 to keep a superior stock in a condition of uniformity, it is quite as 

 important to recognize varieties by the characters of their leaves and 

 brandies as by those of the bolls and seeds. Indeed, selection by 

 vegetative characters can be made even more efficient than selection 

 by fruit characters because it enables degenerate variations to be 

 recognized and removed early in the season, thus avoiding the danger 

 of spreading mferior characters through cross-pollmation.^ 



Selection is our means of keepmg undesirable characters from 

 coming mto expression, but it does not prevent the transmission of 

 sucli characters. Even though all the lines of descent that show ten- 

 dencies to the expression of undesirable cliaracters be rejected, the 

 possibilities of such expression remain in the other lines and are 

 likely to be reawakened if selection be relaxed. One of the most 

 important problems in the selective breeding of cotton and other 

 seed-propagated field crops is to make selection more efficient by 

 more adequate knowledge of the characteristics and behavior of the 

 plants, so that deviations from a type can be more easily recognized 

 and removed from the stock and the exciting causes of such devia- 

 tions avoided. 



DIMORPHISM A PHENOMENON OF ALTERNATIVE EXPRESSION. 



The most miportant of the general facts or principles of heredity 

 that may be illustrated by the phenomena of dimorphism is the 

 fundamental distinction between expression and transmission. Unless 

 this distinction is appreciated it is impossible to understand the 

 measures of selective breeding that are required to preserve the uni- 

 formity and maintam the agricultural value of superior varieties of 

 cotton and other seed-propagated crop plants. Many efforts are 

 being made to solve the problem of heredity by seeking m the proto- 

 plasm of germ cells for microscopic organs or mechanisms that are 

 supposed to transmit the characters from the parents to the offspring. 

 While the discovery of such a meclianism would be of great scientific 

 interest, tlie facts of lioreiHty that promise to be of most value from 

 the standpoint of agricultin-al application are facts of expression. 

 Even without determuiing the mechanism of transmission it is pos- 

 sible to investigate the effects of breeding and en^^.ronment upon the 

 expression of characters.- 



The doctrine elaborated by Weismann that there is a funda- 

 mental distinction between the germ plasm and the protoplasm of 

 the somatic or vegetative tissues has doubtless tended to prolong the 



• Cook, O. F. Cotton Selection on the Farm by the Characters of the Stalks, Ivcaves, and Bolls. Cir- 

 cular 06, Bureau of Plant Industry, U. S. Dept. of Agriculture, 1911. 



2 Cook, O. F. Transmission Inheritance Distinct from Expression Inheritance. Science, n. s., vol. 

 25, 1907, p. 911. 



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