10 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



varieties of cross-fertilized plants like cotton and corn, but the idea 

 of an absolutely fixed or constant expression of characters does not 

 accord with the facts of biology. 



The successive formation of the different organs of the plant repre- 

 sents a series of changes in the expression of the characters, often as 

 definitely contrasted as differences between varieties or species. 

 Even in purely vegetative organs like the leaves speciahzed dimor- 

 phic changes of expression may be established m some species, 

 instead of more gradual or continuously varied changes that appear 

 in related species or even in other varieties of the same species. 



In the study of heredity, as in many other fields of scientific explo- 

 ration, there is a tendency to give special values to evidence drawn 

 from remote or difficult sources and to overlook the significance of 

 familiar facts or of those that are capable of eas}^ and direct observa- 

 tion. Yet it must be recognized that any underlying principles or 

 general facts of heredity that are to be of practical use must have 

 relation to readily visible external characteristics of our most familiar 

 domestic animals and plants. The more familiar the facts, the more 

 ready and reliable should be the interpretation, were it not for the 

 greater interest generally secured by more remote and more doubt- 

 ful considerations. 



Though some of the facts described in this paper may not have 

 been previously recorded in connection with the cotton plant and its 

 relatives, similar facts are common enough in other genera and f amihes 

 of plants. The dimorphic leaves and branches of cotton and other 

 related plants do not represent extreme types of specialization, but 

 this may give them the greater interest from the standpoint of 

 heredit}'^ because of the intermediate position between the more 

 definite and less definite forms of alternative inheritance. 



It is usual to think of plants as simple individuals, but in reaUty 

 they are com[)ound individuals built up by the association of many 

 individual internodes or metamers, each of which may be capable 

 of an indei^endont existence. The internode individuals are not 

 all of one kind. In addition to the s})ecialization of some of them 

 as floral organs definit(^ difl"erences are often to be found among the 

 vegetative metamers. The fact that many plants seem to lack 

 definite specializations among the vegetative internodes only ren- 

 ders such peculiarities the more interesting when they occur, for 

 they throw another light on the facts of evolution and heredity. 



The development of any individual })lant may be viewed as a 

 progressive change of ex])ression of characters, the juvenile charac- 

 ters giving place to the adult, but the changes are generally so gradual 

 as to suggest no analogy with the Mendelian form of definitely con- 

 trasted alternative inheritance. Abrupt changes from juvenile to 



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