CONCLUSIONS. 51 



are without lobes. The change ii'oni one form of leaves to the other 

 is usually quite abrupt. 



The various types of cotton and okra show the same general range 

 of diversities of leaf forms as the Deccan hemp, and some of the 

 varieties have the same tendency to dimorphic expression of the 

 leaf characters. In other words, there is a general parallelism of 

 variation in leaf characters extending through the many species and 

 varieties of cotton, as well as the related genera of plants. 



The definite changes of characters involved in passing from one 

 form of leaves or branches to another are analogous to the abrupt 

 transformations that take place in mutative variations. The facts 

 of dimorpliism and of bud variation indicate that mutative changes 

 of characters are not necessarily connected with conjugation or 

 with the early stages of sexual reproduction from new germ cells. 



Dimorphic differences and mutations show that abrupt changes of 

 characters are to be consideretl as phenomena of alternative expres- 

 sion. It is obvious that such changes are not determined by alterna- 

 tive transmission, as often alleged for Mendelian segregation of con- 

 trasted characters. The same kinds of characters show dimorphic 

 specialization in individual plants and Mendelian segregation in 

 hybrids. Dimorphism and Mendelism may both be interpreted as 

 phenomena of alternative expression. 



The general interest of such phenomena is in their relation to the 

 recognition of a fundamental distinction between transmission and 

 expression as a general law or principle of heredity. The facts of 

 heredity and breeding can be better understood if transmission be 

 considered as including the whole ancestral series of characters. 

 Transmission inheritance is a comprehensive process, while expression 

 inlieritance is partial and alternative, different characters being 

 expressefl in different individuals or in different stages of individual 

 development. 



The facts of dimorphism are worthy of being taken into account in 

 breeding, as affording additional varietal characters and as one of 

 the means of recognizing variations from the standard or typical 

 form of a select variety. Dimorphism must also receive attention in 

 the study of the influence of environmental conditions on the expres- 

 sion of characters. In cotton and other tropical crop plants the modi- 

 fication of dimorphic differences represents one of the most serious 

 disturbances of normal heredity induced by external conditions. 



221 



