SIGNIFICANCE OF PARALLELISM IN HEREDITY. 



a broad-lobed variety included in tlie same plantin*^. In the latter 

 the lower leaves were almost entire, as often occurs in broad-lobed 

 types of cotton. 



SIGNIFICANCE OF PARALLELISM IN THE STUDY OF HEREDITY. 



The parallel series of leaf forms of cotton and related plants are of 

 interest in connection with many ])roblems of heredity and breeding. 

 In view of the fact tliat the same 

 wide range of divereity in leaf forms 

 exists in Gossy])ium, Ilibiscns, and 

 Abelmoschns, it becomes easier to 

 look upon such diiferences as 

 within the usual range of variation 

 for this group of plants. Changes 

 of characters to wdder or narrower 

 leaves do not require us to believe 

 that a new character has originated 

 or that hybridization with a differ- 

 ent type of cotton has occurred. 



The theory of hybridization as 

 a cause of diversity of leaf forms is 

 rendered the more unnecessary be- 

 cause the w^ide range of leaf differ- 

 ences appears not only in the same 

 species or variety, but on the same 

 individual plant. This is well 

 shown in a wild relative of cotton, 

 IngenJiousia triloba, native in Ari- 

 zona. (Fig. 16.) The young plants 

 have entire and broad-lobed leaves, 

 while the leaves of adult plants 

 have long narrow lobes. The 

 branches of Ingenhousia, grown 

 under greenhouse conditions, do 

 not show the same tendency as in 

 cotton to return to simple leaves, 

 but the three-lobed leaves at the 



base of the branches have very Fig. lO.—Plant of Itigenhousta triloba, showing 



short and broad lobes, quite unlike ; He^^iJeed.r™ ""'""' '° "''''''' "''"'"' ^'^^''' 

 the tapering long-pointed lobes of 



subsequent leaves of the same shoot. The upper leaves have five 

 lobes, as in cotton, okra, and Hibiscus cannabirms. 



The fact that a wide variation in leaf forms occurs on the same 

 individual ])lants in i)rmiitive wild species makes it entirely unneces- 



221 



