106 



with flowers appearing very similar at a little distance, and 

 probably pollinated by the same insects.^ But just what ad- 

 vantage two weeds growing in the same habitat could derive 

 from having similar foliage is not obvious. In the insect world 

 cases are recorded of species protected from predatory enemies 

 by their resemblance to another species with an offensive odor or 

 taste, and there may possibly be some such factor involved here. 

 The Carara, which is widely distributed in the United States, 

 is commonly supposed to be native of Europe; but Mohr men- 

 tions its occurrence in Brazil also, and it is possible that both 



'Jywonoitj'lft: 



Gymnostyles nasturtiifolia (left) and Carara didyma (right), collected close 

 together in Tuscaloosa, Ala., May 13, 1937, and photographed in the ofiSce on 

 May 31. About one-third natural size. Note the close similarity of the leaves. 



it and the Gymnostyles originated there, and developed a simi- 

 larity of foliage in response to some environmental factor at 

 present unknown. 



Another somewhat similar case is that of Rumex pulcher 

 and Erysimum officinale (formerly called Sisymbrium officinale) . 

 Both are roadside weeds, supposed to be natives of Europe, 



^ One of the most striking cases is that of Vida pedata and Iris verna, 

 described in Torreya 6: 192-193, 1906, and observed in other Alabama coun- 

 ties in later years. 



