170 PRINCIPLES AND CONCLUSIONS. 



Since other odors were unattractive or actually repellent, with the exception 

 of thyme and sage, which were weakly attractive, Plateau's theory is 

 refuted by his own experiments. 



The discovery that bees still visited flowers rendered inconspicuous by 

 removing the petals or the colored part of the corolla seemed at first to 

 overthrow the accepted principle of attraction by color. However, careful 

 consideration of these experiments led to the conclusion that Plateau's 

 inferences were not justified and that another explanation was permissible. 

 The mutilation of Digitalis by cutting away the corolla, together with the 

 stamens and style, to leave a stump only 1 cm. long seemed to warrant the 

 conclusion that neither color, size, nor form was essential to their attraction. 

 Knuth argues, however, that the nectar is more exposed and the fragrance 

 more widely diffused and that this compensates for the loss of the corolla; 

 moreover, the visits should thus have been more numerous than to the 

 normal flowers, which was not the case. With other species the mutilated 

 flowers were visited less than the normal ones, a fact that Knuth regarded 

 as proving that the corolla also plays a part in attraction. In the case of 

 Antirrhinum majus the lack of visitors to the cut flowers seems to indicate 

 that the odor is not effective and that the form and color of the corolla play 

 the chief part. Moreover, the frequent visits of Megachile to heads of Cen- 

 taurea cyanus deprived of ray-flowers, explained by Plateau as due to the 

 odor, may be equally well explained by the memory of the bee as applied to 

 the honey-bearing disk-flowers. 



Plateau has given a one-sided meaning to his experiments, without regard 

 to the work of earlier investigators. Thus, he has entirely overlooked the 

 experiments of Forel, who showed that blinded insects are unable to recog- 

 nize the landing-place of the flower, while those that have the antennae 

 cut away fly confidently from flower to flower. He further ignores the 

 results of Mueller, Loew, MacLeod, and Knuth himself, which have estab- 

 lished the following principles: 



1. Other things being equal, a flower is visited by insects in proportion to its conspic- 



uousness. Among related species that agree closely in form and color as well as 

 in floral mechanism, the most conspicuous receive the most visits and the 

 least conspicuous the smallest number. 



2. In a number of cases odor has more to do with the attraction of insects than the 



color and size of the corolla. 



3. Dull yellow flowers are not visited as a rule by beetles, while nearly related flowers 



that are white or of some other conspicuous color attract these insects even 

 when nectarless. Reddish-blue or violet flowers are preferred by bees, butter- 

 flies, and hoverflies, which are highly specialized, while the most frequent visi- 

 tors to white or yellow flowers are less intelligent insects with a short proboscis. 

 Bumble-bees appear least dependent upon the color of flowers, and as Mueller 

 indicated, are influenced more by food-value than by the external appearance 

 of flowers. 



4. Strong-smelling flowers attract flies more particularly, while those with sweet 



aromatic odors attract bees strongly without excluding other insects. The 

 delicate odor, strongly exhaled toward evening, of many white flowers with 

 long corolla-tubes attracts hawk-moths especially, as well as other nocturnal 

 Lepidoptera. 



