148 PRINCIPLES AND CONCLUSIONS. 



author remarked that if he had worked only with Antirrhinum he would 

 have been persuaded of the attractive role of the corolla, but regarded this 

 difference in response to two related genera to be due to the fact that the 

 cut flowers of Digitalis are still directed downward, while in the snapdragon 

 the stub is erect and much more difficult to reach. 



One experiment was carried out on Centaurea cyanus to determine the 

 value of the sterile marginal flowers of the head, which were regarded as 

 attractive parts by Mueller and others. The show flowers were removed 

 from 10 heads scattered among many normal ones. During an hour and 

 a half, 1 honey-bee visited a mutilated head, while Megachile revisited them 

 to a total of 19 heads, behaving exactly as at the normal ones, and pass- 

 ing readily from one kind to the other. Observations of Darwin, Mueller, 

 Bonnier, and Van Tieghem were cited to show that insects visit flowers 

 of which the corolla has fallen, while Van Tieghem had similar results with 

 Nicotiana, from which he had cut the upper part of the corolla. The con- 

 clusion of Errera and Gevaert (1878:141) that bees are guided by the odor 

 of the nectar was extended to include all pollinating insects and to be true 

 even when there were no normal flowers to assist in the attraction. 



The effect of masking flowers was further tried on a very fragrant Hera- 

 cleum, the single plant available being several kilometers from any others 

 and thus apparently eliminating the possible action of habit. An umbel 

 was covered with leaves of rhubarb, but in spite of this received 7 visits in 

 a half-hour. Several umbels were brought close together and covered with 

 leaves in a similar fashion, receiving 45 visits during the period of obser- 

 vation. The evidence furnished by this umbellifer was in entire accord 

 with that obtained from Dahlia and was regarded as proving that the 

 insects were guided by the sense of smell. 



Response to different colors. — In connection with his experiments on 

 color, (1897:17), Plateau emphasized the importance of the work of Graber 

 (1884,1885). In the case of the honey-bee, the latter found that the choice 

 of color depended upon the degree of refrangibility. When the bee was 

 given a choice between bright red and deep blue, it went to the latter, 

 which is more refrangible though less luminous; between bright red and 

 deep yellow it chose the latter, which is likewise more refrangible. Be- 

 tween dark green and bright yellow it exhibited no preference, owing to 

 the similar refrangibility of the two colors, in spite of the fact that the 

 yellow is more luminous. In consequence, Plateau concluded that insects 

 do not see color as such at all, in the way that we do, but that they respond 

 to the different ones merely in relation to differences in refrangibility. His 

 observations as to choice of color were made upon several species with 

 flowers of various colors. In the case of Centaurea cyanus he grew blue, 

 rose, white, and dark-purple varieties in mixture. Both Apis and Mega- 

 chile were observed to pass readily from one color to another, though 22 

 of the 30 visits were to blue or purple, and this was explained by the greater 

 number of the blue heads. Unfortunately, the number of each kind was 

 not counted, and no endeavor was made to arrange the installation so that 

 the number of each would be the same. With Dahlia variabilis the colors 

 were scarlet, purple, rose, salmon, and white, among which the visiting 



