Insects and Flowers 



573 



stigma with pollen acquired from a previous visit to another flower. 

 Miscellaneous small insects alighting on the keel are not heavy enough 

 to depress it, and thus are prevented from entering and stealing the nectar. 

 In the salvias (sages) the corolla is similarly tubular below and two- 

 lipped above, the lower lip serving as an alighting-platform for the 

 insect visitors (usually bees), while the arched upper lip covers and pro- 

 tects the stamens and pistil. In Salvia officinalis (Fig. 764) the stamens 

 do not come immediately into contact with the bee as it enters, but they have 

 to be moved in a particular manner, which is accomplished as follows: "Two 

 of the stamens are minute and rudimentary. In the other pair the two 

 anther-cells, instead of being, as usual, close together, are separated by a long 

 connective. Moreover, the lower anther-cells contain very little pollen; 

 sometimes, indeed, none at all. This portion of the stamen, as shown in 

 Fig. 764, hangs down and partially stops up the mouth of the corolla-tube. 

 When, however, a bee thrusts its head into the tube in search of the honey, 

 this part of the stamen is pushed into the arch, the connectives of the two 

 large stamens revolve on their axis, and consequently the fertile anther-cells 

 are brought down onto the back of the bee." 



In the scarlet sage (Salma sp.) cross-pollination is accomplished by 

 humming-birds, which, hovering in front of the narrow mouth of the 

 flower-cup, thrust deeply into it their long bills in the search for small insects 

 which may have entered for nectar. Other flowers regularly visited and 

 cross-pollinated by humming-birds are the scarlet currant, various painted 

 cups (Castilleias), the scarlet mimulus, the wild columbine, the trumpet-creeper, 

 the spotted touch-me-not, the cardinal-flowers, cannas, and fuchsias. Red 

 seems to be the attractive color for humming-birds. As the only humming- 

 bird species east of the Rocky Mountains is the ruby- throat (Trochilus ruber), 

 this one species is to be credited with being the chief pollinating agent of a 

 considerable number of flowers; in California and the southwest there are 

 several species to do the work. 



Another marked and easily seen variant in this specialization of flowers 

 to insure cross-pollination by insects is that shown by the milkweeds of the 

 genus Asclepias. Stevens has described this so well (Introduction to Botany, 

 p. 191 et seq.) that I simply quote here most of his account. "Asclepias 

 cornuti, common everywhere in this country, is perhaps the best species for 

 demonstrating this [peculiar specialization of the milkweeds]. As shown in 

 Fig. 765, the sepals and petals are reflexed; the stamens are joined throughout 

 their length, and are united to a thick and flat structure at their apices, 

 known as the stigmatic disk, which is also united with the top of the two 

 pistils. The pistils are entirely enclosed by the stamens and the stigmatic disk. 

 Five spreading, hollow receptacles for the nectar grow out and upward from 

 the bases of the stamens. 



