196 



Introduction to Botany. 



\ 



141. Cross Pollination of Yucca. — Thus far we have taken 

 for our illustrations flowers which 

 are more or less profoundly modi- 

 fied to secure cross pollination as 

 a necessary incident attending 

 the visits of insects. We shall 

 now examine an instance of quite 

 another character, and in some 

 respects even more wonderful 

 than those which have been 

 FIG. 112. described, The flowers of the 



Photograph of a honey bee that has genus Yticctt, representatives of 



died from exhaustion in its efforts 



to free itself after its legs had been which are COmmonly found in 



caught in several stigmatic cham- gardens, depend almost entirely 



bers at once. o ' j. j 



upon the Pronuba moth for their 

 pollination. The structure of the flower is very simple and 

 readily understood. The 

 perianth is of the lilia- 

 ceous type, there being 

 three sepals and three pet- 

 als, all of a creamy white 

 color. In some of the 

 Yuccas these droop for- 

 ward and form bell-shaped 

 flowers, while in others 

 they are more widely 

 spreading. The six sta- 

 mens consist of fleshy, 

 outward-curving filaments 

 surmounted by small an- 

 thers. The pistil extends 

 beyond the stamens, and 

 the three carpels are im- 



FIG. 113. 



Photomicrograph of a pair of poUinia of As- 

 clepias cornuti attached to their corpuscu- 

 lum, as they appear when withdrawn from 

 their pollen sacs. Photographed by trans- 

 mitted light, and on account of the opacity 

 of thecorpusculnm the slit in it is not shown, 

 but a portion of a leg of a small insect is 

 pendent from the slit. The pollen grains 

 of wliich the pollinia are composed can be 

 made out. X 15. 



