244 



A TEXT-BOOK OF BOTANY 



146. Yucca and Pronuba. This is a remarkable case of 

 self-pollination by means of an insect. Yucca is a plant 



of the southwest- 

 ern arid regions 

 of North America, 

 and Pronuba is a 

 moth; and the two 

 are very dependent 

 upon each other. 

 The bell - shaped 

 flowers of Yucca 

 hang in great ter- 

 minal clusters. In 

 each pendent flower 



FIG. 237. The pendent flower of Yucca, showing (Fig- 237) there are 

 position of stamens and the ribbed ovary. After gjx hanging sta- 



mens, and an ovary 

 ribbed lengthwise, with a fun- 

 nel-shaped stigmatic opening 

 in its top (Fig. 238). The 

 numerous small ovules occur 

 in rows beneath the furrows. 



RILEY and TRELEASE. 



FIG. 238 Longitudinal section of an 

 ovary of Yucca, showing the funnel- 

 shaped stigmatic opening (s), and the 

 rows of ovules attached to the wall 

 (o). After RILEY and TRELEASE. 



FIG. 239. The position of Pronuba on 

 the stamen of Yucca when collecting 

 pollen (.4) and when thrusting it 

 into the stigmatic funnel (B). Af- 

 ter RILEY and TRELEASE. 



