FLOWERS AND INSECTS 



245 



During the day the small female Pronuba rests quietly 

 within the flower, but at dusk becomes very active. She 

 travels down the stamens, and resting on an open pollen 

 sac scoops out the somewhat 

 sticky pollen with her front legs 

 (Fig. 239, A). Holding the little 

 mass of pollen against her body, 

 she runs to the ovary, stands 

 astride one of the furrows, and 

 piercing through the wall with 

 her ovipositor deposits an egg 

 in an ovule. After depositing 

 several eggs in this way, she 

 runs to the top of the ovary and 

 begins to crowd into the funnel- 

 shaped stigmatic cavity the mass 

 of pollen she has collected (Fig. 

 239, B). These actions are re- 

 peated several times, until many 

 eggs have been deposited and 

 repeated pollination has been 

 effected. As a result of this, 

 seeds are formed which develop 

 abundant nourishment for the 

 moth larvae, which become mature and bore their way out 

 through the wall of the capsule (Fig. 240). 



147. Cross-pollination. In those flowers in which cross- 

 pollination is the rule, self-pollination is hindered in a 

 variety of ways. In the cases about to be considered, 

 stamens and carpels are together in the same flower; of 

 course, in dioecious plants there can be no such thing as 

 self-pollination. It is necessary to remember also that 

 when the stigma is ready to receive the pollen, it excretes 

 upon its surface a sweetish, sticky fluid, which holds and 

 feeds the pollen, inducing the development of pollen tubes. 



FIG. 240. A mature capsule of 

 Yucca, showing perforations 

 made by larvse of Pronuba in 

 escaping. After RILEY and 

 TRELEASE. 



