Insects and Flowers 



577 



municates with the cavity of a carpel. Accordingly, when pollen is once 

 deposited on the inner surface of the main stigmatic tube, the pollen-tubes 

 find easy access to the ovules in each of the three carpels. The pollen is 

 sticky and hangs together in masses, so that it is not adapted to being carried 

 by the wind, and it is apparently impossible for it to get to the stigmatic 

 tube without some outside agent. 



"A small amount of nectar is secreted, but it is excreted at the very base 

 of the pistil, so that insects seeking it would be far removed from the stigmas. 

 Indeed, the low position of the nectar would seem rather to lead insects away 

 from the stigmas. The flowers are borne in compound racemes high aloft 

 on a strong woody shaft, and, because of their rather strong odor when new 

 buds are opening in the evening and their white color, they are quite cer- 

 tain to make their presence known to insects flying in the twilight. 



"If we take these facts as our clew and attentively watch these flowers 

 about eight o'clock in the evening, the method of cross-pollination will be 

 made clear. A white moth, known as the 

 Pronuba-moth, is seen to mount a stamen, 

 scrape together the sticky pollen, and 

 pack it against the under side of its head 

 by means of a spinous structure known 

 as the maxillary tentacle, which seems 

 to have been specially developed for this 

 purpose, for in other moths it is a mere 

 vestige. In gathering the pollen it hooks 

 its tongue over the end of the stamen, 

 evidently to secure a better hold. Having 

 become well loaded with pollen, as shown 

 in the photomicrograph of the moth's 

 head, it descends the stamen and flies 

 to another flower. There it places itself 

 on the pistil between two of the stamens 

 (see Fig. 768) and thrusts a slender ovipositor through the wall of the ovary 

 and into the cavity occupied by the ovules. 



" Having deposited an egg, it ascends the pistil, and by means of the 

 maxillary tentacles and tongue, which at other times are coiled around the 

 load of pollen, it rubs pollen down the inner surface of the stigmatic tube. 

 Fig. 769 is a [drawing made from a] flashlight photograph of a moth performing 

 this act. The moth then descends the pistil, and standing between another 

 pair of stamens it deposits another egg within the ovary; then it ascends 

 the pistil and rubs pollen on the stigmatic surface as before. This process is 

 repeated until it may be that each of the six lines of ovules is provided with an 

 egg, and the process of pollination has been as many times accomplished. 



FIG. 768. Pronuba-moth depositing 

 eggs in ovary of Yucca. (After 

 Stevens; natural size.) 



