FLOWERS AND THEIR WORK 



45 



The milkweed or butterfly weed (Asdepias cornuti) is another 

 example of a flower adapted to insect pollination. 1 



Still another example of 

 cross-pollination is found 

 in the yucca, a desert-lov- 

 ing semitropical lily (to be 

 seen in most botanic gar- 

 dens) . In this flower the 

 stigmatic surface is above 

 the anther, and the pollen 

 is sticky and could not be 

 transferred except by in- 

 sect aid. This is accom- 



Milkweed, showing the flower cluster called an 



plished in a remarkable umbel. 



manner. A little moth, 



called the pronuba, gathers pollen from an anther, flies away with 



this load to another flower, there deposits an egg in the ovary of the 

 pistil, and then rubs its load of pollen over the 

 stigma of the flower. The young hatch out 

 and feed on the young seeds 

 which have been fertilized by 

 the pollen placed on the stigma 

 by the mother. They eat 

 some of the developing seeds 

 and then bore out of the seed 

 pod and escape to the ground, 

 leaving the plant to develop 

 the remaining seeds without 

 further molestation. 



The fig insect (Blastophaga 

 grossorum) is another member 

 of the insect tribe that is of 

 considerable economic impor- 



Pod of yucca pierced by 

 the pronuba. 



Pronuba polli- 

 nating pistil of 

 yucca. 



1 For an excellent account of cross-pollination of this flower, the reader is re- 

 ferred to W. C. Stevens, Introduction to Botany. Orchids are well known to botan- 

 ists as showing some very wonderful adaptations. For simple reference reading, 

 see Coulter, Plant Relations. A classic easily read is Darwin, On the Fertilization 

 of Orchids. 



