FLOWERS AND THEIR WORK 



45 



Milkweed, showing the flower cluster called an 

 umbel. 



The milkweed or butterfly weed (Asdepias cornuti) is another 

 example of a flower adapted to insect pollination.' 



Still another example of 

 cross-poUination 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 poUen 

 is sticky and could not be 

 transferred except by in- 

 sect aid. This is accom- 

 plished in a remarkable 

 manner. A httle 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 

 T^ and feed on the young seeds 

 ^^L which have been fertihzed by 

 .I^B tlie pollen placed on the stigma 

 W flB by the mother. Thej- eat 

 tI^W 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 memljer 

 of the insect tribe that is of 

 considerable economic impor- 



Pod of yucca pierced by 

 the pronuba. 



Pronuba polli- 

 nating pistil of 

 yucca. 



' For an excellent acccjiint of cross-pollination of this flower, the reader is re- 

 ferred to W. C. Stevens. Inlroduclion to Bolani/. drcliids are well known to botan- 

 ists as showing some verj- wonderful adaptatiims. For simple rofcroiu'c reading, 

 see Coulter, Plant Relalions. A classic easily read ia Darwin, On the Fertilization 

 of Orchids. 



