42 INTERRELATIONS OF PLANTS AND ANIMALS 



The condition of stamens and pistils on the spiked 

 loosestrife {Lythrum salicaria). 



in the other. Pollination will be effected only when some of the 

 pollen from a low-placed anther reaches the stigma of a short- 

 styled flower, or when the pollen from a high anther is placed upon 



a long-styled pistil. 

 There are, as in the 

 case of the loosestrife, 

 flowers having pistils 

 and stamens of thtee 

 lengths. Pollen only 

 grows on pistils of the 

 same length as the 

 stamens from which it 

 came. 



The milkweed or 

 butterfly weed already 

 mentioned is another example of a flower adapted to insect pol- 

 lination.^ 



A very remarkable instance of insect help is found in the polli- 

 nation of the yucca, a semitropical lily 

 which lives in deserts (to be seen in 

 most botanic gardens). In this flower 

 the stigmatic surface is above the 

 anther, and the pollen is sticky and 

 cannot be transferred except by insect 

 aid. This is accomplished in a re- 

 markable manner. A little moth, 

 called the pronuba, after gathering 

 pollen from an anther, 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 grown because of the 

 pollen placed on the stigma by the mother. The baby cater- 



The pronuba moth within the 

 yucca flower. 



^ 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. A classic easily read is Darwin, 

 On the Fertilization of Orchids. 



