INSECTS AND FLOWERS 203 



Fourteen days before the time of flowering of 

 the Yucca, it begins to show signs of life, and 

 the moment the flowers of that plant open, the 

 silvery moths escape from their pupal envelopes. 

 An important element in the interpretation of 

 the relations subsisting between the Yucca and 

 the Yucca-moth is the fact that without the assist- 

 ance of insects the sticky pollen of the plant in 

 question could not get to the stigma. In the 

 case of Yucca aloefolia alone there seems to be 

 sometimes a transfer of pollen to the stigma 

 through the instrumentality of the petals or of 

 the elongating filaments ; but in most species 

 of this genus, that is to say, in those wherein 

 the fruit is capsular, this certainly does not take 

 place. With the exception of the moth referred 

 to, insects but seldom fly to them, and those 

 which alight by chance on the flowers do not 

 cause a deposition of pollen on the stigmas. If 

 it were not for the transport of the pollen by 

 Pronuba yuccasella the ovaries and ovules of 

 Yucca would not ripen into fruits and seeds. 

 As a matter of fact, all the fruits of the capsular 

 species are rendered abortive if moths are kept 

 away from the flowers by means of a gauze cover- 

 ing. Also in gardens where there are no Yucca- 

 moths, the production of fruit is suppressed. 

 Yucca Whipplei, which in California, its native 

 land, is visited by a particular moth and develops 

 an abundance of dehiscent capsular fruits, has 

 repeatedly flowered in the Botanic Gardens of 

 Vienna, but the moth does not exist in the 

 gardens, and, in consequence of its absence, not 

 a single fruit has ever ripened there. On the 



