MISSOURI BOTANICAL GARDEN BULLETIN 87 
removes to another blossom and repeats the interesting per- 
formance. Each flower is in condition to be fertilized only 
during a brief period of time, since after the first evening 
the hornlike processes of the pistil close over the stigmatic 
chamber and thus preclude the possibility of further polli- 
nation. In the course of about a week the égg hatches and 
the resulting larva feeds on the developing ovules which 
constitute its only food. When it has reached a mature size 
it burrows a passage to the exterior of the capsule and falls, 
or lets itself down by a silk thread, to the ground. Here 
it bores several inches below the surface and forms a tough 
silken cocoon intermixed with soil. It remains as a larva, 
or grub, within its cocoon during the fall, winter and spring 
months and transforms to the pupa or chrysalis state only 
a few days before emerging as an adult moth when the 
Yuccas bloom again in June. 
It has been found that the Yucca flower is incapable of 
fertilizing itself; moreover, it is impossible for the wind to 
carry pollen from one plant to another. Extensive obser- 
vations by many scientists in different parts of the country, 
since the early discoveries by Drs. Engelmann and Riley, 
prove that no other insect does this work which is so abso- 
lutely necessary to the production of seed. The amount 
of pollen placed in the stigma chamber by the Pronuba 
moth is sufficient to fertilize all the numerous ovules of 
that flower, while each larva hatched from the eggs de- 
posited eats but comparatively few ovules—only ten or a 
- dozen—thus allowing a vast majority of them to mature. 
Were it not for the intervention of the moth the Yucca 
plant could never be Red itself except by offshoots or an — 
accidental division of the parent plant. The plant depends 
entirely on the little moth to aid it in the natural method of 
reproduction. On the other hand, this moth does not asso- 
ciate with the flowers and fruits of any other plant than 
the Yucca. It feeds during its larval or “worm” stage only 
and then wholly upon the seeds of Yucca developing within 
the fruit on the plant. Should the adult moth deposit its 
eggs in the ovary and then fly away, the eggs would hatch, 
but the young larvae would soon perish from lack of food. 
To meet this demand the moth has developed a form of 
instinct which impels it to place pollen in the stigma of 
each pistil in which it has deposited an egg, insuring by 
_this means the fertilization of the ovules and consequently 
a liberal supply of food for its offspring. Thus the insect 
is wholly dependent on the Yucea for its existence. Through 
