ECOLOGY OF FLOWERS; POLLINATION 177 
210. Pollination in Yucca.— The yuccas and allied genera 
are xerophytes (some of them low plants and others tree- 
like) which abound especially in the desert and semi-desert 
portions of the southwestern United States and in Mexico 
(Plate VI). The flowers are white or whitish, borne in 
great clusters, and are very conspicuous at night. The 
stamens are shorter than the pistil, and the pollen is sticky. 
The pistil consists of three carpels which form a tube ~ 
stigmatic on its inner surface. 
_ Pollination is impossible without the aid of insects. It is 
effected by a small moth (Pronuba) which throughout the 
day remains at rest within the flower (Fig. 182).1 At dusk 
the female moth begins to fly from one cluster of yucca 
flowers to another. She clings 
to the stamens and collects a 
mass of pollen (Fig. 183). Fly- 
ing with the load of pollen to 
another flower cluster, the moth 
stings one of the ovaries and 
deposits an egg in an ovule, 
then mounts? to the tip of the 
‘el aud a i all Fic. 183. Head of Yucca 
pist1 a crowds some p en. Moth. (Magnified. ) 
into the mouth of the stigmatic 
tube. She then descends to the by spinous appendages of the 
p, mass of pollen held in position 
: moth’s head. 
ovary and deposits another egg, ‘ 
ascends to the stigma to pollinate it, and soon. Each grub 
when hatched from the egg feeds on the ovule in which 
the egg was laid; but the ovules are very numerous, so 
that many of them are left uneaten and ripen into seeds. 
1 In the first flower (the lowest) the moth is gathering pollen; in the second 
she is pollinating the stigma; in the third she is in the position of rest during 
the day; in the fourth in the position of rest when disturbed; in the fifth ovi- 
positing. 2 That is, travels away from the receptacle. 
