170 Botany

wherever the flower is available. An insect entering
the flower is, by the narrow front petal which acts on
a kind of spring, shut up against the column, the
upright body in the interior of the flower, formed by
the fusion of the androcecium and gyncecium. It can
escape only by crawling up between the wings of the
column. In so doing it comes first into contact with
the stigma and then, on emerging, brushes against the
anther and carries off the pollen masses on its back.
On visiting another flower it deposits some or all of
the pollen on the stigma.

7. The pollination of the yucca is one of the most
marvellous things in naature. The female of the moth
pronuba, which lives in the flower, collects from the
anthers a considerable mass of pollen. She then
thrusts her egg-laying tube (ovipositor) into the ovary
of the flower and deposits an egg. Finally she passes
to the stigma and thrusts into it the pollen she has
gathered. In this way pollination (not cross-pollina-
tion it will be noted ) takes place, and the ovules of the
flower develop, some of them serving the larva of the
moth with food till it bores its way out of the fruit.
Thus pronuba and the yucca are mutually useful.
Oronuba provides for fertilization, and thus secures
the development of the yucca's seed; while the yucca,
at the sacrifice of a portion of its seeds, supplies
pronuba's offspring with food.

Unsuitable Insects - Besides attracing insects and
providing a mechanism for cross-pollination, flowesr
must also protect themselves against the visits of
unsuitable insects; otherwise they may be robbed of
their nectar and pollen and yet remain unpollinated.
Ants, for instance, are fond of sweet food, but, as they
walk from flower to flower and plant to plant, there is
danger of the pollen being brushed off. The most suit-
able instects are butterflies, moths, bees, and other
insects that fly from flower to flower. The following