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 andrecium and gynecium. It can 
escape only by erawling 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 earries 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 nature. 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 eross-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 yucea are mutually useful. 
Pronuba provides for fertilization, and thus secures 
the development of the yucea’s seed; while the yucea, 
at the sacrifice of a portion of its seeds, supplies 
pronuba’s offspring with food. 
Unsuitable Insects.—Besides attracting insects and 
providing a mechanism for eross-pollination, flowers 
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 insects are butterflies, moths, bees, and other 
insects that fly from flower to flower. The following 
