THE FLOWER AND ITS PARTS 107 
shaped flowers, which are closed all day; spread out their 
petals when dusk comes on. As the petals spread, the 
anthers split and expose a sticky mass of pollen. The 
moth comes round, and promptly gets to work. It crawls 
over the stamens and picks up piece after piece of the 
pollen, carrying it all, rolled into one mass, in what one 
may call, perhaps, its jaws. So sticky is the pollen that 
there seems no chance of its getting naturally to the 
stigma. You will remember that deep down in the pistil 
are formed ovules, which the fertilisation by means of 
the pollen-tubes is to turn into seeds, but which, without 
the arrival of the tubes, will wither away and die. Well, 
after collecting this ball of pollen from the stamens, the 
moth flies to the pistil. Deep down in the base, close to 
the ovules, she lays some eggs, which in course of time 
will turn to caterpillars. Then comes her question, what 
are they to feed on? The reply is, “The Yucca seeds.” 
And now one sees the object of the pollen collection. 
To ensure that the ovules shall become fertilised the 
moth now flies up to the stigma and fixes the ball of 
pollen to it; the tubes run down the shaft of the pistil, 
and reach the ovules. You may say, “Where does the 
plant get any advantage? What is the good of making 
seeds in order that a caterpillar may eat them?” The 
plant’s reply would be, if it were able to argue the 
question, “The caterpillar will be fully fed by twenty or 
thirty of my seeds, but that ball of pollen will send 
down tubes to two or three hundred ovules, and I can 
well spare a few seeds to keep up this most useful race of 
moth.” The moth in its turn might contend that it was 
very well suited by the arrangement, that its children 
were supplied with suitable food and complete shelter, 
and that, if it suited the plant, so much the better ! 
