After flowering the Willow Herb develops long, pod-like processes.
During damp and stormy weather these pods remain tightly closed. On a
day when the air is dry and the breezes are light, the sides of the case
split open and reveal a prodigious number of perfect flying machines.
The seed itself weighs a mere trifle, and to this is attached a
beautiful arrangement of feathery hairs. The whole thing is so well
adapted for an aërial voyage that it mounts rapidly upward on the
faintest puff of air. It should be here explained that by experiment it
has been shown that the air currents tend to move upward. So light are
some of these flying fruits that they often rise to an immense height.
It is not an uncommon thing for them to be found on mountains thousands
of feet above sea-level.
Of course, many foreign seeds have remarkable flying appendages. That of
the South African Stapelia has a vast mass of fluffy hairs which will
support it on quite a long aërial voyage. In the case of the cotton
plant man has turned to good account the hairs by which the seed flies.