Simple seeds which have wings are rather rare, but we find them on pine seeds (see fig. 125), and the seeds of the willow herb are covered with a number of silky hairs, which make them so light that they fly in the wind. It you watch a spray of willow herb ripening, you will find that the old carpels, or fruits, split up into four parts and let out a number of fluffy white seeds. These are true flying seeds (see fig. 84).
Fig. 84. Fruit of the Willow herb, opening and allowing the flying seeds to escape.
Other seeds get scattered by the wind although they do not fly. For example, in the poppy the fruit is the hardened ripe carpels which have become quite dry, and together look like a little round box, within which there are many tiny dry seeds. When the box or capsule is quite ripe openings come in it, just below the projecting top, and then, when the weather is dry and they are open, a strong wind may bend the stalk of the fruit and shake the capsule strongly. The seeds come scattering out like pepper from a pepper-pot, and may get carried some distance from their parent plant (see Plate II. and fig. 85).
Fig. 85. Ripe Poppy capsule, showing the little pores at the top which let out the ripe seeds when the capsule is shaken.
Some fruits are covered with spines and hooks, which catch on to the wool of animals, and so get carried quite a distance before they are dropped. This gives the seedlings a good chance of reaching a new spot where they can grow away from the parent, and so not be too crowded. Well-known fruits of this kind are the burs, which stick tightly to one another with their dozens of little hooks, the “bur” being really a cluster of many fruits together. Simple fruits of the same kind are the bidens, each with its two long spines, and the small fruits of the goose grass, which are covered with the finest hooks.