Muggles' Guide to Harry Potter/Magic/Dirigible Plum

Overview
Dirigible Plums are a possibly magical plant mentioned in connection with the Lovegood family. It is described as resembling an orange radish.

Extended Description
Dirigible Plums first appear in Harry Potter and the Order of the Phoenix, where we see Luna Lovegood wearing them as earrings. Harry, seeing them, wonders why anyone would wear radishes on their ears.

We also see her wearing them numerous times in Harry Potter and the Half-Blood Prince. To Harry's relief, she leaves them behind for the Christmas party in Professor Slughorn's rooms.

In Harry Potter and the Deathly Hallows, the Trio see them in Xeno Lovegood's garden, where they are named, for the first time, by a sign that reads:

PICK YOUR OWN MISTLETOE

KEEP OFF THE DIRIGIBLE PLUMS

In discussion with Xeno Lovegood, the Trio discover that he is attempting to replicate Rowena Ravenclaw's lost Diadem. This diadem was supposed to give Ravenclaw additional intelligence. One of the components of the Diadem under construction is a Dirigible Plums attached at the forehead, which Xeno explains is to "enhance the ability to accept the unexpected."

Analysis
One wonders why these fruits would be called "dirigible plums" as "dirigible" implies some sort of ability to set a direction. From the limited description we have, however, we can deduce that like radishes, dirigible plums come to a point. Presumably, their "dirigible" nature is indicative of the fact that, because of this point, they can be given an orientation, a direction.

Xeno's explanation of the use of Dirigible Plums in his recreation of Ravenclaw's Diadem clarifies why Luna is always wearing them as earrings. However, we have already seen that Xeno has unconventional ideas, including belief in the existence of creatures like the Crumple-Horned Snorkack and Nargles, and in magical effects caused by Gnome bites; it is quite likely that Xeno's belief in their effects is just as misplaced.