Friday, September 23, 2011

Hooked on Hops

Hops Humulus lupulus have an impressive ability to climb supports - either up other plants or, in the case of cultivated hops, up poles in hop gardens. Charles Darwin devoted a lot of time to studying the way in which their shoot tips rotate as they grow (by the process of circumnutation), seeking out objects to coil around (you can read more about his experiments here). There's more to hops' climbing ability than circumnutation and rapid growth, however - their stems are clothed in very distinctive epidermal hairs (trichomes) that act as grappling hooks, securing their grip on supporting structures.

The hop trichomes that are adapted for climbing have a very distinctive anvil shape - you can see them here, at low magnification, on either side of a hop leaf petiole.

At higher magnification the anvil shape is very distinctive, something noted ....

.... by the botanist Anton Kerner von Marilaun in his Natural History of Plants (1895).

Hops have been cultivated for centuries, primarily for the resins produced by their epidermal glands, mainly at the base of the bracts in the female flowers but also on other parts of the plant, including the underside of the leaf. In the photograph above you can see the minute gold drops of resin on the lower surface of a hop leaf. The resins are converted to bitter isohumulones during the brewing process, adding a distinctive flavour to beer.

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