A witch's broom or witches' broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest.
One example of this would be cytokinin, a phytohormone, interfering with an auxin-regulated bud. Usually auxin would keep the secondary, tertiary, and so on apexes from growing too much, but cytokinin releases them from this control, causing these apexes to grow into witch's brooms.
Witch's broom growths last for many years and can be caused by many different types of organisms, such as fungi, oomycetes, insects, mistletoe, dwarf mistletoes, mites, nematodes, phytoplasmas and viruses. Human activity is sometimes behind the introduction of these organisms; for example when a person prunes a tree improperly, leaving the tree susceptible to disease.
Witch's brooms occasionally result in desirable changes. Some cultivars of trees, such as Picea orientalis 'Tom Thumb Gold', were discovered as witch's brooms. If twigs of witches' brooms are grafted onto normal rootstocks, freak trees result, showing that the attacking organism has changed the inherited growth pattern of the twigs.
Witch's brooms are used by various animals for nesting including the northern flying squirrel.
Famous quotes containing the words witch and/or broom:
“A witch is one who worketh by the Devil or by some curious art either healing or revealing things secret, or foretelling things to come which the Devil hath devised to ensnare men’s souls withal unto damnation. The conjurer, the enchanter, the sorcerer, the diviner, and whatever other sort there is encompassed within this circle.”
—George Gifford (16th century)
“If the juggler is tired now, if the broom stands
In the dust again, if the table starts to drop
Through the daily dark again, and though the plate
Lies flat on the table top,
For him we batter our hands
Who has won for once over the world’s weight.”
—Richard Wilbur (b. 1921)