At the moment, I am teaching plant hormones in our course on plant cell and developmental biology (BS1003). Fortunately, hormones and development link well with Halloween this year: we have witches, sitting on a broom, with a pumpkin, and for good measure some brightly colored leaves falling around them. Where is the connection?
The broom is a good point to start : many species of trees have “witch’s brooms” on them, structures where a large number of small twigs arise from one region of a branch or the main trunk. The uncontrolled outgrowths are caused by other organisms which either make their own phytohormones which induce the host plant to produce the multiple branches, or by the other organisms altering the plant’s own regulation of hormones.
Insects (including wasps or mites), fungi, viruses or even hemiparasitic plants like mistletoe cause the upset in the balance between the major hormones including auxins and cytokinins so the plant generates new meristems within the stem or trunk which grow out as shoots.
What about the witch herself? The state of being a witch is partly a behavioural anomaly and partly physical, and many of the symptoms including hallucinations, seizures and skin effects are those from ergot poisoning or Saint Anthony’s Fire. Although the symptoms “are not actually caused by bewitchment”, Wikipedia discusses the Medical Explanations of Bewitchment, and ergot poisoning is one widely suggested cause, particularly among the infamous Salem Witches of Massachusetts in the 17th century. The fungus Claviceps purpurea infects the ears of rye plants, and the fungal fruiting body (sclerotium) replaces the seed. This parasitic lifestyle involves taking nutrients from the plant (as though they were going to a seed), and the sclerotium then also takes advantage
of the dispersal mechanism and it is carried around along with the seeds. When the rye is milled for bread, the sclerotium is included and the alkaloids including psychedelic drug relatives and toxins in the ergoline family are eaten, possibly giving rise to witch-like behaviour.
The link of plant hormones and plant development to the pumpkins is slightly more stretched – through sex determination. The first flowers of the ground-spreading vines of many cucurbits are male, and later ones are female or a mixture of male and female. Hormones are responsible for the changing sex of the flowers, but interestingly, one hormone can have a opposite effects in different plants: gibberellin application will change cucurbit flowers to males, while making maize flowers female.
Now, for chromosome biology, all I need is a witch with a tortoiseshell or calico cat, rather than their favoured black cats, so that I can discuss inactivation of the genes on the X chromosome leading to the coat variegation!
Addition 1 Nov: The comments give a number of interesting links to other stories about egotism. The Scientist Gardener has an impressive post on Claviceps / Ergot from the end of 2010.