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The thigmotastic monkey flower

Tim Entwisle

Tim Entwisle

September 3, 2014

You must remember the Monkey Hand Tree, well this is the Monkey Flower. In this case the flower is not really evocative of a monkey, or any part of a monkey. It doesn’t even appear in any of the lists of ‘plants that resemble animals’ (yes such lists exist).

The flower is perhaps more like a theatre mask, the kind you might wear to a masked ball, which is where the botanical name Mimulus comes from (a small ‘mimic actor’).

Monkey flowerStill, while not living up to its common name entirely, the flower does does have some special powers. You might therefore stretch the analogy and make reference to Saiyūki, a special kind of Monkey…

Anyway, the special power is thigmotaxis or thigmonasty (or seismonasty if you prefer). This is when you or I, or some other creature or thing, touch a particular part of the plant and it responds in some physical way (thigmo is Greek for ‘touch’). Think trigger plant for a particularly dramatic example.

Monkey flower2In the case of Mimulus, the lobes of the female bits (tucked inside the vase part of the flower, and revealed below after I pealed away the petals) close together when to touched. In nature this tends to be the bill or head of a hummingbird which bumps it en route to the nectar below. All going well, pollen from its previous visit to a Mimulus flower sticks to the hairy surface of the lobes before they shut.

Hairy surface on the lobes of MimulusThis system means that any pollen it gathers on its beak while foraging in that particular flower won’t be deposited on its own receptive female lobes on the way out. You can see how near the pollen are to the (now closed) female lobes in the second dissected flower picture below.

In a fresh flower it takes about two seconds for the lobes to come together. In older flowers, or in the cold, it may take up to 10 seconds. This is pretty quick among those plants that employ this technique. The quickest response time is one second but it can take up to an hour, which seems a little pointless unless you have a particularly enthusiastic hummingbird or other pollinator. Or, it may be achieving some other purpose such as as improving the capture of pollen on the receptive lobes.

Pollen and female lobes Mimulus flowerA study by scientists at the University of California on Mimulus aurantiacus found that pollen gently applied led to much slower closing of the lobes – up to one and a half hours – but once there the lobes remained firmly shut for a day or even for the full life of the flower. Touching the lobes caused them to close within seconds but they would reopen a few hours later.

These scientists concluded that thigmotaxis in this species at least did not help in the capture of pollen or in any way assist with seed set. Their discoveries were consistent with the system inhibiting self-pollination – cross-pollination allows a plant to adapt and change over time, and to produce more robust offspring.

Thigmotaxis of the female lobes is found only in the snapdragon family Scrophulariaceae and some other closely related families such as the Bignoniaceae and Lentibulariaceae. Once a card-carrying member of the Scrophulariaceae, Mimulus is now included with a group of mostly obscure genera – including a few from Australia such as the nondescript Elacholoma and its single species from inland areas – in the Phrymaceae.

Sticky monkey flowerThere are four native Australian species of Mimulus (the Creeping Monkey-flower, Mimulus repens, growing in wetlands around Melbourne)* but most come from North America, with some scattered around other regions of the world. The one photographed here is Mimulus aurantiacus var. puniceus, in its natural habitat straddling the US-Mexico border from southern California to northern Baja California.

Its common name is, quite legitimately, the Sticky Monkey Flower. Hence I found myself strangely attracted then attached to its flowers as I wandered through our Californian Bed at Melbourne’s Royal Botanic Gardens in June.

[*Although at least one of these, Mimulus prostatus, has been moved to the genus Elacholoma (thanks Neville Walsh). I haven’t chased down the taxonomy/nomenclature of the others.]
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