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Sensitive sorrels

September 18, 2010

Have you ever seen a small plant resembling an upside down cocktail umbrella, folding its leaves upwards around its peach-coloured blossom? In that case you have Biophytum petersianum (Oxalidaceae) in your garden. This charming native plant is a new addition to Dokmai Garden. It is an annual, preferring dry and sunny places. Unlike the introduced sensitive mimosa (Mimosa pudica, Fabaceae), the sensitive sorrels (there are five species in Thailand) are not prickly.

Many people wonder how sensitive plants can fold their leaves. It is due to a rapid transport of water out of certain cells at the base of the leaf and branch stalks. The mechanism is similar to keeping water balloons under your armpits, forcing the arms outwards, until the balloons are punctured and the arms fall down. The other question, why they have such cells, is harder to explain. Some hypothesise that vegetarian insects landing on such plants are scared away by the movement. If so, such insects would be scared every time the wind shakes a plant. Others hypothesise that this is a way to avoid damage of the leaves during heavy rains. This can not be true for the sensitive sorrels, as they fold upwards, creating more mechanical tension when hit by rain drops. Many other plants can move their leaves too, but much slower. The raintree (Samanea saman, Fabaceae) and starfruit (Averrhoa carambola, Oxalidaceae) fold their leaves at twilight, to minimise water losses during the night when the leaves are not used for photosynthesis. Maybe the sensitive mimosa and sensitive sorrels fold their leaves of the same reason, just much quicker. If touched, such cells may leak their water, which is more of a mistake, without selective benefits for the individual? If this sensitivity to touching had a great selective advantage, the phenomenon would probably be common among beans and sorrels, but it is not.

Eric Danell

Biophytum petersianum, now in blossom at Dokmai Garden.

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