When I was growing up in Luanda, Angola, I remember a peculiar bush. When you touched it, the area where contact was made wilted immediately, with all the leaves on that branch drooping. This lasted for only 2 or 3 minutes, after which the leaves slowly became erect and returned to normal. Does anyone know the name of this bush and why it behaves in this way?
鈥� The peculiar bush which wilted when touched was almost certainly the 鈥渟ensitive plant鈥�, . This is a spindly tropical shrub with delicate feathery leaves, which fold up very quickly if the plant is touched or shaken. Even in colder climates you can grow it easily from seed. It makes an unusual houseplant with attractive pink fuzz-ball flowers, but the main attraction is, of course, its sensitive nature. The plant will survive outside in warm weather, where a sudden gust of wind or a few raindrops will cause the whole plant to close down. Presumably this sensitivity evolved as a defence against damage by tropical rainstorms. On the other hand, in breezy conditions the plant loses its sensitivity because a breeze, unlike the touch of an animal, holds no threat. The plant then refuses to react even to a good shaking.
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The plants are certainly efficient photosynthesisers because they take only a few months to grow to around 1 metre in height even in the UK, but the leaves are not robust and would easily be damaged if rigidly attached.
A leaf consists of four or six fronds, each about 6 centimetres long and attached to a thin stalk. Each frond carries an average of 17 pairs of small leaflets, which are the first to react when the leaf is touched (see top Photo). If left undisturbed for a few hours in hot and humid conditions, the plant becomes almost hypersensitive as the warmer conditions are more conducive to sensitivity, so touching just one leaflet will cause the whole leaf to progressively close up (see bottom Photos).
With choreographic accuracy the leaflet pairs close in sequence along one frond, then the leaflets on the remaining fronds follow suit. With each leaflet pair taking about half a second to react, the fronds are fully closed in around 15 seconds. Then the fronds close like the fingers of a hand and, after a pause, the leaf stem itself angles downwards when a thickened area close to the stem gets the message. Sometimes other leaves on the same stem will react in a half-hearted sort of way.
At a rough estimate the message from the initial touch point travels at about half a metre per minute. This is not much by mammal standards but good going for a snail and definitely a lightning reaction for a plant. If this isn鈥檛 evidence for the presence of nervous systems in plants then I don鈥檛 know what is. Does anyone know how it carries it out?
John Rowland, Derby, UK
鈥� Many of the species in the Mimosa genus are sensitive to touch. It is native to Brazil but can be found in most tropical regions of the world now. M. pudica is remarkable in its genus for being , meaning it is sensitive to vibrations and also touch. Once touched the leaflets move rapidly together, closely followed by the whole leaf collapsing.
This is caused by a reaction in the cells of the thickened tissue, or pulvini, at the base of the leaflets and leaves. These cells are sensitive to internal or turgor pressure. When a leaf is touched an electrical signal causes this turgor pressure to drop rapidly and the pulvini collapse 鈥� the leaves follow the leaflets because they are connected. The actual mechanism for this movement is still not fully understood. There is evidence that simple turgor mechanism may be an oversimplification 鈥� more study is needed.
Peter Scott, Plant conservationist, School of Life Sciences, University of Sussex, Brighton, UK