MIMOSA PUDICA (sensitive plant, humble plant)

This species may not be one of the most decorative houseplants. but it is certainly the most fascinating. Children are particularly intrigued by the remarkable phenomenon of plant movement that it so dramatically demonstrates. Both the foliage and the flowers have a resemblance to mimosa (Acacia dealbata). However. in M. pudica the flowers are of little consequence, only a few being produced. They are purplish to pinkish in colour – they will appear on young plants the lirst year from spring sowings. usually in July and August. The striking feature of the plant is its sensitive foliage. The leaves are composed of a number of small leaflets. At the base of each leaflet is a tiny, pale. raised spot. This is the touch-sensitive organism, and is called a pulvinus. If it is touched with a needle point, the leaflet will instantly fold upwards. Usually, the leaflet opposite will also fold and the movement may continue along the entire leaf, each pair folding. A sharper vibration will, of course, cause the whole plant to collapse dramatically, the leaf stalks also folding downwards so that it appears wilted and dead. If left alone, the plants slowly regain normality. The time taken for this is variable and may be half an hour or more. The performance can then be repeated.

The movement is believed to be caused by a hydraulic effect due to cells in the pulvinus taking in or discharging sap from the surrounding tissues, but the exact mechanism is still a mystery. The plant may fold its foliage in the evening. This is called sleep movement and it is common in plants of the pea family.

The sensitive plant is a native of tropical America although it grows widely in a number of warm climates. It is perennial. but needs a congenial warmth to survive the winter. Moreover, old plants are less sensitive and do not react so spectacularly to touch. It is, therefore. more convenient to buy seedlings in spring, or grow your own from seed. The seed germinates easily under window-sill conditions, and is sown from April to May. Prick out the seedlings into 7.5cm (3in) pots, later transferring them to 13cm (5 in.) pots. The height reached by autumn is only about 30cm (1ft) or so. Keep the plants moist and give a slightly shaded position.

There are usually no problems associated with this plant, but red spider mites are a possibility. If difficulty is experienced in germinating the seed, it may be because it is not reasonably fresh. Try soaking it overnight before sowing, using tepid water.

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