The Power of Movement in Plants
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THE chief object of the classic Darwin essay, The Power of Movement in Plants, is to describe and connect together several large classes of movement, common to almost all plants. The most widely prevalent movement is essentially of the same nature as that of the stem of a climbing plant, which bends successively to all points of the compass, so that the tip revolves. This movement has been called by Sachs "revolving nutation;" but we have found it much more convenient to use the terms circumnutation and circumnutate. The work concerns itself with how plants respond to external stimuli and examines these processes in individual plants to gain understanding of some general principles governing their growth and life. This continues Darwin’s work of elucidating how natural selection works and specifically how plants have adapted to differing environments whilst at the same time answering some objections of his day that evolution could not account for changes in behavioural responses. In his conclusions, Darwin presents the key features of plants from an evolutionary perspective indicating that gradual modification of these processes in response to natural selective forces like light and water could enable extensive ability to adapt. The process that creates the circular or elliptical movement of the stem and tips of plants (circumnutation) was identified as a significant one in enabling plants to evolve and adapt to almost any environment on the planet. Darwin also drew attention to the similarities between animals and plants, e.g., sensitivity to touch (thigmotropism), light sensitivity (phototropism), and gravity (geotropism). Darwin used various methods of enquiry: usually setting up rigorous controlled experiments which are clearly explained in the text, reporting the results and then drawing general conclusions. Studies of nyctinasty were particularly burdensome, to Darwin's rest as well as to the plants: "I think we have proved that the sleep of plants is to lessen the injury to the leaves from radiation. This has interested me much, and has cost us great labour, as it has been a problem since the time of Linnaeus. But we have killed or badly injured a multitude of plants.