Members of several flowering plant families, particularly the Legume Family (Fabaceae), which includes peas and beans, exhibit movements in which either leaves or petals fold as though "going to sleep." The folding and unfolding usually takes place in regular daily cycles, with folding most frequently taking place at dusk and unfolding occurring in the morning. Such cycles, which have been more extensively documented in the Animal Kingdom, have come to be known as circadian rhythms.
In prayer plants (Maranta—Fig. 11.17) and others where the leaves or flowers fold up and reopen daily, turgor movements are involved, as are the external stimuli of light and temperature. These circadian rhythms are controlled by
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a biological "clock" on cycles of approximately 24 hours. In a few instances, nastic movements apparently are initiated by external stimuli alone.
Circadian rhythms appear to be controlled internally by the plants, although they are also geared to changing day lengths and seasons. They do not generally accelerate when temperatures increase. The actual timing of circadian rhythms varies with the species, although most plants exhibiting sleep movements do so at dusk and at dawn. The flowers of several species pollinated by night-flying moths, however, open at different hours of the night.
About 200 years ago, the famous Swedish botanist Linnaeus planted wedge-shaped segments of a circular garden with plants that exhibited sleep movements. The plants were arranged in successive order of their sleep movements throughout a 24-hour day. One could tell the approximate time simply by noting which part of the garden was "asleep" and which was "awake." A few others copied the garden clock idea, but the expense and difficulty of obtaining all the plants from different parts of the world and replanting them each year proved to be too great for the practice to be continued.
The movements are produced by turgor changes caused by the passage of water in and out of cells at the bases of the leaves or leaflets. The function of these movements is not clear, and the rhythms are also not confined to sleep movements. Species of certain warmer marine-water algae called dinoflagellates (discussed in Chapter 18) glow in the dark through bioluminescence, a process by which chemical energy is reconverted to light energy. One species always glows brightly within 2 or 3 minutes of midnight, even if it is maintained in culture in continuous dim light. This particular dinoflagellate also glows when culture containers in which it is suspended are jarred, and it displays two other types of circadian rhythms. One rhythm involves cell division, with peak mitotic activity occurring just before dawn, and the other rhythm pertains to photosynthesis, which reaches a maximum around noon.
Another type of rhythm is seen in the giant bamboos of Asia, which send up huge flowering stalks every 33 or 66 years, even if the plants have been transplanted to other continents and are growing under different conditions. These flowering stalks use up all the energy reserves of the bamboo, and they die shortly after appearing. This has especially been a problem in cities where nearly all of the bamboo plantings were propagated from a single source and then all died simultaneously after flowering.
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