From the start, plants figured prominently in the development of Darwin's ideas of evolution by means of natural selection. Examples from the plant world abound in On the Origin of Species and Variation of Plants and Animals Under Domestication (1868). Not only were plants easily studied and bred, but they showed a stunning assortment of adaptive features. They were to prove one of Darwin's favorite objects of study, becoming the basis of no less than seven books, most of which appeared in the latter part of Darwin's life.
Darwin's first book on plant evolution was titled Fertilization in Orchids (1862). He chose to study orchids because of the range of adaptations they displayed with respect to fertilization. Darwin recognized that these elaborate adaptations served to facilitate cross-pollination by insects such as bees. For this reason, cross-pollinated plants had flowers with bright colors and fragrant nectaries to attract bees and other insects, while wind-pollinated plants, which did not have to attract insect pollinators, had flowers with little or no color.
Darwin also observed that plants that seemed to have one or few flowers had the tendency to be hermaphrodites, having flowers of both sexes on the same plant. Bigger trees with a large number of small flowers, however, usually had flowers of only one sex. To Darwin, this implied that flowers had adapted mechanisms to ensure cross-pollination. This likely increased the variability, and also increased the vigor of the offspring. Darwin performed numerous experiments to understand the manner in which cross-pollination took place in plants and to understand the adaptive function of increased variability. In the process he noted the phenomenon of heterosis, or hybrid vigor, in the progeny of cross-pollinated plants. He also began to unravel the adaptive functions of sexual reproduction.
Darwin's work in pollination mechanisms appeared in two books. The first was Effects of Cross and Self Fertilization (1876), which was followed by Different Forms of Flowers on Plants of the Same Species (1877).
Darwin was also interested in the adaptive functions of climbing plants. He found that the phenomenon of twining, or the differential bending of plants in a clockwise or counterclockwise manner around an object, permitted young or weak plants to raise themselves higher up off the ground. This maximized exposure to air and sunlight in a relatively short time, and without the costly and time-consuming investment of woody supportive structures. The various means used by plants to climb were explored in his book Climbing Plants (1875). The mechanisms by which this and other plant movements took place was explored in Power of Movement in Plants (1880). In this book Darwin explored plant tropisms, or the manner in which plants were able to grow toward light. He determined that the stem bends toward the light because of differential growth rates: the illuminated side grew more slowly than the unilluminated side so that shoot tips appeared to bend toward the light. He postulated the existence of a substance that was diffused from the apex downward that affected growth rates. These investigations anticipated the existence and action of plant hormones.
Darwin then directed his attention to other types of movements in plants, including mechanisms for prey capture in insectivorous plants such as the sundew, Drosera rotundifolia (1880). After detailed observation and experimentation, Darwin concluded that carnivorous plants had acquired the ability to live in nitrogen-poor soil with little or no root structure by feeding on prey. In addition to developing sensory apparatus to detect and capture prey, plants had also developed a digestive system capable of breaking down proteins. Most of these observations, including experiments with the sundew plants, were the focus of his book Insectivorous Plants (1875).
Darwin's last book relating to plants was a work with a strangely ecological theme: the action of worms in turning up soil. After experiments that ran for more than fifty years, Darwin postulated that earthworms played a vital ecological role: they fed on dead leaves and other organic matter and excreted this back into the soil. In so doing, earthworms served to aerate the soil and recycle vital nutrients. These results, including quantitative estimates of how much soil was processed by earthworms, were included in The Formation of Vegetable Mould Through the Action of Worms (1881).
Darwin's botanical work is notable for its detailed observations and simple, elegant experiments. These were performed in the confines of Darwin's backyard or at greenhouses at his home in Downe. Despite the fact that some of these are now classic experiments reproduced by students the world over, they were judged harshly by the leading German plant physiologist of the late nineteenth century, Julius von Sachs (1832-1897). A revolutionary experimentalist who introduced powerful analytical laboratory methods to botanical science, launching the "New Botany," Sachs thought Darwin's naturalist tendency and simple backyard experiments to be antiquated and amateur. Nonetheless, Darwin's botanical work remains the cornerstone of his studies on variation and mechanisms of adaptation in plants and is significant for his keen insights.
The book on earthworms was published just six months before Darwin's death. Until his end, Darwin remained a productive scientist. Some of his most imaginative work was performed toward the end of his long life. His was a happy and productive life in a home filled with the voices of his ten children and numerous grandchildren. On his death in 1882, he received a rare honor for a scientist: he was given a state burial and was buried at Westminster Abbey. see also Carnivorous Plants; Compost; Evolution of Plants; Evolution of Plants, History of; Hooker, Joseph Dalton; Mendel, Gregor; Orchidaceae; Phylogeny; Sachs, Julius von; Tropisms and Nastic Movements.
Vassiliki Betty Smocovitis
Allan, Mea. Darwin and His Flowers: The Key to Natural Selection. New York: Taplinger, 1977.
Bowler, Peter J. Charles Darwin: The Man and His Influence. Cambridge, UK: Cambridge University Press, 1990.
Browne, Janet. Charles Darwin: Voyaging, Vol. 1. New York: Alfred A. Knopf, 1995.
de Beer, Gavin. "Charles Darwin." In Dictionary of Scientific Biography, Vol. 3, ed. Charles Coulston Gillispie. New York: Scribner's Sons, 1970.
Desmond, Adrian, and James Moore. Darwin. London: Michael Joseph, 1991.
Huxley, Julian, and H. B. D. Kettlewell. Charles Darwin and His World. New York: Viking Press, 1965.
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