The Different Forms of Flowers on Plants of the Same Species is a classic botany essay by Charles Darwin. Using the four classifications established by Carl Linnaeus (hermaphroditic, monoecious, dioecious, polygamous), Darwin concentrated on two divisions of the hermaphroditic class, namely the cleistogamic and heterostyled. Darwin's reflections indicate the economy of nature through a process of gradual modification of plants, their structures being modified and degraded for the purpose of the large scale production of ...
Read More
The Different Forms of Flowers on Plants of the Same Species is a classic botany essay by Charles Darwin. Using the four classifications established by Carl Linnaeus (hermaphroditic, monoecious, dioecious, polygamous), Darwin concentrated on two divisions of the hermaphroditic class, namely the cleistogamic and heterostyled. Darwin's reflections indicate the economy of nature through a process of gradual modification of plants, their structures being modified and degraded for the purpose of the large scale production of seed which is necessary and advantageous for survival. Darwin states (p. 227): "Cleistogamic flowers ... are admirably fitted to yield a copious supply of seed at a wonderfully small cost to the plant." Francis Darwin indicated that the work on heterostylism had given his father extreme pleasure, especially as it had been one of the most puzzling bits of work he ever carried out. Darwin thought that hardly anyone had seen the full importance of heterostylism. In 1883, Alfred Russel Wallace wrote a tribute to Darwin (entitled 'The Debt of Science to Darwin') who had died the year before. One such tribute appeared in 'The Century', an illustrated monthly magazine. As part of this article he included a summary of Darwin's work relating to this book. "The cowslip (Primula veris) has two kinds of flowers in nearly equal proportions: in the one the stamens are long and the style short, and in the other the reverse, so that in one the stamens are visible at the mouth of the tube of the flower, in the other the stigma occupies the same place, while the stamens are halfway down the tube. The fact had been known to botanists for 70 years, but had been classed as a case of mere variability, and therefore considered to be of no importance. In 1860 Darwin set to find out what it meant, since, according to his views, a definite variation like this must have a purpose. After a considerable amount of observation and experiment, he found that bees and moths visited the flowers, and that their proboscis become covered with pollen while sucking up the nectar, and further, the pollen of a long stamened plant would most surely be deposited on the stigma of the long styled plants, and vice versa. Now followed a long series of experiments, in which cowslips were fertilised with either pollen from the same kind or from a different kind of flower, and the invariable result was that the crosses between the two different types of flowers produced more good capsules, and more seed in each capsule; and as these crosses would be most frequently effected by insects, it was clear that this curious arrangement directly served to increase the fertility of this common plant. The same thing was found to occur in the primrose, as well as in flax (Linum perenne), lungworts (Pulmonaria), and a host of other plants, including the American partridge-berry (Mitchella repens). These are called dimorphic heterostyled plants.
Read Less
