Harmony and Conflict in the Living World - Alexander F. Skutch [56]
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his book Insectivorous Plants than to any other genus and demonstrated their movements.
When, attracted by 130 to 250 glittering reddish droplets, a tiny insect alights on a short tentacle in the center of a leaf of the widespread Round-leaved Sundew and cannot free itself, this tentacle does not move but transmits to surrounding tentacles, as far as the leaf's margin, a stimulus that causes them to bend inward and apply their sticky heads to the struggling victim, binding it more firmly. If an insect is caught on a long outer tentacle, this sweeps inward to deposit the victim on short tentacles near the leaf's center, where it is joined by other tentacles. The rapidity of response varies with temperature and other factors; at best, movement visible to the naked eye may begin ten seconds after a tentacle just within the leaf's margin is directly stimulated, and sweep through 180 degrees in twenty seconds, but usually it is much slower. After contacting the prey, the glands in the tentacles' heads pour out proteolytic enzymes to digest it, as revealed by changes in the protoplasm. The tentacles remain in contact with the corpse until all its nutriment has been extracted, which may take up to five days; then they separate and return to their original positions. The same tentacle may close upon three successive victims, after which its capacity for bending, which is effected by unequal growth of the sides of its stalk, appears to be exhausted.
The sundew's tentacles respond not only to insects or tiny particles of food such as meat or cheese placed upon them but likewise to stimulation without nourishment, as by stroking them gently with a fine brush. Darwin was amazed to find that a fragment of soft thread only one-fiftieth of an inch long, or a much shorter particle of human hair, caused a tentacle to sweep through an angle of more than eighty degrees. After stimulation that yields no food, the tentacles bend outward again much more promptly than when they are fed. Experiments by Charles Darwin's son, Francis, and other investigators showed clearly that sundew plants nourished by insects are more vigorous, produce more and larger inflorescences, more seeds, and greater dry weight than those, otherwise similarly treated, that are prevented from catching prey. While the
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drosera plant digests the proteins of insects, it continues to carry on photosynthesis at a normal rate for its carbohydrates.
Folding Leaves
From sundews we turn to another plant in the same family, Venus's-flytrap, which in an unguarded moment Charles Darwin called ''the most wonderful plant in the world." At least it is unique, the only member of its genus, and with no remotely similar plant except Aldrovanda vesiculosa. Dionaea muscipula, as the flytrap is known to botanists, grows only in sandy bogs and wet pinelands in North and South Carolina. It is an inconspicuous herb that from a short rhizome produces at ground level a rosette of small leaves, from the center of which arises a long, leafless stalk crowned with a cluster of small white flowers. Each leaf consists of a petiole broadened into a green blade and, separated from this by a short slender stalk, the lamina divided into two lobes, or valves, that can fold together. Charles Darwin's grandfather, Erasmus, regarded this as a contrivance to protect the plant from insects by crushing or piercing any that might crawl over it. Actually, these leaves are not defensive but devices to snare, kill, and digest small victims. From the upper surface of each lobe spring three long, sensitive bristles, arranged in a triangular pattern. The rest of this surface is densely