Harmony and Conflict in the Living World - Alexander F. Skutch [60]
Page 124
the animal enter the bladder, since no creature voluntarily incarcerates itself? The rat pushes into the trap for the bait; but what within the sealed bladder can be sensed by an animal outside and entice it to enter? This problem began to engage botanists after the Crouan brothers, pharmacists and amateur botanists of Brest in France, in 1858 communicated to the scientific world their discovery that the Utricularia bladders held imprisoned animalcules. Before this, these vesicles, which when removed from the water often contain much air, were believed to be floats to hold the inflorescences above the surface. A few aquatic species, including U. inflata of the eastern United States and U. stellaris of Malaysia, have at the base of the flower stalk a whorl of hollow, air-filled petioles to sustain it, but other species lack such buoys. All parts of the Greater Bladderwort and similar species are buoyant with air in intercellular spaces, although their submerged trap bladders contain no air. The numerous epiphytic bladderworts certainly have no need of floats.
Darwin was the first naturalist to consider seriously how and why animals that are trapped in such large numbers enter the bladders. In spite of prolonged and patient observations, which he describes in detail in Insectivorous Plants, published in 1875, he failed to reach a convincing explanation of how the trap works. He suggested that small creatures might be attracted to the bladder to feed on the mucilage around the orifice, and might even ''habitually seek to intrude themselves into every small cavity, in search of food and protection," but he ends with the unsatisfactory conclusion that "animals enter merely by forcing their way through the slit-like orifice, their heads serving as a wedge." And in the years that followed, other able investigators endeavored to penetrate the mystery of the bladderwort, with results equally inconclusive.
A Mystery Solved
About the year 1910, the Swiss entomologist Frank Brocher became dissatisfied with Darwin's generally accepted explanation. Believing that nobody had followed closely enough the entry of an animal into
Page 125
a bladder, he decided to watch with great care. On the upturned surfaces of valves he pushed around small crustaceans, injured so that they could not too readily swim away, while he watched through a microscope. Sometimes they suddenly vanished, to be found later inside the bladder. Next he tried shooting these creatures against valves from a small pipette, usually with no enlightening result. Once, however, the animal disappeared into the bladder, and with it went a bubble of air to which it had become attached while in the pipette. This minute bubble was the key to the mystery; it told Frank Brocher that the bladder expands as it captures its prey
Following this clue, he was able to observe that a bladder suddenly became distended when with a needle he touched one of the four bristles on the valve, or the valve itself near their bases. The impact of a small creature had the same effect. As it dilates, a bladder sucks in a current of water, and with it the animal that collided with the releasing mechanism. Darwin had come close to solving the riddle when minute fragments of blue glass that he pushed gently over the valve's surface suddenly vanished and were afterward found inside; as did his son, Francis, when tiny cubes of green boxwood were similarly engulfed.
Published in 1911 in a journal of small circulation, Brocher's discovery did not claim wide attention, with the consequence that the same observation was made independently thrice more before the bladderwort's secret became better known. In 1916, a youth of eighteen, C. L. Withycombe, noticed while watching bladders with a hand lens that