Sunday, August 23, 2009
Friday, August 7, 2009
Diaspora is a Greek word meaning the scattering of seeds. We are now heading into late summer and plants are setting seed. One of the things I most enjoy at this time of year is marveling at the variety of forms produced by seed bearing plants.
Seeds hold an embryo and carry the genetic material of a new plant. There are three methods evolved by plants to disperse their seeds-by wind, by water, and by animals.
The structures used by plants to get their seeds aloft and carried by the air currents of the earth are masterpieces of engineering. The designs include gliders, parachutes, whirlybirds, and spinners. Gliding seeds are said to have inspired the designs of some early aircraft. I like to think that spinner or whirlybird seeds may have contributed to one of Leonardo Da Vinci's concepts for a flying machine.
An outstanding example of the parachute design is the ubiquitous Common Dandelion (Taraxacum officinale) a member of the Composite family. The seed (achene) is attached by a thin stalk to a radiating plume of bristly hairs (pappus). So equipped, the seeds become airborne in response to the slightest breeze. As is often the case with parachuting seeds, they are arranged in a globular puff. Another name for the dandelion is blowball. The word pappus is Latin for old man.
The Composite family includes sunflowers, daisies, coneflowers, chicory, and thistles. In members of this family, the pappus is modified in a multitude of ways, often to promote the effective dissemination of seeds. For example, in sticktights, the pappus is barbed so that it adheres to passing animals. The characteristic of the pappus is important in identifying the particular species of a Composite blossom.
In his last years, Thoreau was working on an exhaustive research project to determine all of the dominant patterns of seed dispersal within an hour's walk of his home in Concord, Massachusetts. He called it "learning the language of the fields." Thoreau was one of the first American field ecologists to apply Darwin's ideas of natural selection to the subject.
Thoreau died from a respiratory infection before he could finish his manuscript entitled The Dispersion of Seeds. The work is an argument against the then-prevalent theory that some plants grew spontaneously without any root, seed, or cutting from a parent plant. Typically, Thoreau combines keen observation with a view to a larger perspective. His description of the milkweed ends with these thoughts :
I am interested in the fate or success of every such venture which the autumn sends forth. And for this end these silken streamers have been perfecting themselves all summer, snugly packed in this light chest, as perfect adaptations to this end--a prophecy not only of the fall, but of future springs...Who could believe in prophecies...that the world would end this summer, while one milkweed with faith matured its seeds?
Wednesday, August 5, 2009
The pitcher plant (species Sarracenia pupurea venosa)and the sundew are two of many plants I saw on my visit to Cranberry Glades (see previous post The Boreal Bog). These plants lure insects, trap them, then slowly digest them. I had to learn more-here is what most impressed me. If you are so inclined, while you read, ponder the Adaptation process that resulted in such elegant solutions.
Carnivorous plant species number more than 500 and grow all over the world. They often live in waterlogged areas such as swamps and bogs. These places are deficient in nitrogen and other trace minerals important to plant physiology. So it is thought that carnivorous plants have developed strategies to get some of those needed substances from the bodies of animals-such as insects, spiders, and even bigger prey. Their strategies often require glands that either fill with water or must remain moist. The details of their dining habits bring to mind the narrow escapes of heroes like Luke Skywalker and James Bond. If they could talk we might hear something like: "Welcome, Mr. Bug, hahahahahahaha!"
The Pitcher Plant
The leaves of the pitcher plant form cups, indeed another name for them is "hunter's cup. " Supposedly the rainwater contained in the cup is safer for a human to drink than the acidic bog water in which it grows. On the rounded lip of the cup are red veins that attract the attention of insects--possibly due to the resemblance to carrion.
Alighting on the slippery edges of the cup, an unfortunate critter then finds it easy to lose its footing and slide into the gaping maw. Perhaps it even experiences Vertigo. A spiky fur points downward to deter any attempts to escape. Imagine trying to climb the slippery walls while treading water in a deep cistern and you've got an idea of the insect's experience. But that's only the start of the macabre process.
In a sarracenia purpurea, the insect drowns and over time the water dissolves the insect's soft tissues. Special cells at the base of the cup absorb the nutrients. Some other species of pitcher plant actually contain a fluid similar to that found in mammals' gastric systems. That would certainly help digest something like a large rat. The biggest known pitcher plant does just that. The cup of the Nepenthes Rajah of Asia can grow up to 20 inches long, 6 inches wide, with an opening of 3 inches.
Biologists who do things like dissect pitcher plants are amazed at the quantity of indigestible exoskeletons that collect in a "boneyard" at the bottom of the cup. I wonder if the live insect can see the gruesome remains of previous victims at the moment of his or her descent into insect "hell." To an entomologist however, I'm sure those are Lovely Bones.
Sundews belong to the largest genus of carnivorous plants Drosera, with more than 200 species. The way the sundew hides menace behind whimsy, you might suspect it came from outer space. At Cranberry Glades, sundews dot the surface of the moss like countless simpering, "happy faces."
But each tiny leaf is far from being a Little Miss Sunshine and more like the product of a Bad Seed-- at least from an insect's point of view. There is a glistening drop of "superglue" at the tip of each of the fine hairs on this little damsel. Insects alight expecting a sweet reward of nectar. But touching even one hair can ensnare. The greater the bug's Frenzy to escape, the more it entangles itself in other hairs.
The rest of the hairs on the leaf then bend lovingly over the prize until it is clenched firmly as if in a many fingered hand. The process is not as fast as the abrupt snap of a Venus Fly Trap, but still amazingly swift--for a plant. It takes about one hour for the insect to be fully embraced in a deadly kiss. Pressed firmly against the enzymes on the surface of the leaf, the insect innards are liquified. Digestion may take several days. Afterward the hairs return to their original positions. For a time the hairs stay dry and the remains of the day are soon gone with the wind.
The biggest species of Sundew is South Africa's Drosera Regina with leaves up to more than 22 inches long. In Australia a sundew of similar size makes a banquet of frogs and lizards. Crocodile Dundee types boast of finding sundews growing in clumps that feast on the occasional rabbit or squirrel.
Darwin was the master and commander of evolutionary thought, but nevertheless was humbled by the sundew. He wrote to a scientist friend that "I care more about Drosera than the origin of all the species in the world." Its not surprising that the ingenuity of carnivorous plants continues to seduce and ensnare our fascination. We are willing victims.