填空题

Seed Dispersal
A. They fly, float, hitchhike—and even explode. But the many clever ways seeds get around make sense: after all, a plant’s life depends on finding fertile ground in which to grow. The quest for survival has even led plants to develop delightful and devious ways of fooling us into working for them as they send their seeds out to conquer new lands. Birds help some fruits like the like the cherry by eating the fruit around the seed. The mistletoe’s seeds are covered with a sticky substance that substance that sticks to the bird that tries to eat it. The mistletoe’s seeds have to land on the or they die. Some seeds travel by sailing in the wind. Others hitchhike on animals. Some just get blown in the wind. Seeds can also grow in a flower and drop to the ground. When the seed leaves the flower of whatever it was in, it’s called dissemination. This is one of the most important steps in the seed’s life. Some tree seeds drop directly below the parent and die because there is not enough light or food for them to grow.
B. The seeds that travel by wind must be light-weight. Some of these seeds have wings, like the maple seed. Some of these seeds have gone 32 miles on a windy day. Another flying seed is the dandelion which gets planted because ants carry the seeds down into their hole.
C. Many plants use wind or water to carry their seeds, which are attached to fine, fluffy fibers that act as parachutes. Milkweed and thistle are among the plants whose seeds fly through the air. Trees such as willow, cottonwood, and sycamore, which grow along streams or creeks, drop many of their seeds into the water, where the fluff keeps them afloat on the current so it can carry them to new spots. Coconuts may drift for several months and travel for up to 1,200 miles (2,000 kin) before reaching dry land. The coconuts are able to float because of special fibers around their seeds.
D. The hitchhikers are built with spikes, such as: the squirting cucumber. Then they travel to another place to grow. Some plants have sacks that explode, bursts open and shoot its seeds up to 27 feet (8m) away from the parent plant. The seeds zoom off and may travel as fast as 62 miles (100kin) per hour. Then there are the leapers, seeds that bounce or jump away from the parent plant. Jewelweed and witch hazel have seed cases that are so constructed that when they dry out, the slightest touch causes the case to burst and project the mature seeds through the air. Black walnuts have spongy hulls so that they bounce when they hit the ground and roll away from the tree.
E. At first glance some seeds’ designs make plants seem downright intelligent. Take apples, for instance. These sweet fruits have evolved to be bright and shiny for good reason: they attract people and other animals. Drawn in by their effective advertising, we do the work of carrying apple seeds to new territory where the species can gain a toehold and expand. Indeed, we like apples so much that we’ve planted orchards especially for our favorite fruit. The practice has prompted some biologists to ask who really is the boss in this relationship, do the apple trees work for us—or do we work for them
F. Similar examples can be found throughout nature, from fig-eating bats that become unwitting cargo planes for fig seeds, to squirrels and woodpeckers that unknowingly help oak trees spread their acorns. An African melon that grows a gourd-shaped bladder of water deep underground. In the dry season, aardvarks sniff out the watery melons, digging deep to quench their thirst. In the process, however, the thirsty aardvarks also sip up a few pit-like seeds, which they later deposit inside fertilizing manure. It’s hard to say who gets the better end of the deal: the melon or the mammal.
G. Both plant and animal, of course, get something out of these mutually beneficial relationships. Apple trees, for instance, didn’t set out to fool people into picking their fruit. But somewhere along the line, certain apple trees ended up with a combination of genes that made their fruit a bit brighter or sweeter than all the other apples. Since we liked these apples so much, we began selectively planting the trees, and learned how to breed even sweeter varieties. In exchange for the tender, nutritious fruit, the trees get steady care and even protection from potential enemies, such as insects and browsing deer.
H. Evolutionary accidents may explain how other types of seeds developed, too. On the island of Mauritius, for instance, there once were trees that dropped their tasty fruits full of seeds to the ground. Then, a new bird arrived on the island. It loved the fruits, but the tree’s seeds couldn’t survive the trip through the bird’s stomach. As a result, the tree was in trouble, since fewer of its seeds were surviving. Then, perhaps through a random genetic mutation, one tree, produced fruit with tougher seeds that could survive being eaten by the birds. Given this significant advantage, the tougher calvaria soon began to thrive. Eventually, they crowded out their ancestors completely.
I. However evolution can sometimes produce a plant that is too reliant on a particular animal for survival. That’s exactly what happened on Mauritius. There, some biologists believe that lonely 300-year-old calvaria trees await a bird that will never return: the dodo. In 1598, Dutch explorers established a colony on Mauritius. In the search for food to eat and sell, the settlers plundered the island’s natural resources, killing giant turtles, lizards, and the huge, flightless dodo birds with abandon. When the settlers did in the dodo, however, they may have also put the death of the calvaria in motion. Some biologists believe the dodos ate the tree’s fruit, and that the trip through the bird’s stomach helped prepare the seeds for germination. But now that their partner in life is gone, only a few calvaria survive. They are silent reminders of a lost past, with their seed-bearing fruit littering the ground and inviting a feast that will never come.
Squirting cucumber with hooks attaches to the seed.