NARRATOR:Listen to part of a lecture in a marine biology class.
FEMALE PROFESSOR:OK, today we're going to continue our discussion of plant life in coastal salt marshes of North America.Salt marshes are among the least inviting environments for plants.The water is salty, there's little shade, and the ocean tide comes in and out, constantly flooding the marsh... so the variety of plants found in salt marshes is limited, but there is a plant genus that thrives there- the spartina.
In fact, the spartina genus is the dominant plant found in salt marshes.You can find one type of spartina, salt marsh cordgrass, growing in low marsh areas.In higher marsh areas you're likely to find a spartina commonly called salt meadow hay.So how is the spartina able to survive in an environment that would kill most plants?
Well, it's because salt marsh grasses have found ways to adapt to the conditions there.First of all, they're able to withstand highly saline conditions.One really interesting adaptation is the ability to reverse the process of osmosis.
Typically the process of osmosis works... well ...When water moves through the wall of a plant cell, it will move from the side containing water with the lowest amount of salt into the side containing the highest amount of salt.So, imagine what would happen if a typical plant suddenly found itself in salt water.The water contained in the plant's cells- that's water with very little salt- would be drawn out toward the seawater-water with a lot of salt.So you can see, the freshwater contained in the plant would be removed and the plant would quickly lose all its water and dehydrate.
But, what about the spartinas?Well, they allow a certain amount of salt to enter their cells, bringing the salt content of the water within the plant to a slightly higher concentration than that of the surrounding seawater.So instead of freshwater moving out of the plant's cells, salt from the seawater enters,-reverse osmosis- and this actually strengthens the cells.
Another adaptation to the salty environment is the ability to excrete excess salt back into the environment.That's why you might see a spartina shimmering in the sunlight.What's reflecting the light is not salt from seawater that has evaporated- although that's a good guess. But it's actually the salt that came from within the plant.Pretty cool, huh?You can really impress your friends and family with that little tidbit the next time you're in a salt marsh.But coping with salt is not the only challenge for plants in a salt marsh. Soil there is dense and very low in oxygen. So spartinas have air tubes.Air enters through tiny openings on the leaves.The tubes provide a direct pipeline for the oxygen, carrying it down the leaves, through the stems and into the roots, where it's needed.If you pull up a spartina, you might even notice some reddish mud on some of the roots.This is caused by oxygen reacting with iron sulfide in the soil and it produces iron oxide, or rust.
Now although the spartinas have adapted several chemical and physical mechanisms that allow them to thrive in salt water and to feed oxygen to their roots, there's yet another aspect of the harsh environment that they've had to adapt to: the force of the tides and occasional violent storms.Wind and water are constantly crashing into these plants.
So as you might have guessed, they've developed a means of solidly anchoring themselves into the soil.How? Well, they have tough sort of ... uh ...underground stems called rhizomes.
Rhizomes from one plant grow through the muddy soil and interlock with those of other nearby plants.The plants form a kind of colony, a community that will survive or perish together, because alone, as single plants, they cannot survive.
Of course, the plants in these colonies also need tough, resilient stems above the soil, stems that can bend a lot but not break as water constantly crashes into them.So in addition to the interlocking underground rhizomes, they have yet another adaptation and it's, well,we're back to reverse osmosis again.By adjusting the osmotic pressure so that its cells are always fully inflated the plant is able to withstand great pressure before snapping.
So spartinas may look like simple marsh grass, but they're really a wonder of chemistry, physics and structural engineering that allows them to survive, and even thrive, in an environment in which most other plants would wilt and die within hours.
OK, today we’re going to continue our discussion of plant life in coastal salt marshes of North America. ..so the variety of plants found in salt marshes is limited, but there is a plant genus that thrives there— the spartina.
文章开头说salt marshes很少有植物能生长，spartina却在这儿活的很好，下端句尾So引出全文想要讨论的：Spartina能在salt marshes中生活的原因，接下来看到标志词First of all，another等等，说明要分条详细阐述了。