A. factors that relate to the size of the area in which it grows
B. the size of its population over the last few centuries
C. whether anything can be done to ensure its survival
D. why it did not change much over the last one hundred million years
NARRATOR:Listen to part of a lecture in a biology class.
FEMALE PROFESSOR:Ok, I have an interesting plant species to discuss with you today.Um... it's a species of a very rare tree that grows in Australia, Eidothea hardeniana, but it's better known as the Nightcap Oak.
Now, it was discovered only very recently, just a few years ago.Um... it remained hidden for so long because it's so rare, there are only about, oh, two hundred of ‘em in existence.They grow in a rain forest, in a mountain ridge, range in the north part of New South Wales which is er... a state in Australia.So just two hundred individual trees in all.
Now another interesting thing about the Nightcap Oak is that it is, it represents, er... a very old type, er... kind of tree that grew a hundred million years ago.Um... we found fossils that old that bear remarkable resemblance to the tree.So, it's a primitive tree, a... a living fossil you might say.It's relic from earlier times and it has survived all these years without much change.And it, it's probably a kind of tree from which other trees that grow in Australia today evolved.
Just—just to give you an idea of what we are talking about, here's a picture of the leaves of the tree and its flowers.I don't know how well you can see the flowers. they're those little clusters sitting at the base of the leaves.
Okay, what have we tried to find out about the tree since we've discovered it?Hum... or how, why is, is it so rare? That's one of the first questions.Um... how is it, um... how does it reproduce? This's another question. [inviting students to propose a theory]Um... maybe those two questions are actually related. [seeing hand raised] Jim?
MALE STUDENT:Hum... I don't know, but I can imagine that, for instance, seed dispersal might be a factor.I mean if the, er... you know, if the seeds cannot really disperse in the wild area, then, you know, the tree may not colonize new areas, it can't spread from the area where it's growing.
FEMALE PROFESSOR:Right. That's, that's actually a very good answer.Um... of course, you might think there might not be many areas where the tree could spread into, er... because, um... well, it's very specialized in terms of the habitat.But, that's not really the case here, uh... the suitable habitat, that is, the actual rain forest is much larger than the few hectares where the Nightcap Oak grows.
Now this tree is a flowering tree as I showed you, uh uh, it—it produces a fruit, much like a plum, on the inset inside there’s a seed with a hard shell.It, it appears that the shell has to crack open or break down somewhat to allow the seed to soak up water.If the Nightcap Oak remains, if their seeds remain locked inside their shell, they will not germinate.Now actually, the seeds, er... they don't retain the power to germinate for very long, maybe two years, so there's actually quite a short window of opportunity for the seed to germinate.So the shell somehow has to be broken down before this, um... germination ability expires.And, and then there's a kind of rat that likes to feed on the seeds as well.So, given all these limitations, not many seeds that the tree produces will actually germinate.So this is a possible explanation for why the tree does not spread.It doesn't necessarily explain how it became so rare, but it explains why it doesn't increase.
OK, so it seems to be the case that the species, this Nightcap Oak is not very good at spreading.However... it seems, though we can't be sure, that it's very good at persisting as a population.Um... we, there are some indications to suggest that the population of the Nightcap Oak has not declined over the last uh, y’know, many hundreds of years.So it's stayed quite stable; it—it’s not a remnant of some huge population that is dwindled in last few hundred years for some reason.It's not necessarily a species in retreat.Ok, so it cannot spread very well, but it's good at maintaining itself.It's rare, but it's not disappearing.
Ok, the next thing we might want to ask about the plant like that is what chances does it have to survive into the future.Let's look at that.
Hmm, I dunno, but uh I can imagine that…for instance…uh seed dispersal might be a factor I mean if the uh y’know if the seeds cannot really disperse in a wide area then you know the tree may not uh colonize new areas, it it can’t spread from the area where it’s growing.
Right, that’s that’s actually a very good answer.
Uh, uh we, uh, there—there’re some indications to suggest that the population of the Nightcap Oak has not declined over the last uh, y’know, many hundreds of years.
教授提到了两个问题，为什么这种树非常rare，这种树怎么繁殖，又说这两个问题是有关系的。紧接着学生可能seed dispersal可能是一个问题，教授接着这个答案讲下去，就提到了So this is a possible explanation for why the tree does not spread，对应A选项