『新东方在线托福听力』为托福考生准备了ETS托福TPO/Official 72 Lec2听力练习：What causes some sap to have a dark color?原文题目答案解析，TPO/Official 72 Lec2听力原文范文音频翻译。

托福TPO/Official 72 Lec2听力原文What causes some sap to have a dark color?【新东方在线】

旁白：请听一段植物学课堂中的讲座片段。

Narrator: Listen to part of the lecture in a botany class.

教授：好的，在课间休息时，有些同学提了些问题，我们聊天时我提到我家在马萨诸塞州有一个枫糖浆生意。

Professor: OK, during the break, some of you have some questions and when we were talking, I mentioned that my family owns a maple syrup business in Massachusetts.

有同学问我能不能讲讲这个过程，正如大卫所说的，来一堂应用植物学的小课。

And some of you ask if I talk a little bit about how that works, give you a brief lesson in applied botany as David put it.

枫树汁是从几种不同种类的枫树中采集的，这些枫树主要生长在美国东北部和加拿大东南部。

Well, maple sap is collected from several species of maple trees they grow primarily in the northeastern United States and south eastern Canada.

你知道的，你不会直接从枫树上获得可以浇在煎饼上的糖浆。

You know you don’t actually get syrup from a maple tree ready to put on your pancakes.

你得到的是树汁，需要经过加工才能变成糖浆。

What you get is sap that can be made into syrup.

首先，糖是通过光合作用生成的。

So first, sugar is created through photosynthesis.

然后糖会被转化为淀粉，通常储存在树的根部。

Then, it's converted into starch, and it's sent for storage usually in the roots of the tree.

随着春天的到来，这些淀粉会再次转化为糖。

Then as springtime approaches, the starch is converted back into sugar.

糖会溶解在一种水样液体中，我们称之为“树汁”。

The sugar then started to dissolve in its watery liquid that we called sap.

这些树汁通过树内的运输组织——韧皮部，在树体内流动。

And they are moved around to various places in the tree in vessels that are referred to as phloem.

韧皮部可以把糖分往下运输到根部，或者往上运输到叶子，也可以横向运输到树的另一侧。

So phloem can either move sugar down toward the roots or up toward the leaves or horizontally form one side of the tree to another.

春天，在合适的天气条件下，树汁就会开始在树体中上升。

Hmm during the spring in proper weather conditions the sap in the tree begins to rise

而要让树汁流动，天气必须刚刚好。

and for the sap to move throughout the tree, the weather conditions have to be just right.

也就是说，白天气温温和、在冰点以上，而夜晚仍然保持冰点以下。

When the days become mild, above freezing, and the nights are still freezing.

所以需要冷热交替的天气：白天暖和，晚上结冰。

So, their needs to be an alternate warm day and below freezing night.

我们现在还不能完全确定为什么这种天气会促进树汁流动。

We're still not totally sure exactly how this promotes sap movement.

嗯……最普遍的理论和枫树特有的结构有关。

Hmm… the most popular theory involves the unique structures found in Maple trees.

这些韧皮部周围是一些细胞，在大多数树种中，这些细胞是充满液体的，

Hmm… this phloem vessels are surrounded by other cells which in most species of trees are filled with fluid

但在枫树中，它们大多充满了气体。

but in the maple trees these are mostly filled with gas.

夜间天气寒冷时，这些气体细胞会收缩，导致细胞变小。

During the night when it's cold the gas need cells contracts, causing the cells to shrink.

这就会在树体内形成低压，而树根部的高压就会将树汁向上推送。

This creates a low pressure in the tree so the higher pressure in the roots pushes the sap up.

所以在夜里，树汁会流向树干和叶子，对吧？

So, during the night the sap moves up into the trunk and the leaves, OK?

这时候，树体内部的压力比树根处低。

The pressure is lower in the tree than it is in the roots.

接着白天气温升高，细胞中的气体膨胀。

Then during the day, as the temperature rises, the gas in the cells expand.

细胞变热后膨胀，导致树体内部压力增大。

The cells heat up, so they expand, creating greater pressure in the tree.

这时树体内的压力高于根部，使树汁向各个方向流动。

Now the pressure is higher in the tree moving this up in various directions.

对于采集树汁来说，最重要的一点就是树内存在这种压力。

The important thing for collecting the sap is since there's this pressure in the tree.

如果你在树干上钻一个孔，这叫做“插管”，树汁就会因压力自然流出。

If you put a hole into the tree, it's called tabbing, the sap will flow out of the tree due to this pressure.

你知道，在树上钻孔，这会伤到树吗？

You know, doesn’t that hurt the tree when you drill a holes in them?

其实被采集的树汁只是树木总产量的一小部分，而且那个小孔到第二年就会愈合。

Well, the sap you struck is only a small portion of the tree’s total production and the hall, it heals over by the next year.

一棵枫树通常需要大约 40 年才能开始插管，但如果管理得当，一棵健康的树可以连续生产树汁超过 100 年。

So, hmm…a maple tree does need to be about 40 years old before it can be tapped, but a good tree, if taking care of properly, can produce sap year after year for well over 100 years.

So, anyway, there's several species of maples.

其中最常用的是糖枫，其次是黑枫，这是最常见的两种采糖树种。

The most common species that used is the Sugar Maple, followed by the black maple are the most common types tapped.

还有一些其他枫树也可以使用，但它们产出的树汁质量和数量都不如前两种。

There are other maples that can be used with, but they don't produce the same quality or quantity of sap.

此外，也有一些非枫树在钻孔后会产生汁液，

And there are other trees other than Maples that may when drilled produce a sap

但由于其中溶解矿物质的组合不同，由它们产生的糖浆味道通常不太好。

but due to a combination of dissolved minerals the taste of the syrup derived from their sap is not that pleasant.

现在，要制作枫糖浆，需要大量的树汁，大概需要 125 到 250 升树汁。

Now, in order to make maple syrup, it takes a lot of sap, about 125 to 250 L of sap.

然后要将其浓缩提炼成大约 4 升糖浆。

It has to be reduced and concentrated down to about 4 L of syrup.

糖浆的颜色和风味取决于树汁中的矿物质含量。

The color and flavor of the syrup is determined by the minerals in the sap,

在采集季节的早期，树汁通常比较清澈；

and during the early part of the season, the sap usually runs much clearer,

但随着季节推进，树汁中溶解的矿物质越来越多，糖浆颜色也会变得更深。

but as we get later into the season, more and more dissolved minerals are found in the sap, and it tends to be darker.

采集者其实无法控制某一年的糖浆质量或产量，

Now, the harvester can't really control either the quality or the amount of syrup produced in any given year.

一旦天气变暖，比如夜晚不再结冰，或者树芽开始开放，

And once the weather warms up, you know once you don't have the freezing temps at night anymore, and or the bud start opening in the leaves.

这个时候，树汁就不再流动，采集季节也就结束了。

Well, at that point, the sap is no longer running, and the harvest is over.

当然，之后你还得把树汁加工成糖浆。

And then of course, you've got to convert that sap into syrup

The professor makes the point that cells in maple trees are mostly filled with gas. What does this point explain? Click on 2 answers.

What is the professor's attitude toward the drilling of holes in maple trees?

What does the professor imply about black maple and sugar maple trees?

What causes some sap to have a dark color?

What events signal the end of the sap season? Click on 2 answers.

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