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If you were a jackrabbit hopping through the desert, you'd be glad to stumble -- well, maybe not stumble -- across a cactus:
假设你是只蹦跳着穿越沙漠的野兔，你会开心地碰到--好吧，也许不该碰到--一棵仙人掌。
the succulent flesh of these plants is a water source for many desert animals.
这些多汁的多肉植物是许多沙漠动物的水分来源。
Native to the Americas and known for their spines and succulent stems,
原产于美洲大陆，因刺和多汁的茎而闻名，
cacti of all shapes and sizes have evolved to not just survive, but thrive, in some of the harshest desert climates on Earth.
各种形状及大小的仙人掌，在地球上一些最恶劣的沙漠气候中，不止进化得可以生存，还得以繁茂。
So how do they do it? A cactus's spines are one key to its survival -- but not for the reason you might think.
它们是怎么做到的呢？一颗仙人掌的刺是它生存的关键之一，但不是因为你可能想到的理由。
Take a look at the prickly pear. Its spines are highly modified leaves.
我们来看看胭脂仙人掌。它的刺是高度变化过的叶子。
A normal leaf's large surface area would be ill-suited to the desert, transpiring massive amounts of water under the baking sun.
一片普通树叶大大的表面积不适用于沙漠中，大量的水分在烈日下会蒸发出去。
The dramatically reduced surface area of the spines limits water loss.
仙人掌刺极大减小的表面积限制着水分流失。
They also shade the cactus and reflect the sun's rays, reducing the plant's core temperature during the heat of the day.
它们同时为仙人掌制造阴影并反射太阳光，以此在炎热的白天里降低仙人掌的核心温度。
Then, at night, when air temperatures plummet, the spines act as an insulating layer, keeping the cactus from cooling down too much.
然后，在夜里，当气温骤降时，刺又作为保温层，来防止仙人掌过度冷却。
These functions are just as important, if not more, than defending against predators.
这些功能和预防天敌比起来一样，或者说更加重要。
From Cuba to Mexico, and as far south as Brazil and Peru,
从古巴到墨西哥，再南至巴西和秘鲁，
Melon cacti grow on limestone soils in seasonally dry tropical forests, where they're constantly exposed to the beating sun.
甜瓜仙人掌长在季节性干旱热带森林里的石灰岩土中，在那儿它们一直被暴露在烈日下。
They rely on another adaptation common to cacti: a thick skin, which is coated in a waxy substance called a cuticle that limits water loss.
它们依赖着在其它仙人掌中又一普遍的适应特性：一层厚厚的外皮，它被包裹在一层防止水分丢失、被叫做表皮层的蜡状物质中。
Meanwhile, the stomata -- tiny holes that allow the exchange of gases that enable photosynthesis -- remain firmly closed until night when they open.
与此同时，气孔--允许促成光合作用的气体交换的小孔--在夜晚开启前紧紧地保持关闭。
The lower temperatures at night mean the cactus loses less water from the stem when the stomata open.
夜里气温越低，意味着气孔打开时仙人掌通过茎丢失的水分越少。
The bulk of the plant acts as a large barrel of water, storing it for times of need.
仙人掌的主体就和一只大水桶一样，把水存起来应急。
But to survive the desert, a cactus can't just limit water loss
但要在沙漠中生存，仙人掌不能只是限制水分丢失，
it has to be prepared to take full advantage of the rare situations where water is readily available.
它还必须准备好充分利用少有的水分充足的情况。
In North America's Sonoran Desert, the towering Saguaro cactus can grow up to 20 meters tall and live for up to 200 years.
在北美洲的索诺兰沙漠，巨大的树形仙人掌能长到20米高并活到200岁。
Woody tissue, like the kind found in tree trunks, give the Saguaro its height, but the Saguaro survives with way less water than most trees.
木质组织，就像在树干里见到的一样，成就了树形仙人掌的身高，但它们依靠着比大多树少得多的水生存着。
Most of its roots are only a few inches deep. Just below the soil's surface, they spread out laterally for meters and hold the plant in place.
树形仙人掌大部分的根只有几英寸深。在土壤的表面下，这些根茎散开来好几米并将树形仙人掌固定住。
Even its single deepest root, the taproot, extends less than one meter into the ground.
就连它最深的根茎，直根，往地面下延伸也不会超过一米。
After a rain, the lateral roots respond in real time, rapidly growing and spreading.
在雨后，它的侧根即时作出反应，快速地生长并发散。
They produce ephemeral rain roots that quickly take up the available water.
这些侧根长出能快速吸收水分的短生雨根。
The water is then pulled up into the plant body and stored in cells that contain mucilage,
然后水被快速地吸入主体内并被贮存在带有黏质的细胞里，
a gluey substance that clings to water molecules and stops them from evaporating if the plant's tissue is ever damaged and exposed.
黏质是一种像胶水一样的物质，它吸附在水分子上并防止它们在植物组织被破坏和暴露的情况下蒸发。
As the soil dries after the rain, the small rain roots also start to dry and wither away,
在雨后土壤变干的时候，细小的雨根也开始变干枯萎，
and the cactus awaits the next time it can take advantage of a shower.
而仙人掌也开始等待下一次利用降雨的机会。
Taken together, these features make cacti well-equipped to survive their environments, from the driest desert to... a tropical rainforest?
合起来，这些特质将仙人掌们变得适合在它们的环境中生存，从最干的沙漠到...一片热带雨林？
The mistletoe cactus can live on the branches of trees in the rainforest.
榭寄生仙人掌能够在雨林中树木的枝干上生活。
Though there's lots of water around, not much of it reaches the cactus here, and there's nowhere for its roots to go.
尽管周围有很多水分，但却没有多少能够触及这些仙人掌，而它们的根也无处可去。
So even here, the cactus survives using adaptations that long ago helped its ancestors survive the desert.
所以就算在这儿，这些仙人掌利用它们祖先沙漠生存进化出的特性在雨林中生存。
