加利福利亚的红杉可以高达100多米，居地球6万多的树种之冠。但是，这些巨树似乎也有它的极限：记录里，还没有能超过130米的。那么，到底是什么在阻止树木的生长，让它们不能无限止长下去呢？瓦伦丁·哈姆迪研究了为什么树的高度有极限。
Reaching heights of over 100 meters, Californian sequoias tower over Earth’s other estimated 60,000 tree species. Growing in the misty Sierra Nevada mountains, their massive trunks support the tallest known trees in the world. But even these behemoths seem to have their limits. No sequoia on record has been able to grow taller than 130 meters – and many researchers say these trees won’t beat that cap even if they live for thousands of years to come. So what exactly is stopping these trees from growing taller, forever? 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
高达100多米，加利福利亚红杉比地球上其它6万多的树种都要高。生长在雾蒙蒙的内华达山脉群山中，它们巨大的树干，支撑着世界上最高的已知树种。但是，即使这些庞然大物高度似乎也有极限。在现有记录上，没有红杉高过130米——很多研究者说，它们不可能超过这个高度即使它们再生长上千年。那么，到底是什么阻止它们的生长，不能一直长上去呢？文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
It all comes down to sap. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
这一切都跟树液有关。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
In order for trees to grow, they need to bring sugars obtained from photosynthesis and nutrients brought in through the root system to wherever growth is happening. And just like blood circulates in the human body, trees are designed to circulate two kinds of sap throughout their bodies – carrying all the substances a tree’s cells need to live. The first is phloem sap. Containing the sugars generated in leaves during photosynthesis, phloem sap is thick, like honey, and flows down the plant’s phloem tissue to distribute sugar throughout the tree. By the end of its journey, the phloem sap has thinned into a watery substance, pooling at the base of the tree. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
树木要生长，它们要把光合作用产生的糖和根茎系统吸收的营养送到生长的地方。就像血液在人体循环一样，树木本身需要有两种树液通体循环——带着树细胞生长需要的所有物质。第一种是韧皮树液。包含树叶光合作用产生的糖分，韧皮树液很稠，像蜂蜜，沿着植物的韧皮组织向下流动把糖份送到树的各个部分。这个旅程的最后，韧皮树液变成很稀，类似水一样的物质，聚集在树的底部。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
Right beside the phloem is the tree’s other tissue type: the xylem. This tissue is packed with nutrients and ions like calcium, potassium, and iron, which the tree has absorbed through its roots. Here at the tree’s base, there are more of these particles in one tissue than the other, so the water from the phloem sap is absorbed into the xylem to correct the balance. This process, called osmotic movement, creates nutrient-rich xylem sap, which will then travel up the trunk to spread those nutrients through the tree. But this journey faces a formidable obstacle: gravity. To accomplish this herculean task, the xylem relies on three forces: transpiration, capillary action, and root pressure. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
就在韧皮的旁边是树的另外一种组织叫木质。这种组织充满了营养素和像钙、钾、铁之类的离子，由树通过它的根部吸收。在树的根部，这些组织里的微粒分布不均匀，因此，韧皮树液中的水分被吸收到木质中，来调节平衡。这个过程，叫做渗透运动，会产生营养丰富的木质树液，这些树液会沿着树干向上把营养传送到整棵树。但是，这项任务面临着一个巨大的障碍：重力。要完成这样艰巨的任务，木质要依赖于三种力量：蒸腾作用，毛细作用和根压。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
As part of photosynthesis, leaves open and close pores called stomata. These openings allow oxygen and carbon dioxide in and out of the leaf, but they also create an opening through which water evaporates. This evaporation, called transpiration, creates negative pressure in the xylem, pulling watery xylem sap up the tree. This pull is aided by a fundamental property of water called capillary action. In narrow tubes, the attraction between water molecules and the adhesive forces between the water and its environment can beat out gravity. This capillary motion is in full effect in xylem filaments thinner than human hair. And where these two forces pull the sap, the osmotic movement at the tree’s base creates root pressure, pushing fresh xylem sap up the trunk. Together these forces launch sap to dizzying heights, distributing nutrients, and growing new leaves to photosynthesize – far above the tree’s roots. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
作为光合作用的一部分，树叶会打开和关闭它上面的细孔——叫气孔。这些气孔让氧气和二氧化碳从叶子进出，水分也通过这些气孔蒸发。这种蒸发，叫作蒸腾作用，在木质中产生一种负压，把液体的木质树液往树上方拉。这种拉力还得到水的一种基本特征——毛细作用的帮助。在微小的管道里，水分子间的吸引力和水与周边物质的粘结力加起来超过重力的作用。在木质纤维中毛细作用无处不在，这些木质纤维比人的头发还细。这两种力量推动树液向上，树根部的渗透作用也产生树压，把新鲜的木质树液推上树干。所有这些力量一起把树液推到惊人的高度，进行营养分配，促进新叶子生长进行光合作用——在远远高出树根的地方。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
But despite these sophisticated systems, every centimeter is a fight against gravity. As trees grow taller and taller, the supply of these vital fluids begins to dwindle. At a certain height, trees can no longer afford the lost water that evaporates during photosynthesis. And without the photosynthesis needed to support additional growth, the tree instead turns its resources towards existing branches. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
尽管有这么精密的系统，树木的每一厘米增长都要和重力做斗争。随着树越来越高，这种生命之液也越来越少。到了一定的高度，树再也提供不了光合作用所蒸发的水分。没有了再生长所需的光合作用，树就把它的营养送到现有的枝叶上了。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
This model, known as the “hydraulic limitation hypothesis,” is currently our best explanation for why trees have limited heights, even in perfect growing conditions. And using this model alongside growth rates and known needs for nutrients and photosynthesis, researchers have been able to propose height limits for specific species. So far these limits have held up – even the world’s tallest tree still falls about fifteen meters below the cap. Researchers are still investigating the possible explanations for this limit, and there may not be one universal reason why trees stop growing. But until we learn more, the height of trees is yet another way that gravity, literally, shapes life on Earth. 文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
这种理论，被叫做“液压限制假定”，是目前最好的解释，为什么树的高度有限，即使在最理想的生长环境中。利用这个理论，加上生长率以及已知营养及光合作用的需求量，研究人员已经能推断出一些树种的极限高度。到目前为止，这些极限都很准确——虽说世界上最高的树仍然低于极限15米。研究人员还在试图解释这些极限的原因，树木停止长高也许不会有统一的原因。在我们了解更多之前，树的高度应该是地球引力的又一种表现，基本上，它在界定地球生命的形态。文章源自：力哥爱英语（ienglish521.com）力哥爱英语-https://ienglish521.com/2990.html
