地理作业杯具，稀树草原的金合欢树为什么是平顶的？（英语好的进）

Question

啊~地理的作业都快崩溃了。。。要是能回答以下问题我追加一百分都没关系。（后天要交了啊，麻烦高手速战速决)
全部是稀树草原的问题。。。。靠（savanna grassland)
a）why are the crowns of trees often flattened?
b)why are ong lines of low bushes common?
c)how do plants conserve moisture in the dry season?
d)what two features help a baobab tree keep its moisture through the dry season?
e)how might the flattened crowns of tree used by widlife?
f)describe and explain grass growth in the wet season
g)what happens to grass in the dry season
h)how dose a baobab tree collect water in the dry season?

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以上的问题变态了，要是有高手又闲得慌的帮帮忙吧，我都快SHI了
PS：英语看不懂用翻译器
全部给我回答了我就给您磕头了，追加N倍分

Answer 1

全部翻译好了，你照着找吧。
来源：网易博客
http://blog.163.com/hklovelyboy@126/blog/static/2773341220073258577551/

Acacias - Thorn Trees of Africa 金合欢 - 非洲荆棘树
There are few more striking symbols of Africa than a thorn tree - its gnarled branches, graceful form, jagged thorns and abundant blooms, in many ways reflecting the paradoxes of the continent.

很少能有比荆棘树更能代表非洲的了 - 粗糙的树枝，优美的树形，参差的刺针以及绚烂的花朵，从各个方面体现了这个大陆的种种矛盾。

Acacias are the dominant woody plants over much of sub-Saharan Africa, with the exception of forests, grassland and mountain heathland. There are about 700 species in Africa, South America, India and Australia. All belong to the Family Mimosaceae (part of the legume group) and some were previously known as mimosa trees. Although just about all acacias have abundant yellow or white flowers, they are not widely cultivated due to their thorny branches. Several Australian species have, however, become invasive weeds in parts of Africa, where their seeds have no parasites or predators, and can lay dormant for up to 50 years.

在非洲撒哈拉以南除热带雨林，草原和高山灌木林外的地区中，金合欢是木本植物中的优势种。在非洲，南美，印度和澳大利亚分布着700多种金合欢。它们全部属于金合欢属（豆科的一部分），其中一些以前被称为mimosa trees（合欢的另一个英文名字）。尽管几乎所有的金合欢都会开满树的黄色或白色花朵，但是却因为它们多刺的枝干而没有被广泛种植。现在一些澳洲品种已经在非洲的一些地区成为了入侵物种，在这些地区它们的种子没有天敌，并且还能休眠长达50年的时间。

Adapted for the Hot and Dry

对干热气候的适应

African savannahs experience hot and often dry conditions, so plants must find ways of retaining moisture. The way in which acacias have evolved to do this, is to have their leaves divided into dozens of tiny leaflets (pinnae) which can be held horizontally to capture sunlight or vertically to reduce transpiration. Leaves with a large surface area are prone to dry out in the absence of shade or moisture, so are not suitable for savannah trees. Acacia foliage is sought after by many browsing animals - from giraffe to beetles - and here too it helps for them to be divided, as the tiny leaflets left behind by the browsers can continue to perform their vital photosynthesis function.

非洲稀树草原的气候酷热且常年干旱，因此植物必须使用各种方法来保存水分。金合欢所使用的方法是将叶片分成许多小叶（羽片），这样既可以展开以吸收更多的阳光，又可以合上以减少水分蒸发。过大的叶面在缺少遮蔽或水分时会导致植株很快脱水，因此不适合稀树草原气候。非洲的动物 - 从长颈鹿到甲虫 - 都喜欢啃食树叶， 这也是导致金合欢采用复叶的原因，当一些小叶被吃掉后，剩下的小叶还能继续进行重要的光合作用。

Many acacias have exaggerated umbrella-shaped crowns which enable the tree to capture the maximum amount of sunlight, with the smallest possible leaves. Some species have deep tap root systems, allowing them to survive in semi-arid terrain along dry watercourses (they tap into the water table) while others have shallow but extensive splayed root systems to capture limited rainfall in dryer areas. The well-known Fever Tree which grows along lake fringes and on poorly-drained clay soils also has a shallow root system, in order to avoid waterlogging.

许多金合欢都长着夸张的伞形树冠，这样植株就能以最少的树叶来获得最多的阳光。一些种类具备了发达的垂直根系，能够在干涸河床周围的半干旱地带中生长（它们的根能从地下水位吸收水分），而另一些则有着浅而宽的水平根系，能在更干旱的地区接收有限的雨水。著名的金鸡纳树生长在湖边排水不良的粘土地带，它同样有着很浅的根系，以免受水涝的危害。

Improving the Soil

改良土壤

Acacias belong to the group of pod-bearing plants known as legumes and have the remarkable ability to enhance soil fertility. The activity of microbes associated with roots of acacias (and other legumes such as Lucerne) fixes nitrogen to soil particles and so improves fertility. Experiments have also shown that soil fertility is higher below the canopies of acacia trees, due to leaf-fall and decomposition, and the accumulation of droppings from roosting birds and from herbivores seeking shade.

金合欢能结豆荚，属于豆科植物，能够显著的改善土壤肥力。金合欢（和苜蓿等其他豆科植物）根部附生的细菌能将氮固定到土壤中，以此来提升肥力。实验证明金合欢树冠下的区域土壤肥力较周围更高，这是因为落叶分解及在此栖息或寻找遮蔽的鸟兽的粪便所致。

Food for the Masses, but Varied Defences

大众食物的自卫方法

Just about all parts of an acacia tree are edible to one kind of animal or another, and in many parts of Africa their small leaves provide the only greenery in the dry season, so are bound to be eaten. The roots may be sought after by porcupines, the tiny leaflets by macro- and micro-herbivores, the sticky sap by galagos, the flower nectar by bees and butterflies, the seeds by bruchid beetles, and the stringy bark by elephant. Along with grasses, acacias are the "building blocks" of African savannahs - propping up a colossal food pyramid with super predators at the apex.

金合欢身上的几乎每一个部分对于动物们来说都是可食用的，在非洲的许多地方，它们的树叶是干旱季节里唯一可见的绿叶，因此注定会被当作食物。它们的各个部分都受到动物们的追捧 - 豪猪挖掘树根，食草动物嚼食树叶，丛猴摄取树汁，蜜蜂蝴蝶吸食花蜜，象鼻虫喜欢种子，而大象则中意树皮。金合欢和草是非洲稀树草原的基石，它们一起支撑起了巨大的以超级食肉动物为顶端的食物链金字塔。

With so many creatures making a living from their growth efforts, it is no wonder that acacias have evolved some of the most effective and cunning defences in the plant kingdom. Acacia thorns may be straight or hooked depending upon the species (only the Umbrella Thorn has both) and these are capable of keeping all but the largest browsers at bay. Interestingly, straight thorns have been shown to pose little deterrent to impala, while the much smaller hooked thorns succeed in keeping them at bay. Giraffe, however, have leathery tongues which feel no apparent pain against either hooked or straight thorns, and millions of browsing insects and their larvae can simply crawl around the thorns to feed on the leaves. To combat large mammalian browsers some acacia trees - such as the Knob Thorn - release chemicals in the form of toxic tannins which render the leaflets unpalatable and cause giraffe or kudu to move on to another tree. To reduce the impact of browsing insects, some acacia species have developed an association with biting ants - ant-plant mutualism - which is one of the most intriguing relationships in the natural world.

有这么多动物靠金合欢为生，这也就难怪它们能发展出植物王国中最巧妙有效的防御系统。金合欢的刺因品种不同而分为直刺和勾刺（只有Umbrella Thorn平顶树*1同时拥有这两种刺），这足以将稍小的造访者拒之门外。有趣的是，直刺只能对黑斑羚造成很小的威胁，反而是那些小得多勾刺对它们起到了阻挡作用。而是长颈鹿则长有皮质的舌头，因此不论是直刺还是勾刺都不能对它造成伤害，同样数以百万计的昆虫和它们的幼虫则能够轻松绕开那些刺而吃到树叶。为了同大型哺乳动物作斗争，一些荆棘树如Knob Thorn黑叶木兰/黑相思*2能够释放有毒的丹宁类化学物质使叶片味道变得难以食用，使得长颈鹿和大羚羊不得不转而食用其他树木。为了降低昆虫的影响，一些种类的金合欢与蚂蚁建立起了联盟，这是自然界最有趣的联盟 - 蚁树共生。

Ants and Acacias

蚂蚁与金合欢

Among the most important insect browsers of acacia trees are beetles, grasshoppers and the larvae (caterpillars) of butterflies and moths. In a masterful strategy to ward off these micro-herbivores, some acacias provide food and shelter, in return for defence, to cocktail ants. The ants live inside the swollen bases of thorns (Whistling Thorn is perhaps the best-known example) and feed on sweet sap exuded from pores at the tips of the acacia leaves. In order to maintain such ideal living and foraging quarters, the ants patrol the branches and will gang up and physically bite any herbivore that threatens the plant. Insects quickly retreat, and even giraffe and rhino have been seen to become irritated and move off with the unwanted attentions of aggressive ants. Amazingly, the ants also nip off the growth tips and tendrils of strangling vines which threaten to suffocate the tree - and thus their home. Acacias still rely on insects for cross-pollination, however, so it remarkable to note that the patrolling cocktail ants are least active at the hottest time of day - when browsers are seeking shade but bees are out looking for nectar; in this way, they do not disturb the pollen-gathering bees from their important task!

金合欢的所有访客中甲虫，蝗虫以及蝴蝶和蛾的幼虫（毛虫）占大多数。为了避开这些微型食草动物，一些金合欢采用了一种巧妙的策略，它们为cocktail ants举尾蚁/举腹蚁*3提供食宿，以换取它们的保护。蚂蚁居住在基部胀大的刺中（Whistling Thorn合欢荆棘树*4也许是最著名的例子）并以金合欢叶尖分泌的甜汁为食。为了保护如此理想的居住和觅食区域，蚂蚁们会在枝干上巡逻并对任何威胁树木的动物发起攻击。昆虫很快就撤退了，甚至还有人看到连长颈鹿和犀牛都因忍受不了好斗的蚂蚁而选择离开。更令人惊讶的是，如果其它藤蔓会令树木衰弱甚至窒息，蚂蚁就会切断它们的须叶以保护自己的家园。不过金合欢依然需要昆虫来进行授粉，值得注意的是在一天中最热的时候举腹蚁最不活跃，其他昆虫也在乘凉，而此时蜜蜂会外出采蜜，如此一来，蚂蚁就不会打扰到蜜蜂进行重要的传粉工作了。

Valuable Fuel

有用的燃料

The wood of many acacia trees is renowned for its excellent fuel properties and can also produce good charcoal. Since many African people still rely on wood as fuel for cooking and heating it is essential that acacia woodlands are managed effectively in order that the trees do not disappear altogether. In areas that have been overgrazed, bush-encroachment by acacia and the related Dichrostachys (as well as other species) is commonplace and restorative land management can harvest this resource as attempts are made to restore savannah conditions (i.e. trees and grass). Acacia sap or gum is another important product and the Sweet Thorn and Scented Thorn produce copious amounts which can be used as a glue. "Gum Arabic" was originally derived from the Three-hook Thorn and synthetic alternatives are now used as a binding compound in cosmetics, medicines, fast foods and paints.

许多种类的金合欢木材都是很好的燃料，并能制作上好的木炭。至今在非洲人们仍然依靠木头来做饭取暖，因此，有效地管理金合欢树林而不让它们全部消失掉是非常重要的。在那些过度放牧的地区，金合欢与Dichrostachys代儿茶属植物*5（同样还有其它种类的植物）等灌木占据这里是很平常的，而采用恢复性的土地管理措施可以充分利用这一资源，同时这也是恢复稀树草原环境（比如树和草等）的一种尝试。金合欢树汁也是一种非常重要的产品

Further Reading

扩展阅读
Guide to the Acacias of South Africa by Nico Smit. (1999) Briza Publications, Pretoria.

'Ants, pollinators and acacias in Mkomazi' by Willmer, Stone and Mafunda (In: Mkomazi - the Ecology, Biodiversity and Conservation of a Tanzanian Savanna.) Edited by Coe, McWilliam, Stone & Packer (1999) published by the Royal Geographic Society, London.

Posted: Plants, Date: 21 November 2006

*1 species种
umbrella thorn平顶树
Acacia tortilis (Forsskal) Hayne（拉丁）

kingdom Plantae - plants 植物界/ divisio Magnoliophyta - Flowering Plants 被子植物门 / class Rosopsida - Eudicots 双子叶植物纲/ order Fabales 蚕豆目/ family Fabaceae 豆科

*2 species种
knobthorn黑相思（英）
Acacia nigrescens Oliv. 黑相思（拉丁）

kingdom Plantae - plants 植物界/ divisio Magnoliophyta - Flowering Plants 被子植物门 / class Rosopsida - Eudicots 双子叶植物纲/ order Fabales 蚕豆目/ family Fabaceae 豆科

*3 Cocktail Ants 举腹蚁 （英）Crematogaster 举腹蚁（拉丁）

genus属
Crematogaster举腹蚁属

kingdom Animalia - animals 动物界/ phylum Arthropoda - Arthropods 节肢动物门/ class Insecta - Insects 昆虫纲/ order Hymenoptera - Bees, Ants and Wasps 膜翅目/ family Formicidae 蚁科

*4 species种

Whistling Thorn合欢荆棘树
Acacia drepanolobium Harms ex B.Y.Sjöstedt

kingdom Plantae - plants 植物界/ divisio Magnoliophyta - Flowering Plants 被子植物门 / class Rosopsida - Eudicots 双子叶植物纲/ order Fabales 蚕豆目/ family Fabaceae 豆科

参考资料：http://blog.163.com/hklovelyboy@126/blog/static/2773341220073258577551/