plants in the deserts
some cacti, like the saguaro, grow to tree size, but true trees need more moisture than mostdesert environments can supply, so they are scarce on deserts. close to streambeds,cottonwoods can sometimes be found. though these streams are dry most of the year, waterflows there longest and is usually available fairly close to the surface. elsewhere, trees mustsend taproots deep into the hard baked desert soil to draw on underground water. perhaps themost widespread family of trees on the world's deserts is the acacia, whose taproots drilldown as far as 25 feet (7. 5 meters). the mesquite common on north american deserts in bothtree and shrub forms,does not begin to grow above ground until its root system is completelydeveloped, ensuring the plant a supply of moisture. the roots of shrubs and trees help to holdthe desert soil in place. their stalks and branches also act as screens to keep the wind fromsweeping great drifts of sand along the surface. these services are vital if a desert is tosupport life. scientists estimate that a desert needs year round plant cover over 20 to 40percent of its surface. if shrubs are too far apart-separated by a distance greater than fivetimes their height - soil around them is likely to blow away. without the shelter of establishedshrubs, new seedlings will have difficulty in getting a start.
on the other hand, plants that are too close together may compete for underground moisture.to protect themselves from this competition some shrubs give off a substance that killsyoung plants that sprout too close to them. in addition to a few varieties of trees and toughshrubs, most deserts have grasses, herbs, and other annual plants. these do not compete formoisture with the longer lived growth. they spring up quickly after rains, when the surface ismoist. then, for a brief time, the desert can be literally carpeted with color. almost as quickly asthey appeared,these small plants die away. but they have developed special ways of ensuringthe life of another generation when rains come again.
一些仙人掌，如撒瓜罗，能长到象树那么高。 但真正的树却需要比大多数沙漠所能提供的更多的水份，所以树在沙漠里是鲜见的。在小溪河床附近，有时能发现三角叶杨。 尽管一年的大多数时间里这些小溪都是干涸的，那里却是水流得时间最长的地方而且水份相当靠近地表。 其它地方树木的主根必须深入受炙烤而坚硬的沙漠底部的土壤以吸取地下水。 在沙漠里分布最广的树或许是刺魏，其主根能深达25 英尺(合7 5 米)。 牧豆树属植物不论是乔木和灌木，在北美沙漠中常常可见，在它根部系统完全生长发达到能保证提供充足的水分时才长出地面。 灌木和树的根有助于固定沙漠中的土壤，它们的茎和树枝同时起屏障的作用，防止风从沙漠表面吹起大堆的沙。 如果沙漠要支持生命，这种作用必不可少。 科学家估计一年之中植物必须覆盖沙漠表面的 20%到 40%。 如果灌木间隔太远--大于它们高度的5 倍--周围的土壤就可能被吹走。 如果没有这些灌木作为保护，新的种子很难生长。 另一方面，植物相互靠得太近却会引起竞争地下水分。
为避免竞争，一些灌木会释放一种物质来杀死那些距他们太近发芽的植物。 除了一些种类的树木和顽强的灌木外，大多数沙漠里还有青草、草本植物和其它年生植物。 它们并不与长期生长的树木竞争水份。当雨后地表还潮湿时它们就迅速发芽，然后在一个短时间里，给沙漠铺上绿色地毯。这些小植物很快就消失了，几乎就象它们长出来时那样迅速，但它们已发展了特殊方式来保证在下次降雨来时下一代的生命。
scattered through the seas of the world are billions of tons of small plants and animals calledplankton. most of these plants and animals are too small for the human eye to see. they driftabout lazily with the currents, providing a basic food for many larger animals.
plankton has been described as the equivalent of the grasses that grow on the dry landcontinents, and the comparison is an appropriate one. in potential food value, however,plankton far outweighs that of the land grasses. one scientist has estimated that while grassesof the world produce about 49 billion tons of valuable carbohydrates each year, the sea'splankton generates more than twice as much.
despite its enormous food potential, little effect was made until recently to farm plankton aswe farm grasses on land.now marine scientists have at last begun to study this possibility,especially as the sea's resources loom even more important as a means of feeding anexpanding world population.
no one yet has seriously suggested that "planktonburgers" may soon become popular aroundthe world. as a possible farmed supplementary food source,however,plankton is gainingconsiderable interest among marine scientists.
one type of plankton that seems to have great harvest possibilities is a tiny shrimplike creaturecalled krill. growing to two or three inches long, krill provide the major food for the great bluewhale, the largest animal ever inhabit the earth. realizing that this whale may grow to 100feet and weigh 150 tons at maturity, it is not surprising that each one devours more than one
ton of krill daily.
数十亿吨的被称为"浮游生物"的小动物、植物散布在世界的海洋中。 这些小的动、植物大多太小而难以被人眼看到。 它们随波逐流，为许多较大的动物提供了基本的食物。 浮游生物曾被描述为生长在大陆陆地上的各种草类的海洋对应物。 这种比喻是恰当的。 然而就潜在的食物价值而言，浮游生物远胜于草类。 一位科学家曾经估计，世界上的草类每年生产大约490 亿吨有用的碳水化合物，而海洋里的浮游生物每年生产的碳水化合物
多于此数的两倍。 尽管浮游生物具备巨大的食物潜能，但直到最近人们还很少象种植草类那样付出努力养殖浮游生物。 现在，海洋科学家们至少已开始研究这种可能性。 全球人口不断扩张，海洋资源作为食品的重要性日益突出。 现在还没有人认真说过"浮游生物汉堡" 会很快在世界上流行起来。 然而，作为一种可能养殖的补充性食物资源，浮游生物正引起了海洋科学家们相当大的兴趣。 一种似乎具有很大收获可能性的微小的虾状浮游生物被称为鳞虾。鳞虾长至2～3 英寸长时即成为地球上曾居住过的最大动物--蓝鲸的主要食物。
成熟的蓝鲸可以达到 100 英尺长，150 吨重，所以每头鲸每天吞食 1 吨多的鳞虾一点也不让人吃惊。
the ancestry of the piano can be traced to the early keyboard instruments of the fifteenth andsixteenth centuries -- the spinet, the dulcimer, and the virginal. in the seventeenth century theorgan, the clavichord, and the harpsichord became the chief instruments of the keyboardgroup, a supremacy they maintained until the piano supplanted them at the end of theeighteenth century. the clavichord's tone was metallic and never powerful; nevertheless,because of the variety of tone possible to it, many composers found the clavichord asympathetic instrument for intimate chamber music.the harpsichord with its bright,vigorous tone was the favorite instrument for supporting the bass of the small orchestra ofthe period and for concert use, but the character of the tone could not be varied save bymechanical or structural devices.
the piano was perfected in the early eighteenth century by a harpsichord maker in italy(thoughmusicologists point out several previous instances of the instrument). this instrument wascalled a piano e forte (soft and loud), to indicate its dynamic versatility; its strings werestruck by a recoiling hammer with a felt-padded head. the wires were much heavier in theearlier instruments. a series of mechanical improvements continuing well into the nineteenthcentury, including the introduction of pedals to sustain tone or to soften it, the perfection ofa metal frame, and steel wire of the finest quality, finally produced an instrument capable ofmyriad tonal effects from the most delicate harmonies to an almost orchestral fullness ofsound, from a liquid, singing tone to a sharp, percussive brilliance.
钢琴的家系可以追溯至 15 至 16 世纪早期的键盘乐器，包括小型拨弦琴、洋琴和维金娜琴。 17 世纪时风琴、敲弦琴和拨弦琴成为键盘乐器类的主要成员。 这种至高无上的地位一直为它们所保持，直到 18 世纪末期钢琴将它们取代。 敲弦古钢琴的音调有金属的音质，缺乏雄劲。 然而由于它的音调变化多，许多作曲家发现对于亲切的室内乐是一种得体的乐器。 人们最喜欢用具备明快有力音调的拨弦古钢琴来配合当时小型管弦乐团的低音乐器以及在演奏会上演奏。 但它的音调难以变化，除非使用机械或构件装置。18 世纪早期的意大利，钢琴在一位拨琴钢琴制造者手中得到完善(尽管音乐理论家们指出有更早的例子) 。 这种乐器被称为pianoeforte (意大利语，柔和而响亮的)，以显示它有力的多样性。 演奏者用一个头部带皮毡的弹击乐锤敲击琴弦。 更早的这种乐器之上的金属丝要重得多。
从此，持续到 19 世纪的一系列机械上的改进，包括引入踏板以维持音调或使其柔和，改善金属框架，以及使用最佳性能的钢丝，最终产生了一种具备无数音调效果的乐器。 这些效果涵盖了从最精致的和声到几乎全部的管弦乐音响，从明快流畅的吟唱的音调到尖锐的打击乐器的清晰动人的恢宏气势。