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Orchids are unique in having the most highly developed of all blossoms, in which the usual male and female reproductive organs are fused in a single structure called the column. The column is designed so that a single pollination will fertilize hundreds of thousands, and in some cases millions, of seeds, so microscopic and light they are easily carried by the breeze. Surrounding the column are three sepals and three petals, sometimes easily recognizable as such, often distorted into gorgeous, weird, but always functional shapes. The most noticeable of the petals is called the labellum, or lip. It is often dramatically marked as an unmistakable landing strip to attract the specific insect the orchid has chosen as its pollinator. To lure their pollinators from afar, orchids use appropriately intriguing shapes, colors, and scents. At least 50 different aromatic compounds have been analyzed in the orchid family, each blended to attract one, or at most a few, species of insects or birds. Some orchids even change their scents to interest different insects at different times. Once the right insect has been attracted, some orchids present
all sorts of one-way obstacle courses to make sure it
does not leave until pollen has been accurately placed
or removed. By such ingenious adaptations to specific
pollinators, orchids have avoided the hazards
of rampant crossbreeding in the wild, assuring
the survival of species as discrete identities.
At the same time they have made themselves irresistible
to collectors. One of the most important social developments that helped to make possible a shift in thinking about the role of public education was the effect of the baby boom of the 1950's and 1960's on the schools. In the 1920's, but especially in the Depression conditions of the 1930's, the United States experienced a declining birth rate -every thousand women aged fifteen to forty-four gave birth to about 118 live children in 1920, 89.2 in 1930, 75.8 in 1936, and 80 in 1940. With the growing prosperity brought on by the Second World War and the economic boom that followed it, young people married and established households earlier and began to raise larger families than had their predecessors during the Depression. Birth rates rose to 102 per thousand in 1946, 106.2 in 1950, and 118 in 1955. Although economics was probably the most important determinant, it is not the only explanation for the baby boom. The increased value placed on the idea of the family also helps to explain this rise in birth rates. The baby boomers began streaming into the first grade by the mid-1940's and became a flood by 1950. The public school system suddenly found itself overtaxed. While the number of schoolchildren rose because of wartime and postwar conditions, these same conditions made the schools even less prepared to cope with the flood. The wartime economy meant that few new schools were built between 1940 and 1945. Moreover, during the war and in the boom times that followed, large numbers of teachers left their profession for better-paying jobs elsewhere in the economy. Therefore, in the 1950's and 1960's, The baby boom hit an antiquated
and inadequate school system. Consequently, the
"custodial rhetoric" of the 1930's and
early 1940's no longer made sense; that is, keeping youths aged
sixteen and older out of the labor market by keeping them in
school could no longer be a high priority for an institution
unable to find space and staff to teach younger children aged
five to sixteen. With the baby boom, the focus of educators
and of laymen interested in education inevitably turned
toward the lower grades and back to basic academic skills and
discipline. The system no longer had much interest in
offering nontraditional, new, and extra services to older youths.
Nineteenth-century writers in the United
States, whether they wrote novels, short stories, poems, or
plays, were powerfully drawn to the railroad in its golden year.
In fact, writes responded to the railroads as soon as the first
were built in the 1830's. By the 1850's, the railroad was a
major presence in the life of the nation. Writers such
as Ralph Waldo Emerson and Henry David Thoreau saw the railroad
both as a boon to democracy and as an object of
suspicion. The railroad could be and was a despoiler
of nature; furthermore, in its manifestation of
speed and noise, it might be a despoiler of human nature as
well. By the 1850's and 1860's, there was a great distrust
among writer and intellectuals of the rapid industrialization
of which the railroad was a leading force. Deeply philosophical
historians such as Henry Adams lamented the role that
the new frenzy for business was playing in eroding
traditional values. A distrust of industry and business continued
among writers throughout the rest of the nineteenth century
and into the twentieth. By the 1820's in the United States, when steamboats were
common on western waters, these boats were mostly powered by
engines built in the West (Pittsburgh, Cincinnati, or Louisville),
and of a distinctive western design specially suited
to western needs. The first steam engines in practical
use in England and the United States were of low-pressure
design. This was the type first developed by James Watt,
then manufactured by the firm of Boulton and Watt, and
long the standard industrial engine. Steam was accumulated
in a large, double-acting vertical cylinder, but
the steam reached only a few pounds of pressure per square
inch. It was low-pressure engines of this type that were first
introduced into the United States by Robert Fulton. He imported
such a Boulton and Watt engine from England to run the Clermont.
But this type of engine was expensive and complicated,
requiring many precision-fitted moving parts. The engine that became standard on western steamboats was of
a different and novel design. It was the work primarily of an
unsung hero of American industrial progress, Oliver
Evans(1755-1819). The self-educated son of a Delaware
farmer. Evans early became obsessed by the possibilities
of mechanized production and steam power. As early as 1802 he
was using a stationary steam engine of high-pressure
design in his mill. Engines of this type were not unknown, but
before Evans they were generally considered impractical and
dangerous.Within a decade the high-pressure engine, the
new type, had become standard on western waters. Critics
ignorant of western conditions often attacked it as wasteful
and dangerous. But people who really knew the Ohio, the Missouri, and the
Mississippi insisted, with good reasons, that it was
the only engine for them. In shallow western rivers the
weight of vessel and engine was important; a heavy engine added
to the problem of navigation. The high-pressure engine was far
lighter in proportion to horsepower, and, with less than
half as many moving parts, was much easier and cheaper to repair.
The main advantages of low-pressure engines were safe
operation and economy of fuel consumption, neither of
which meant much in the West. Volcanic fire and glacial ice are natural enemies.
Eruptions at glaciated volcanoes typically destroy
ice fields, as they did in 1980 when 70 percent of Mount Saint
Helens ice cover was demolished. During long dormant
intervals, glaciers gain the upper hand cutting deeply
into volcanic cones and eventually reducing them to rubble.
Only rarely do these competing forces of heat
and cold operate in perfect balance to create a phenomenon
such as the steam caves at Mount Rainier National Park.
Located inside Rainier's two ice-filled summit craters,
these caves form a labyrinth of tunnels and vaulted
chambers about one and one-half miles in total length.
Their creation depends on an unusual combination of factors
that nature almost never brings together in one place. The cave-making
recipe calls for a steady emission of volcanic
gas and heat, a heavy annual snowfall at an elevation
high enough to keep it from melting during the summer, and a
bowl-shaped crater to hold the snow. Snow accumulating yearly in Rainier's summit craters is compacted
and compressed into a dense form of ice called firn,
a substance midway between ordinary ice and the denser
crystalline ice that makes up glaciers. Heat rising from
numerous openings (called fumaroles) along the
inner crater walls melts out chambers between the rocky walls
and the overlying ice pack. Circulating
currents of warm air then melt additional opening in the firn
ice, eventually connecting the individual chambers and, in the
larger of Rainier's two craters, forming a continuous
passageway that extends two- thirds of the way around
the crater's interior. To maintain the cave system, the elements of
fire under ice must remain in equilibrium. Enough snow
must fill the crater each year to replace that melted from below.
If too much volcanic heat is discharged, the crater's
ice pack will melt away entirely and the caves will vanish
along with the snow of yesteryear. If too little heat is produced,
the ice, replenished annually by winter snowstorms,
will expand, pushing against the enclosing crater walls and
smothering the present caverns in solid firn ice. 重点单词： |
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