教甄◆英文科題庫下載題庫

        Although the fossil record holds few clues to the evolution of cells, recent advances in biochemistry and molecular biology have provided powerful new means of reconstructing the past by probing the present. Hardly 300 years have elapsed since the day when a living cell was first glimpsed by the human eye. Throughout that period, every milestone about cell discovery bears the name of a new tool or instrument.

        The world of cells remained entirely unknown and unexplored until the middle of thecentury, when individuals of prying minds served by skilled hands started grinding lenses and using them to extend their power of vision. One of the first designers of microscopes was the English scientist Robert Hooke—physicist, meteorologist, biologist, engineer, architect—a most remarkable product of his time. In 1665, he published a popular collection called Micrographia； among the beautiful drawings of his observations was one of a thin slice of cork showing a honeycomb structure, an array of what he called “microscopic pores” or “cells.” In his description of it, Hooke used the word “cell” in its original meaning of small chamber, as in the cell of a prisoner or a monk. The word has remained, not to describe the little holes that Hooke saw in dead bark, but rather to designate the little blobs of matter that are the inmates of the holes in the living tree. 

       One of Hooke’s most gifted contemporaries was the Netherlander Antonie van Leeuwenhoek, who made almost three hundred microscopes of a very distinct design--- a small bead of glass inserted in a copper plate. By holding this contraption close to his eye and peering through the glass bead at an object held of a needle he manipulated with a screw. Leeuwenhoek succeeded in obtaining magnification 270 times that of the naked eye. He was able to see for the first time what he called  “animalcules” in blood, sperm and the water of marshes and ponds. Amazingly, he even saw bacteria, which he drew so accurately that specialists can identify them today. 

       Not all early users of microscopes were as perceptive. The images they were able to observe with their simple instruments—especially when it came to objects as small as living cells—were so blurred that most details had to be filled in by the imagination. Many showed admirable restraint in the use of this faculty. Others took full advantage of it, as did the French scientist Gautier D’Agoty, who believed that a fully formed baby existed within a sperm cell.

        For a long time, microscopy did little more than hover around the world of cells until, in 1827, the Italian physicist Giovanni Battista Amici succeeded in correcting the major optical aberrations Section 2 How to Prepare for Academic Reading and Listening of lenses. Through three pairs of matched lenses that could deflect light without separating it into colors, the sharpness of the images was dramatically increased； so much so that only a few years later the generalized theory was formulated that plants and animals are made of one or more similar units—cells. 

       This theory was proposed for plants in 1837 by the German botanist Mathias Schleiden and was extended to animals by his friend, the physiologist Theodor Schwann. The theory was subsequently completed by the pathologist Rudolf Virchow, when he proclaimed in 1855: “ Every cell arises from a cell,” an altered version of “ Every living being arises from an egg.” The latter was an assertion made by William Harvey, the English physician who discovered blood circulation and who had died shortly before Robert Hooke’s discovery. By the turn of the century, a number of important cell parts had been described and named. 

       Later investigators found themselves confronting a new obstacle, seemingly insurmountable, as it was set by the very laws of physics. Even with a perfect instrument, no detail smaller than about half the wavelength of the light used can be perceived, which puts the absolute limit of resolution of a microscope utilizing visible light at .25 millionth of a meter. In the world of cells, such a dimension is quite large, relatively speaking. Just think of what we would miss in our own world if no detail smaller than inches could be distinguished, and what classical microscopists would have seen had they been able to magnify the living cell a millionfold.