Because of their small size, cells can only be observed with the aid of a microscope, an instrument that provides a magnified images of a tiny object. We do not know when humans first discovered the remarkable ability of curved-glass surfaces to bend light and form images. Spectacles were first made in Europe in the thirteenth century. By the mid-1600s, a handful of pioneering scientist had used their handmade microscope to uncover a world that would never have been revealed by the naked eye. The discovery of cells (Figure 1.1) is generally credited to Robert Hooke, an English microscopist who, at the age 27, was awarded the position of the Royal Society, England's foremost scientific academy. One of the many questions Hooke attempted to answer was why stoppers made of cork (part of the bark of trees) were so well suited to holding air in a bottle. As he wrote in 1665: "I took a good clear piece of cork, and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and . . . then examining it with a Microscope, me though I could perceive it to appear a little porous . . . much like a Honeycomb." Hooke called the pores cells because they reminded him of the cells inhabited by monks living in a monastery. In actual fact, Hooke had observe the empty cell walls of dead plant tissues, walls that had originally been produced by the living cells they surrounded. Meanwhile, Anton van Leeuwenhoek, a Dutchman who earned a living selling clothes and buttons, was spending his spare time grinding lenses and constructing simple microscope of remarkable quality.
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For 50 years, Leeuwenhoek sent letters to the Royal Society of London describing his microscopic observations-along with a rambling discourse on his daily habit and the state of his health. Leeuwenhoek was the first to examine a drop of pond water under the microscope and, to his amazement, observe the teeming microscope "animacules" that darted back and forth before his eyes. He was also the first to describe various forms of bacteria, which he obtained form water in which pepper had been soaked and from scraping of his teeth. His initial letters to the Royal Society describing this previously unseen world were met such skepticism that the society dispatched its curator, Robert Hooke, to confirm the observation. Hooke did just that, and Leeuwenhoek was soon a worldwide celebrity, receiving visits in Holland from Peter the Great of Russia and Queen of England. It wasn't until the 1830s that the widespread importance of cells was realized. In 1838, Matthias Schleiden, a German lawyer turned botanist, concluded that, despite differences in the structure of various tissues, plants were made of cells and that the embryo arose from a single cell. In 1838, Theodor Schwann, a German zoologist and colleague of Schleiden's published comprehensive report on the cellular basis of animal life. Schwann concluded that the cells of plants and animals are similar structures and proposed these two tenets of the cell theory:
a. All organism are composed of one or more cells.
b. The cell is the structural unit of life.
Schleiden and Schwann's ideas on the origin of cells proved to be less insightful; both agreed that cells could arise from non-cellular materials. Given the prominence that these two scientist held in the scientific world, it took a number of years before observation by other biologists were accepted as demonstrating that cells did not arise in this manner any more than organism arose by spontaneous generation. By 1855, Rudolf Virchow, a German pathologist, had made a convincing case for the third tenet of the cell theory:
c. Cells can arise only by division from a preexisting cells.
a. All organism are composed of one or more cells.
b. The cell is the structural unit of life.
Schleiden and Schwann's ideas on the origin of cells proved to be less insightful; both agreed that cells could arise from non-cellular materials. Given the prominence that these two scientist held in the scientific world, it took a number of years before observation by other biologists were accepted as demonstrating that cells did not arise in this manner any more than organism arose by spontaneous generation. By 1855, Rudolf Virchow, a German pathologist, had made a convincing case for the third tenet of the cell theory:
c. Cells can arise only by division from a preexisting cells.