Life at the Cell and Below-Cell Level. The Hidden History of a Fundamental Revolution in Biology
"Dr. Ling is one of the most inventive biochemist I have ever met."
How It Began on the Wrong Foot---Perhaps Inescapably
Around 1609 Zacharias Janssen and Galileo Galilei (1564-1642) independently invented the compound microscope.367 Through such a microscope, Robert Hooke (1635-1703)—described as "a Person of a prodigious inventive Head, so of great Virtue and Goodness"459, 3 p 295—saw tiny air-filled pores in a piece of cork and called them cells. In the same volume, "Micrographia," in which Hooke published his microscopic observations in 1665,368 he also used the same word, cells, to describe fluid-filled "pores" in the pith of carrot, fennel, fem and the like, believing that these "cells" represent avenues of communication in plants.
A decade later, Antony van Leeuwenhoek (1632-1723), seeking the cause of the hotness of pepper, discovered under a single-lens microscope, little animalcules (bacteria) in a watery extract of hot pepper.367 He did not know that each of these animalcules comprised just a single cell. For, among other reasons, it was long before the word, "cell," acquired the meaning it has today.
In fact, between the later part of the 17th century and the early 19th, different views emerged on what were called cells. von Haller,309 p 393 Grew3 p 180 and later Brisseau de Mirbel460 believed as Hooke himself had believed, that cells are fluid-filled cavities or spaces. Malpighi and Moldenhawer, on the other hand, contended that cells are closed sacs or utricles.3 p 180-184
When G. R. Treviranus demonstrated in 1805 that the membrane between two neighboring cells in the buds of a buttercup plant is in fact double and that the neighboring cells could be torn apart without damaging either one,461 the cells-as-entities theory gained ground. Treviranus's discovery undoubtedly accelerated the broad acceptance of cells as entities. Nonetheless, one hesitates to describe this event as a triumph of one view over another. It is possible that they were not dealing with the same object.462;463 One group might have focused on the cross-sections of what were later named xylem and phloem; the other group on true living cells as we now know them.
Between 1835 and 1840, two basic defining biological concepts were formally introduced: Theodor Schwann's Cell Theory,1;335 according to which cells are the basic units of all animals and plants and Felix Dujardin's sarcode,2 as an even more fundamental building block of life. Their historical importance notwithstanding, neither concept arose de novo at the time (see Chapter 2 and Figure 72).
From Robert Hooke on, mature plant cells were a favorite material in early microscopic studies of living matter. Their large size, their well-defined boundaries, their ready availability and their good keeping quality offered compelling reasons for their choice in cell studies. And with it, a hidden pitfall—as will be made clear presently.
Schwann's focal interest was on (mature) plant cells.1 Dujardin, on the other hand, studied the kind of Infusoria323 known as protozoa today. These unicellular organisms, when crushed, spilled out a gelatinous, water-immiscible material, which Dujardin described as gelée vivante (living jelly) and named sarcode.2
Schwann believed that the containing membrane of a cell (and that of its nucleus) is of overriding importance over (what he believed to be) a homogeneous, transparent liquid filling in the space (Zwischenraum) between the nuclear and cellular membrane.3 pp 193-194 True, a mature plant cell4 like that shown in Figure 1A resembles Schwann's notion of “one hollow cell inside a hollow cell.”3 p 193 However, young plant cells—like all animal cells—are solid bodies. About a decade after Dujardin announced his sarcode, two botanists, Karl von Nageli and Hugo von Mohl also described a viscous fluid in young plant cells, which von Mohl called protoplasm. 6,7 Following an extensive comparative study, Ferdinand Cohn concluded that the animal sarcode and the plant protoplasm are the same.8 Robert Remak then suggested that both be called protoplasm.9
Figure 1. A mature plant cell undergoing plasmo lysis. The central vacuole is marked as "vacuole" in A. In B. the shrunken cytoplasm, nucleus, etc. still enclosed in the plasma membrane makes up the "protoplast." (Glasstone13)
By the turn of the century, there was agreement among microanatomists on the solid nature of most living cells. Thus, in introducing his monumental treatise, "The Cell in Development and Heredity," American cytologist, E. B. Wilson pointed out in 1928 that the name, cell, is a misnomer, because cells "do not, in general, have the form of hollow chambers as the name suggests but are typically solid bodies." 12 p 4