The Cell-Theory: a Restatement, History, and Critique Part III
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*57 The Cell-theory: A Restatement, History, and Critique Part III. The Cell as a Morphological Unit By JOHN R. BAKER (From the Department of Zoology and Comparative Anatomy, Oxford) SUMMARY A long time elapsed after the discovery of cells before they came to be generally regarded as morphological units. As a first step it was necessary to show that the cell-walls of plants were double and that cells could therefore be separated. The earliest advances in this direction were made by Treviranus (1805) and Link (1807). The idea of a cell was very imperfect, however, so long as attention was con- centrated on its wall. The first person who stated clearly that the cell-wall is not a necessary constituent was Leydig (1857). Subsequently the cell came to be regarded as a naked mass of protoplasm with a nucleus, and to this unit the name of protoplast was given. The true nature of the limiting membrane of the protoplast was discovered by Overton (1895). The plasmodesmata or connective strands that sometimes connect cells were prob- ably first seen by Hartig, in sieve-plates (1837). They are best regarded from the point of view of their functions in particular cases. They do not provide evidence for the view that the whole of a multicellular organism is basically a protoplasmic unit. Two or more nuclei in a continuous mass of protoplasm appear to have been seen for the first time in 1802, by Bauer. That an organism may consist wholly of a syn- cytium was discovered in i860, in the Mycetozoa. The syncytial nature of the Siphonales was not revealed until 1879. The existence of syncytia constitutes an exception to the cell-theory. No wholly syncytial plant or animal reaches a high degree of organization. Natural polyploidy was discovered by Boveri (1887), who was also the first to produce it experimentally (1903). Although many organisms contain some polyploid constituents and others are polyploid throughout their somatic tissues, yet diploid and haploid protoplasts (haplocytes and diplocytes) are the primary components of plants and animals and are still retained as such by most organisms. The haplocyte is more evidently unitary than the diplocyte. Haplocytes and diplocytes are not composed of lesser homologous units, and with the necessary reservations required by the existence of syncytial and polyploid masses of protoplasm, they may therefore be said to be the fundamental morphological units of organisms. [Quarterly Journal of Microscopical Science, Vol. 93, part 2, pp. 157-90. June 1952.] 158 Baker—The Cell-theory: CONTENTS PAGE INTRODUCTION 138 EARLY RESEARCHES BEARING ON THE MORPHOLOGICAL SEPARATENESS OF CELLS . .158 THE DISCOVERY OF THE CELL-MEMBRANE . .163 CONNECTIVE STRANDS BETWEEN CELLS . ... 168 SYNCYTIA 177 POLYPLOIDY ............. 183 THE INDIVISIBILITY OF CELLS INTO LESSER HOMOLOGOUS UNITS . .186 COMMENT ............. 186 ACKNOWLEDGEMENTS . .186 REFERENCES 187 INTRODUCTION N the first of this series of papers (1948) the cell-theory was restated in I the form of seven propositions. The second of these was as follows: Cells have certain definable characters. These characters show that cells (a) are all of essentially the same nature and (b) are units of structure. Part II of the series (1949) was devoted to the statement labelled (a) in this proposition; that is to say, to the fundamental similarity of all cells as revealed by the discovery of protoplasm and the nucleus. We are now con- cerned with the part of the proposition labelled (b); that is, with the idea of the cell as a morphological unit. The idea of the cell as a functional unit cannot escape mention here, but this aspect of our problem will be more fully considered under the heading of Proposition V. EARLY RESEARCHES BEARING ON THE MORPHOLOGICAL SEPARATENESS OF CELLS The cellular nature of plants is much more obvious than that of animals, and the earliest cytological observations were naturally made mostly on plants. Since it was the cell-wall and not the cell itself that called attention to the existence of cellular structure, attention was concentrated on the wall as a matter of course. The wall that separates two cells appears single on super- ficial examination, and the early observers were not inclined to regard cell- walls as separate boxes enclosing material within. The idea of a cell as a morphological unit only originated when it was found that the wall separating two cells was in some cases demonstrably double. The history of this advance will now be related. It has been mentioned in Part I that Grew described the parenchyma of plants as 'nothing else but a Mass of Bubbles' (1672, p. 79). In his later work he formed a wrong impression and made his well-known comparison with lace. This false simile had a profound influence that remains with us today in such erroneous names as tissue (and its counterparts in other languages) and histology. We are all accustomed today to speaking of the 'tissues' of the body and may tend to forget that until relatively recent times this word meant A Restatement, History, and Critique 159 nothing else than a textile fabric woven of threads. Until late in the seven- teenth century no one could have conceived how such a name could be applied to a part of the body of an organism, and indeed the word was not used in this sense in literary English until about 120 years ago. It is strange to reflect that the modern English usage derives indirectly from a fallacy about the microscopical structure of plants published by Grew in 1682. FIG. I. Part of Grew's drawing of a piece of a branch of the sumach, highly magnified. The supposed fibres, corresponding to the threads of lace, are well shown. (Enlarged from Grew, 1682, plate 40.) Grew considered that all the parenchymatous parts of a plant, including its fruit, consisted of 'Threds or Fibres', variously woven together. In a well- known passage (1682, pp. 121-2) he compares these fibres to the threads of lace. The comparison is actually to lace while it is being made in a horizontal piece upon a cushion. The pins, inserted vertically into the latter, are imagined to be hollow, and thus represent the vessels of the wood. The lace is supposed to be made in many thousands of layers, one on top of another. The holes between the threads represent the cavities of the cells; and it is understandable that the layers of threads could be added in such a way as to make closed vesicles instead of holes. 'And this', remarks Grew, 'is the true Texture of a Plant: and the general composure, not only of a Branch, but of all other Parts from the Seed to the Seed.' Of Grew's many figures, the one that illustrates best the ideas just expressed is his representation in perspective of a piece of a branch of the sumach, highly magnified. This figure is here reproduced as fig. 1. It shows clearly his belief in the essentially fibrous nature of plant parenchyma. It follows 160 Baker—The Cell-theory: from what Grew writes and also from this illustration, that if there is a unit in plant tissue, that unit is a fibre and not a cell; for the latter is merely a space left here and there by the intertwining of the fibres. For well over a century after the time of Grew, attention continued to be focused mainly on the cell-wall rather than on the contents of the cell. The belief that the wall consisted of microscopically-visible fibres did not persist (though the nomenclature based on that mistake was retained); but another kind of error began to be generally accepted. It was thought that the system of cell-walls was a continuous substance throughout the whole of a plant. Spaces (cells and vessels) were supposed to appear in this continuous sub- stance ; nourishment was thought to flow through them to the really essential constituent, the continuous membrane or cell-wall. The cells were often regarded as freely open to one another. According to this belief in its extreme form, a plant would consist of two substances, a membranous meshwork and the nourishing fluid filling its meshes; each would be completely continuous. There would indeed be cells, or enlarged meshes; but there would be no unit of structure. One of the most obstinate adherents to this view was Brisseau-Mirbel, who wrote (1808, pp. 14 and 128): 'The first idea, the fundamental idea is that all vegetable organization is formed by one and the same membranous tissue, variously modified. This fact is the base of all the others. The contrary idea is a source of errors. Plants are composed of cells, all the parts of which are continuous among themselves; they present only one and the same membranous tissue.' In very early times the contrary opinion began to appear, though the development of two opposing schools of thought came slowly, and more than a century was to elapse after the publication of Grew's Anatomy of Plants before anyone began to think clearly in terms of the cell as a unit. Grew's contemporary, Malpighi, nevertheless inclined towards what may be called the utricular view: he did not regard the cell as merely a space between interlacing fibres. This appears, for in- stance, in the account of the microscopical structure of the petals of the tulip and other plants, in his Anatome plantarum (1675, pp. 46-47). He remarks that when the IB surface is torn, the outflowing material contains micro- scopical objects resembling icicles in shape, which had dra^ngrfaroW been entangled loosely together in the intact petal.