PROTOPLASM, the name given in modern biology to a substance composing, wholly or in part, all living cells, tissues or organisms of any kind, and hence regarded as the primary living substance, the physical and material basis of life. The term “protoplasm,” from πρῶτος, first, and πλάσμα, formed substance, was coined by the botanist Hugo von Mohl, in 1846, for the “tough, slimy, granular, semi-fluid” constituent of plant cells, which he distinguished from the cell-wall, nucleus and cell-sap. This was not, however, the first recognition of the true living substance as such, since this step had been achieved in 1835 by the French naturalist F. Dujardin, who in his studies on Foraminifera had proposed the term “sarcode” for the living material of their bodies in the following words: “Je propose de nommer ainsi ce que d'autres observateurs ont appelé une gelée vivante, cette substance glutineuse, diaphane, insoluble dans l'eau, se contractant en masses globuleuses, s'attachant aux aiguilles de dissection, et se laissant étirer comme du mucus, enfin se trouvant dans tous les animaux inférieurs interposée aux autres éléments de structure.” To the French naturalist belongs, therefore, the real credit of the discovery of protoplasm, or rather, to be more accurate, of the first recognition of its true nature as the material basis of vital phenomena. Neither Dujardin nor von Mohl, however, had any conception of the universal occurrence and fundamental similarity of protoplasm in all living things, whether animal or vegetable, and it was not till 1861 that the identity of animal sarcode and vegetable protoplasm was proclaimed by Max Schultze, whose name stands out as the framer, if not the founder, of the modern notions concerning the nature of the living substance. From this time onwards the term “protoplasm” was used for the living substance of all classes of organisms, although it would have been more in accordance with the custom of priority in nomenclature to have made use of Dujardin's term “sarcode.”
A living organism, of any kind whatsoever, may be regarded as composed of (1) protoplasm, (2) substances or structures produced by the protoplasm, either by differentiation or modification of the protoplasm itself, or by the excretory or secretory activity of the living substance. The protoplasm of a given organism may be in a single individual mass, or may be aggregated into a number of masses or units, discontinuous but not disconnected, termed cells (see Cytology). Thus living organisms may be distinguished, in a general way, as unicellular or multicellular. An instance of a unicellular organism is well seen in an Amoeba, or in one of the Foraminifera, classic examples for the study of undifferentiated protoplasm, which here composes the greater part of the body, while products of the formative activity of the protoplasm are seen in the external shell and in various internal granules and structures. As an example of a multicellular organism we may take the human body, built up of an immense number of living cells which produce, singly or in co-operation, a variety of substances and structures, each contributing to the functions of the body. This, without attempting to enter into details, the horny epidermis covering the body, the hairs, nails, teeth, skeleton, connective tissue, &c., are all of them products formed by the metabolic activity of the living substance and existing in intimate connexion with it, though not themselves to be regarded as living. In addition to metabolic products of this kind, special modifications of the living substance itself are connected with specializations or exaggerations, as it were, of a particular vital function; such are the contractile substance of muscular tissue, and the various mechanisms seen in nervous and sensory tissue. It is necessary, therefore, in a living body of any kind, to distinguish clearly between simple protoplasm, its differentiations and its products.
Protoplasm from whatever source, whether studied in a cell of the human body, in an Amoeba or Foraminifer, or in a vegetable organism, is essentially uniform and similar in appearance and properties. Its appearance, graphically described by Dujardin in the passage quoted above, is that of a greyish, viscid, slimy, semi-transparent and semi-fluid substance. Its properties are those of living things generally, and the most salient and obvious manifestation of life is the power of automatic movement exhibited by living protoplasm. When free and not limited by firm envelopes, the movements take the character known generally as amoeboid, well shown in the common Amoeba or in the white corpuscles of the blood. When confined by rigid envelopes, as in plant-cells, the protoplasm exhibits streaming movements of various kinds. Even more essentially characteristic of the living matter than the power of movement is the property of metabolism—that is to say, the capacity of assimilating substances different from itself, of building them up into its own substance (anabolism), and of again decomposing these complex molecules into simpler ones (katabolism) with production of energy in the form of heat, movement and electrical phenomena. An important part of the metabolic process is respiration, i.e. the absorption of oxygen from the surrounding medium and oxidation of carbon atoms to form carbonic acid gas and other simple chemical compounds; in ordinary plant and animal protoplasm the process of respiration seems to be of universal occurrence, but some Bacteria constitute apparently an exception to the rule. Metabolism results not only in the generation of energy, but also, if anabolism be in excess of katabolism, in increase of bulk, and consequent growth and reproduction.
Living protoplasm is, therefore, considered from a chemical standpoint, in a state of continual flux and instability, and it follows that if protoplasm be a definite chemical substance or mixture of substances (see below), a given sample of protoplasm cannot be pure, or at least cannot remain so for any length of time so long as its power of metabolism is being exerted, but will contain particles either about to be built up by anabolism into its substance, or resulting from katabolic disintegration of its complex molecules. Hence it is convenient to distinguish the living substance from its metaplastic products of anabolism and katabolism. Such products are to be recognized invariably in protoplasm and take the form generally of granules and vacuoles. Granules vary in size from very minute to relatively large, coarse grains of matter, usually of a firm and solid nature. To the presence of innumerable granules is due the greyish, semi-transparent appearance of protoplasm, which in parts free from granules appears hyaline and transparent. Different samples of protoplasm may vary greatly in the number and coarseness of the granulations. Vacuoles are fluid drops of more watery consistence, which, when relatively small, assume a spherical form, as the result of surface tension acting upon a drop of fluid suspended in another fluid. When vacuoles are numerous and large, however, they may assume various forms from mutual pressure, like air-bubbles in a foam. A good example of frothy protoplasm, due to the presence of numerous vacuoles, is seen in the common “sun-animalcule” (Actinosphaerium). Or when the cell is confined by an envelope, and becomes very vacuolated, the vacuoles may become confluent to form a cell-sap contained in a protoplasmic lining or “primordial utricle,” and traversed by strands of protoplasm, as in the ordinary cells of plant-tissues. In many unicellular organisms, so-called contractile vacuoles are continually being formed as an act of excretion and expelled from the body when they reach a certain size.
While the majority of protoplasmic granules are probably to be regarded as metaplastic in nature, there is one class of granulation's of which this is certainly not true, namely the grains of chromatin, so named from their peculiar affinity for certain dyes, such as carmine, logwood and various aniline stains. These grains may occur as chromidia, scattered through the protoplasm, or they may be concentrated at one or more spots to form a definite nucleus or nuclei, which may or may not be limited from the remaining protoplasm by a definite membrane, and may undergo further differentiations of structure which cannot be considered further here (see Cytology). The protoplasm of an ordinary cell is thus specialized into nucleus and cytoplasm. It was formerly thought that the most primitive forms of life, the Monera of E. Haeckel, consisted of pure protoplasm without a nucleus. It must be borne in mind, however, that chromatin can be present without being concentrated to form a definite nucleus, and that with imperfect technique the chromatin may easily escape observation. It seems justifiable at present to believe, until the contrary has been proved, that all organisms, however primitive, contain chromatin in some form: first, because this substance has always been found when suitable methods for its detection have been employed; secondly, because it has been shown experimentally, by cutting up small organisms, such as Amoeba, that nucleated fragments of protoplasm are unable to maintain their continued existence as living bodies; and, thirdly, because modern research has shown the chromatin to be of very great, perhaps fundamental, importance in regulating the vital processes of the cell and so determining the specific characters of the organism, a property which enables the chromatin to act as the vehicle of heredity and to transmit the characters of parent to offspring. In the present state of our knowledge, therefore, the peculiar chromatin-granules must be regarded as an integral part, perhaps even the most essentially and primarily important portion, of the living substance. At the same time it must be borne in mind that the term “chromatin” does not denote a definite chemical substance, to be recognized universally by hard and fast chemical tests. The chromatin of different organisms or cells may behave quite differently in relation to stains or other reactions; and if it be true that it is the chromatin which determines the nature and activities of the cell, it follows that no two cells which differ from one another in any way can have their chromatin exactly similar. The conception of chromatin is one based upon its relations to the vital activities and life cycle, as a whole, of the organism or cell, and not upon any definable material, that is chemical and physical, properties.
The importance of protoplasm, as the physical and material basis of life, has caused it to be the subject in recent years of much minute and laborious research. It seems obvious that matter so peculiarly endowed must possess a complexity of structure and organization far exceeding that which at first sight meets the eye. Some biologists have attacked the problem of the ultimate constitution of protoplasm from a purely theoretical standpoint, and have framed hypotheses of an ultramicroscopic constitution sufficient, in their opinion, to explain, or at least to throw light upon, the vital activities of the living substance. Others, proceeding by more empirical methods, have attempted to lay bare the structure of protoplasm by means of the refinements of modern microscopical technique, or to solve the question of its constitution by means of chemical and physiological investigation. Hence a convenient distinction, not always easy, however, to maintain in practice, is drawn between speculative and empirical theories of protoplasm.
1. Speculative theories have come with the greatest frequency from those who have attempted to find a material explanation for the phenomena of heredity (q.v.). As instances may be mentioned more particularly the “gemmules” of Darwin, the “pangenes” of de Vries, the “plastidules” of Haeckel, and the “biophores” of Weismann. These theories have been ably brought together and discussed by Delage, who has included them all under the term “micromerism,” since they agree in the assumption that the living substance contains, or consists of, a vast number of excessively minute particles—i.e. aggregates or combinations of molecules, which give to the protoplasm its specific properties and tendencies (“idioplasm” of Nägeli). In other cases the assumption of invisible protoplasmic units has been inspired by a desire either to explain the general vital and assimilative powers of protoplasm, as, for example, the “micellae” of Nägeli and the “plasomes” of Wiesner, or to elucidate the mechanism of some one function, such as the “inotagmas” of Engelmann, assumed to be the agents of contractility. In general, it may be said of all these speculations either that they can only be extended to all vital phenomena by the help of so many subordinate hypotheses and assumptions that they become unworkable and unintelligible, or that they only carry the difficulties a step further back, and really explain nothing. Thus it is postulated for Wiesner's hypothetical plasomes that they possess the power of assimilation, growth and reproduction by division; in other words, that they are endowed with just those properties which constitute the unexplained mystery of living matter.
2. Empirical theories of protoplasm differ according as their authors seek to find one universal type of structure or constitution common to all conditions or differentiations of the living substance, or, on the contrary, are of opinion that it may vary fundamentally in different places or at different times. From these two points of view protoplasm may be regarded either as monomorphic or polymorphic (Fischer). The microscopical investigation of protoplasm reveals at the first glance a viscid, slimy or mucilaginous substance, in which is embedded an immense number of granules, for the most part very tiny. Very rarely are these granules absent, and then only from a portion of the protoplasm, and only temporarily. Hence many authorities have regarded the minute granules—the “microsomes” of Hanstein—as themselves the ultimate living units of protoplasm, in opposition to those who would regard them merely as “metaplastic” substances, i.e. as the heterogeneous byproducts of metabolism and vital activity. The granular theory, as this conception of the living substance is called, has received its extreme elaboration at the hands of Altmann, whose standpoint may be taken as typical of this class of theories. After demonstrating the universal occurrence of granules in protoplasm, Altmann has compared each individual granule to a free-living bacterium, and thus regards a cell as a colony of minute organisms, namely the granules or bio blasts, as he has termed them, living embedded in a common matrix, like a zoogloea colony of bacteria. Of this theory it may be remarked, firstly, that it brings us no nearer to an explanation of vital phenomena than do the plasomes of Wiesner; secondly, that to consider bacteria as equivalent, not to cells, but to cell granules, is to assume for this class of organisms a position with regard to the cell theory which is, to say the least, doubtful; and, thirdly, that the observations of the vast majority of competent microscopists furnish abundant support for the statement that granules of protoplasm do not lie free in a structureless matrix, but are embedded in the substance of a minute and delicate framework or morphoplasm, which in its turn is bathed by a watery fluid or enchylema permeating the whole substance. The upholders of the granular theory deny the existence of the framework, or explain it as due to an arrangement of the granules, or as optical effect produced by the matrix between the granules. Amongst those, on the other hand, who assert the existence of a framework distinct from granules and enchylema, the utmost diversity of opinion prevails with regard to the true structural relations of these three parts and the role played by each in the exercise of vital functions. Some have regarded the framework as made up of a tangle of separate fibrillae (filar theory)—a view more especially connected with the name of Flemming—but most are agreed that it represents the appearance of a reticulum or network with excessively fine meshes, usually from ½ to 1 μ in diameter. The reticulum carries the granules at its nodal points, and is bathed everywhere by the enchylema. Even with so much in common, however, opinions are still greatly at variance. In the first place, the majority of observers interpret the reticulum as the expression of an actual spongy framework, a network of minute fibrillae ramifying in all planes. While, however, Heitzmann, following the speculations of Brücke, considered the framework itself to be actively contractile and the seat of all protoplasmic movement, an opposite point of view is represented by the writings of Leydig, Schäfer and others, who regard the reticulum merely as a kind of supporting framework or spongioplasm, in which is lodged the enchylema or hyaloplasm, considered to be itself the primary motile and living substance. Bütschli, on the other hand, has pointed out the grave difficulties that attend the interpretation of the reticulum as a fibrillar framework, in view of the distinctly fluid consistence of, at any rate, most samples of protoplasm. For if the substance of the framework be assumed to be of a firm, solid nature, then the protoplasm as a whole could not behave as a fluid, any more than could a sponge soaked in water. On the other hand, the hypothesis of a fluid fibrillar framework leads to a physical impossibility, since one liquid cannot be permanently suspended in another in the form of a network. Bütschli therefore interprets the universally present reticulum as a meshwork of minute lamellae, forming a honeycombed or alveolar structure, similar to the arrangement of fluid lamellae in a fine foam or lather, in which the interstices are filled, not with air but with another fluid; in other words, the structure of protoplasm is that of an exceedingly fine emulsion of two liquids not miscible with one another.
It may be claimed for the alveolar theory of Bütschli that it throws light upon many known facts relating to protoplasm. It interprets the reticulum as the optical section of a minute foam-like structure and permits the formation of protoplasmic striations and of apparenf fibrillae as the result of linear or radiating dispositions of the alveolar framework; it reconciles with the laws of physics the combination of a framework with a fluid or semi-fluid aggregate condition, while variations in the fluidity of the framework are compatible with a stiffening of the protoplasm almost to the pitch of rigidity, as seen, for example, in nervous tissue; and, finally, it explains many characteristic structural peculiarities of protoplasm, such as the superficial layer of radiately arranged alveoli, the spherical form of vacuoles, the continuous wall or pellicle which limits both the vacuoles and the protoplasm as a whole, and many other points not intelligible on the theory of a sponge-like structure. Bütschli has succeeded, moreover, in producing artificial foams of minute structure, which not only mimic the appearance of protoplasm, but can be made to exhibit streaming and amoeboid movements very similar to those of simple protoplasmic organisms. Incidentally these experiments have shown that many of the apparent granulation’s and “microsomes” are an optical effect produced by the nodes of the minute framework. In his most recent works Bütschli has extended his theory of alveolar structure to many other substances, and has tried to prove that it is a universal characteristic of colloid bodies, a view strongly combated, however, by Fischer. While it cannot be claimed that Bütschli’s theory furnishes in any way a complete explanation of life, leaving untouched, as it does, the fundamental question of assimilation and metabolism, he at least draws attention to a very important class of facts, which, if demonstrated to be of universal occurrence, must be reckoned with in future treatment of the protoplasm question, and would form an indispensable preliminary to all speculations upon the mechanism of the living substance.
In opposition to the above-mentioned monomorphic theories of protoplasm, all of which agree in assuming the existence of some fundamental type of structure in all living substance, attempts have been made at various times to show that the structural appearances seen in protoplasm are in reality artificial products, due to precipitation or coagulation caused by reagents used in the study or preparation of living objects. These views have been developed by Fischer, who by experimenting upon various proteids with histological fixatives, has shown that it is possible to produce in them a granular, reticular or alveolar structure, according to treatment, and, further, that granules so produced may be differentially stained according to their size and absorptive powers. Fischer therefore suggests that many structural appearances seen in protoplasm may be purely artificial, but does not extend this view to all such structures, which would indeed be impossible, in view of the frequency with which reticular or alveolar structures have been observed during life. He suggests, however, that such structures may be temporary results of vital precipitation of proteids within the organism, and that protoplasm may have at different times a granular reticular or alveolar structure, or may be homogeneous. Fischer’s conception of living protoplasm is therefore that of a polymorphic substance, and a similar view is held at the present time by Flemming, Wilson and others. Strassburger also regards protoplasm as composed of two portions: a motile kinoplasm which is fibrillar, and a nutritive trophoplasm which is alveolar, in structure.
The chemical investigation of protoplasm labours at the outset under the disadvantage that it cannot deal with the living substance as a whole, since no analysis can be performed upon it without destroying the life. Protoplasm consists, to the extent of about 60% of its total mass, of a mixture of various nucleo-proteids—that is to say, of those substances which, in molecular structure and chemical composition, are the most complex bodies known. In association with them are always found varying amounts of fats, carbohydrates, and other bodies, and such compounds are always present in the living substance to a greater or less degree as products of both upward and downward metabolism. Protoplasm also contains a large but variable percentage of water, the amount of which present in any given case affects largely its fluid or viscid aggregate condition. Especial interest attaches to the remarkable class of bodies known as ferments or enzymes, which when prepared and isolated from the living body are capable of effecting in other substances chemical changes of a kind regarded as specifically vital. It is from their study, and from that of the complex proteids found in the living body, that the greatest advances towards an explanation of the properties of living matter may be expected at the present time.
The question may be raised how far it is probable that there is one universal living substance which could conceivably be isolated or prepared in a pure state, and which would then exhibit the phenomena characteristic of vital activity. It is sufficiently obvious, in the first place, that protoplasm, as we know it, exhibits infinite diversity of character, and that no two samples of protoplasm are absolutely similar in all respects. Chemical differences must be assumed to exist not only between the vital fabrics of allied species of organisms, but even between those of individuals of the same species. Kassowitz regards this variability as compatible with the assumption of a gigantic protoplasmic molecule in which endless variations arise by changes in the combinations of a vast number of atoms and atom complexes. It is difficult to conceive, however, of any single substance, however complex in its chemical constitution, which could perform all the functions of life. To postulate a universal living substance is to proceed along a path which leads inevitably to the assumption of biophores, plastidules or other similar units, since the ultimate living particles must then be imagined as endowed at the outset with many, if not all, of the fundamental properties and characteristic actions of living bodies. Such a conception has as its logical result a vitalistic standpoint, which may or may not embody the correct mental attitude with regard to the study of life, but which at any rate tends to check any further advance towards an explanation or analysis of elementary vital phenomena. We may rather, with Kölliker, Verworn and others, ascribe the activities of protoplasm to the mutual interaction of many substances, no single one of which can be considered as living in itself, but only in so far as it forms an indispensable constituent of a living body. From this point of view life is to be regarded, not as the property of a single definite substance, but as the expression of the ever-changing relations existing between the many substances which make up the complex and variable congeries known to us as protoplasm.
Authorities.—For exhaustive historical summaries of the protoplasm question, with full bibliographical references, the reader may be referred to the following works, especially the first five: Bütschli, Investigations on Microscopic Foams and Protoplasm (London, 1894); Untersuchungen über Strukturen (Leipzig, 1898); “Meine Ansicht über die Struktur des Protolplasmas und einige ihrer Kritiker,” Arch. f. Entwickelungsmechanik d. Org. (1901); xi. 499–584, pl. xx.; Delage, La Structure du protoplasm et les théories sur l’hérédité (Paris, 1895); Wilson, The Cell (2nd ed., London, 1900); Fischer, Fixirung, Färbung, und Bau des Protoplasmas (Leipzig, 1899); Kassowitz, Allgemeine Biologie (Vienna, 1899); G. Mann, Protoplasm, its Definition, Chemistry and Structure (Oxford, 1906), p. 59. (E. A. M.)