Individuality in Organisms

The root system as a whole seems to exert an inhibiting influence on the development of roots in other parts of the plant. When the whole root system is removed or its metabolic activity inhibited, new roots usually develop from the basal region of the stem if external conditions permit their growth there; if not, they may appear higher up on the stem. The propagation of plants by cuttings depends on this ability to produce roots on the stem in the absence of the root system. In an experiment described by Goebel and represented diagrammatically in Fig. 92, a bean seedling was placed in nutritive solution, b, which was kept at low temperature, whereby the activity of the root system was largely

inhibited. A part of the stem was then surrounded with water, a, at ordinary temperature to provide the moisture necessary for the growth of roots, and roots arose on this region. Submerging part of the stem in water in this way does not result in the development of roots when the original root system is active. By inclosing a region of the stem in a chamber containing ether vapor, and thus anesthetizing but not killing it, McCallum was able to induce the formation of roots above the anesthetized region, as indicated in Fig. 93. In this experiment the original root system was present and uninjured, but the region above the anesthetized level was apparently cut off from its influence, and, the moisture being sufficient, new roots appeared near the basal end. These experiments with roots seem to indicate that not only does a relation of dominance and subordination exist between the different parts of a root system, but that the root system as a whole dominates the stem to a certain extent, so far as the production of roots is con-

FiGS. 92, 93. — Diagrammatic figures illustrating experiments on root production on the stems of seedlings; only lower parts of plants shown: Fig. 92, formation of roots on stem at a when this region is kept moist after inhibition of original root system, b, by low temperature (after Goebel); Fig. 93, formation of roots above a region of stem inclosed in narcotic atmosphere (after McCallum's description). cerned. If this dominance and the dominance of the stem-tip both result from metaboHc gradients, then there must be in plants possessing roots two metabolic gradients in opposite directions, the apical region of onv being in the stem-tip or tips, that of the other in tluroot-tip or tips.

Two gradients in opposite directions along the same axis cannot exist at the same time without interfering with and partially obliterating each other unless they have different paths of transmission or are of different metabolic character. Concerning the possibility of the simultaneous transmission of different metabolic changes in different directions in the same protoplasm we know nothing, and our knowledge of conducting paths in the plant does not go far beyond the fact that some part ot the vascular bundles seems to transmit some kind of change better than other tissues.

It is possible, however, that the influence of the root system on the stem as a whole may be different in character from the dominance of the main root-tip on lateral roots. This possibility is suggested by the fact that the range of dominance within the root system is rather short, even where the tissues are differentiated, while the apparent dominance of the root system as a whole over the stem and other parts of the plant is apparently unlimited in range or without relation to distance. The root system takes up water and nutritive salts and these are transported to other i)arts of the plant. It is conceivable that the inhibiting inOu- ence of the root system on the formation of roots in other parts of the plant may be rather a transportative than a transmissive correlation, and that the other parts give

rise to roots when this transportation falls below a certain minimum or when they are isolated from it in any way. This alternative is more nearly in accord with the views of most botanists, and it seems at present more satisfactory than the assumption of two opposed and overlapping gradients. If, however, this relation between root system and other parts is transportative rather than transmissive, McCallum's experiment described above of bringing about physiological isolation of the upper levels of the stem from the root system by local anesthesia seems to indicate that the transportation is not a simple physical process but is dependent in some way and to some extent upon the metabolic activity of living cells.

If we accept this alternative and admit at the same time the primary dominance of the stemtip or tips and the secondary dominance within the root system of the root-tip or tips we must regard the root system as a subordinate specialized constitutent individual of the composite plant individual. The root, like the leaf, is primarily determined by relations to other parts of the plant, but requires certain external conditions for its development and differentiation. Like the leaf also, the root or root system shows a certain degree of secondary individuation among its parts.

The formation of roots is the reaction of a plant individual to a certain relation between internal and external conditions, and this relation may apparently be brought about either by the inhibition of activity in, or absence of, the original root system, or in many cases by changes in the external conditions, such as decrease in light and increase in moisture, even though the original root system is present. The root of the plant, like the basal end of the animal body, is the morphological expression of the performance of a certain functional activity primarily subordinate to and dependent upon the activities of other parts. Without the activities of parts representing higher levels in the primary gradient, root formation does not occur, but when it has occurred the products of the special metabolic activity of roots transported to other parts affect the metabolic processes there and so inhibit more or less effectively the formation of roots there.

From this point of view the apparent dominance of the root system over other parts of the plant with respect to root formation is not a feature of the primary and fundamental relation of dominance and subordination in the individual, but rather a secondary relation — transportative rather than transmissive — unlike the primary relation, and resulting from local differentiation which is itself associated with and dependent upon the primary relation.

It is often possible to alter the localization of the new dominant region in the reconstitution of an isolated piece by altering the gradient relations of the piece. A few examples from the flatworm, Planar ia, among the animals and the liverwort, Marchantia, among the plants will illustrate the point. It has been pointed out (pp. 80, 81) that the outgrowth of new tissue on a piece of Planaria isolated by transverse planes of section is most rapid in the median

ventral region of the apical end, this region representing the region of highest metabolic rate or irritability resultant from the three main axial gradients. By altering the shape of the piece in relation to the axial gradients it is possible to alter the position of this outgrowth and so the position of the new head. In a piece cut very obliquely {abed, Fig. 94), the head develops as in Fig. 95, and the side of the head which arises from the more

Figs. 94-98. — Localization of head-formation in the reconstitution of pieces of Planaria as resultant of apico-basal and transverse axial gradients: Fig. 94, diagrammatic outline of part of body of Planaria, indicating shapes of pieces; Fig. 95, asymmetrical position of head in reconstitution of piece, abed; Fig. 96, reconstitution of piece, aehd; Fig. 97, reconstitution of piece, aegi; Fig. 98, reconstitution of piece, afi. apical level of the piece is likely to develop somewhat more rapidly than the other side. This asymmetry of position and development is due largely to the fact that one side of the cut surface represents a higher level in the major axial gradient than the other and so reacts more rapidly. When the cut surface is oblique, the major gradient becomes a factor in determining the position of most rapid dedifferentiation, division, and new development of cells, and this determines the

position of the new head. In a piece achd. Fig. 94, the head develops, as shown in Fig. 96, on the apical cut surface, but in a shorter piece aegi, Fig. 94, the head is likely to appear at an angle to the apical and median cut surfaces, as in Fig. 97. This condition results when the metaboHc rate of the cells on the median cut surface is as high as that of the cells on the apical cut surface, so that both take an equal part in giving rise to the new head. In pieces like afi, Fig. 94, the head often develops nearly or quite in the direction of the transverse axis (Fig. 98). In such pieces there is little difference in metabolic rate between apical and basal cut surfaces, and the cuts are not sufficiently oblique so that the higher level in the major gradient of the lateral as compared with the median region of the cut surface overbalances its lower level in the transverse gradient. Consequently the median regions of both cut surfaces represent the region of highest rate or irritability in such a piece and therefore become the head-forming region. For these and many other experimental modifications of the position of the head in reconstitution no satisfactory general basis of interpretation has heretofore been discovered, but I know of no case which cannot be very simply accounted for in terms of axial metabolic gradients.

In the bilaterally symmetrical liverwort MarcJiautui (Fig. 23, p. 78), the gradientrelations are apparently very similar to those in Planaria. In these plants practically every cell of the body is capable of giving rise to a new plant, but in pieces without the growing tip new growing tips originate in definite relations to the axes, and their presence inhibits the formation of others. In general, on transverse cut surfaces new individuals arise, like

the head in Planaria, in or near the median ventral region of the apical end of the piece just basal to the cut surface (Fig. 99). When the piece is taken from the lateral margin of the plant body and does not contain the median region, individuals usually arise near the apical end and ventrally on the most nearly median region of the piece (Fig. 100). In pieces with oblique instead of transverse apical cut surfaces the position of the new individual varies according as the piece contains part of the midrib or not, according to the obliquity of the plane of the cut, and probably also according to the region of the body. Where the piece does not include the midrib the new individual usually arises ventrally near the inost apical region of the piece, the major gradient being the chief factor in determining its position. Thus in Fig. loi the new plant appears near the lateral margin, undoubtedly because the metabolic level is higher here than elsewhere. The conditions here are apparently much like those which determine the asymmetrical position of the new head in Planaria in Fig. 95. In pieces which contain a part of the midrib this is usually the chief factor in determining the position of the new head. The piece in Fig. 102, for example, is cut from one side of the body and includes part of the midrib at the basal end of the oblique cut, and the new bud arises here. The influence of the midrib in localization in this form depends on the fact that the cells in this region retain their capacity for growth and division much longer than the cells of the lateral regions, and so they represent a relatively high metabolic level and bear much the same relation to the transverse gradient that the apical growing tip does to the major

gradient. Because of the relatively high metabolic 1l'\-l1 of these cells along the midrib this region [)la>'s a more important part in the localization of reproduction than the median region in Planaria. In fact, the experimental evidence seems to indicate that the chief difference in axial relations between Marchantia and Planaria is the higher metabolic level of the apical region of the transverse gradient, the median region of the body. Figs. 99-102. — Localization of new individual as resultant of dilTerent axial gradients in pieces of liverwort, Marchantia: Fig. 99, usual position in median ventral region near apical end of piece; Figs, loo- 102, different positions of new individual apparently determined by the different relations of the axial gradients according to shape of piece and region represented (from Vochting).

With advancing age the region of the midril) undergoes gradual differentiation and so loses to a greater or less extent its high metabolic rate. These experiments and many others which cannot be discussed here are highly significant in that they indicate the essential identity in character of the dilTcrent axes of the physiological individual. In fact, I believe they constitute evidence of the greatest importance for the fundamentally quantitative character of at least the main axes of the body, for if the different axes are qualitatively different, I cannot conceive how the position of a new head or growing tip on an isolated piece can be determined in one case chiefly by the major axis, in another as a resultant of two or more axes, and in a third by one of the minor axes. If, however, all axes are fundamentally gradients in metabolic rate, the facts are very simply accounted for, as 1 have tried to show. The major axis is the major axis, not because its nature is fundamentally different from that of other axes, but because it arises first or because its apical region has the highest metabolic rate of any part of the body, and the minor axes are minor axes because they arise later or their apical regions have a lower rate. When the major gradient is in any way obliterated to a sufficient degree one of the minor gradients may act in exactly the same way as, though often more slowly than, the major gradient where it is present. This is true, of course, only for forms and stages in which the fundamental quantitative character of the axes has not been too greatly altered by progressive differentiation.

The fact that a plant bud may be inhibited by the main growing tip, by another bud, by a growing leaf, or by a lateral branch also indicates that there is nothing specifically different in these different inhibitions and so suggests that these different plant axes act in essentially a quantitative way in dominating other parts. One may be substituted for the other without altering the character of the effect. It is possible to control ;and alter experimentally the spatial relations of parts in the individual by altering the length of the metabolic gradient and so the range of dominance. Parts of the individual may come to lie beyond the range of dominance in consequence of increase in size of the whole, of decrease in range and degree of dominance by decrease in the metabolic rate in the dominant region, of decrease in conducti\ity of the paths of correlation, and of the direct local action of external factors which increase the independence of subordinate parts. Parts thus physiologically isolated may reproduce new individuals if the essential axial gradients exist, or arise in them. In many of the lower organisms the original axis or axes may be experimentally obliterated and a new axis and dominance established in relation to external conditions which determine differences in metabolic rate in different parts of the mass. In general, the range of dominance increases during the development of the individual because the conductivity of the protoplasm increases, and special conducting paths develop as the m()ri:)hological expression of the fundamental correlative conditions in the individual. The essentially quantitative character of different axes of the individual is indicated by the fact that one axis may be experimentally substituted for another in determining the localization of a new individuation.

It has been assumed thus far that dominance depends on a transmitted change, or excitation, rather than on the transportation of substance, and it now becomes necessary to consider what basis there is for this conclusion. As already pointed out (pp. 26, 27), some sort of organization must be present in order that transportative or chemical correlation may occur in a definite and constant manner. If different regions of the body produce specifically different substances they must be specifically different, and if these substances act on certain other parts in a definite specific way those parts must possess a certain constitution. The data of experimental reproduction discussed in earlier chapters show that new individuals arise from parts of old individuals which either cannot possibly possess the "organization" of a complete individual or must possess an indefinite number of such organizations. The latter alternative leads to a conception of the Weismannian sort, and I have tried to indicate how unsatisfactory such conceptions are (pp. 22, 23).

If, on the other hand, the individual is primarily a metabolic gradient in a specific protoplasm, the only primary difference between the dominant and other levels of the gradient is a difference of metabolic rate. At this time the products of metabolism at different levels of the gradient are not specifically different, but differ in quantity. If the transportation of chemical substances is the only means of correlation between the different levels of the gradient, it is impossible to understand either how the gradient can persist or how a relation of dominance and subordination can arise between ievels of higher and those of lower metabolic rate. Specific chemical correlation between parts is possible only when specifically different parts are present, and the definite space relations which we find associated with physiological dominance do not usually appear in such correlation. In short, I beheve it is impossible to conceive of the process of organic individuation with the definite, constant, and orderly character which it actually possesses as having its origin in transportativc or chemical correlation alone.

If, however, the metabolic gradient arises and is maintained by the transmission of excitation from the region of highest metabolic rate, this region becomes dominant simply because its metabolic rate is so high that it determines and maintains the gradient in rate, and the differences in rate at different levels bring about sooner or later differences in constitution and character of the protoplasmic substratum. In regions of high rate only certain relatively stable substances remain as constituents of the substratum, and others are broken down and eliminated. In regions of lower rate, on the other hand, other substances accumulate as parts of the substratum because under these conditions tliey are less readily or less rapidly broken down than where the rate is higher, and it is also probable that the character of synthesis differs with the rate of metabolism. In

this way each level of the gradient develops a characteristic protoplasm and the character of the protoplasm in turn modifies and alters the character of the reactions, and so specific, or what we call qualitative, differences arise, and different specific substances may be produced at different levels of the gradient. At the moment when these specific differences first appear chemical cbrrelation in the commonly accepted sense becomes possible, and from this time on it may play a part in determining the character of further changes at the various levels. After chemical correlation appears it is unquestionably a factor of great importance in determining the character of the various parts and so of the individual as a whole. The point which I wish to emphasize is that chemical or transportative correlation does not and cannot account for the origin of the individual, because the individual must exist as some sort of orderly and definite relation or organization before orderly and definite chemical correlation between its parts is possible. The dynamic conception of the individual is primarily concerned, not with the orderly specificities of chemical correlation, but with the conditions in protoplasm which make those orderly specificities possible.

The occurrence of transmission in living protoplasm is a familiar fact. The existence of a transmissiondecrement and therefore of a limited range of effectiveness has been demonstrated for the transmission of stimuli in plant tissues and in various animal nerves. In many of the lower animals the range of effectiveness in transmission can readily be observed by means of the range of reaction to stimuli of different intensity. In transportative correlation a definite range of effective-

ness cannot exist unless transportation is uniform and constant in rate in all parts at each level and the substance is gradually destroyed or transformed during' the transportation. The dynamic theory affords an adequate basis for the very definite range of dominance which we find in organisms, and a chemical theory does not. Tashiro's recent investigations on carbon-dioxide production and my observations on susceptiljility gradients in the nerve indicate that physiological dominance in the neuron, i.e., the direction of transmission, is associated with the existence of a metabolic gradient. Individuation in what is probably the most highl\' specialized cell individual in the organism apparenll\- starts from the same condition, the metabolic gradient, as in the simplest axiate animal or plant. It is certain that dominance in the neuron depends primarily on transmission and not on transportation. This argument from the highly specialized to the simi)le is perhaps not of great value; still I camiot but beheve that the existence of an axial gradient in metabolic rate in the neuron and in the simple axiate indi\'iduals among the lower organisms is a fact of real significance.

It has been very generally beHeved by ph}'si(>logists that the nerve, at least the meduUated nerve of vertebrates, transmits excitations under normal conditions without a decrement in energy or intensity. It is. however, a well-known fact that even in these nerves a decrement appears when transmission takes place at low temperature or in partially narcotized or compressed nerves; in fact, under various conditions whicii decrease metabolic rate or irritability in the nerve.

Those who hold that the nerve in normal condition transmits without a decrement have usually maintained that under depressing conditions the nerve behaves in a different way from the normal nerve and that the decrement exists only under these conditions. In view of the fact that in the nerves of the lower animals a transmission-decrement undoubtedly occurs normally, and that in protoplasmic transmission in the absence of nerves the decrement is even more marked, the grounds for the belief that transmission without a decrement occurs in the vertebrate nerve do not appear to be adequate. It seems scarcely probable that the higher degree of specialization of the vertebrate nerve has brought about a fundamental change in the character of transmission of such a nature that the decrement is reduced to zero and transmission to an indefinite or infinite distance is possible. The experiments along this line prove only that with the very limited lengths of nerve available the decrement under normal conditions is very slight or inappreciable. Evidently the nerve of the vertebrate, and particularly of the higher vertebrate, is a much better conductor than undifferentiated protoplasm or even than the nerves of lower animals, and within the limits of the individual vertebrate body the decrement is undoubtedly slight or practically absent when the nerve is in good metabolic condition, but the conclusion that there is no decrement in such cases seems unwarranted. It is also highly improbable that the nature of transmission in the cooled, partially narcotized, or compressed nerve is essentially different from that in the same nerve under normal conditions, and since a decrement appears under depressing condi-

tions, the only conclusion justified by the facts seems to be that a decrement must exist in normal transmissicjn, but is much less marked, and the range of transmission is therefore much greater, than under depressing conditions. Undoubtedly in the higher animals the range of transmission is very much greater than the limits of the individual body, for the size of the individual in these forms is limited by other factors than the range of dominance (see pp. 46, 47, 151), but that transmission mthout decrement occurs is far from being demonstrated and, as I have endeavored to show, there is much evidence against such a view.

It is also a highly significant fact that the nervous system, which is the chief conducting organ of the body in those forms which possess it, develops in a definite relation to the axial gradients. The dominant region of the nervous system appears in the apical region of the major axial gradient, and at other levels of the body which contain the central nervous system it represents the region of highest metabolic rate in the minor gradients. If the unity of the organism depends primarily upon transportation, there is no apparent reason why it should change to a unity depending on transmission or why the dominant region of the central nervous system should arise in the dominant region of the primitive individual. If, however, organic unity is fundamentally and from the beginning dependent upon transmission, the general plan and arrangement of the nervous system are very evidently the expression in speciaHzcd structure and function of the primary unity and relation which was the starting-point of individuation, and dominance or control by nervous transmission is mercl)'

a specialized and more effective modification of the dominance which is the foundation of organic unity and order. Moreover, the nervous system dominates or controls the chemical activities of the organism to a very considerable degree. If the primary dominance is purely a matter of chemical correlation, it is difficult to conceive how the functional dominance of the nervous system has come about, but if the primary dominance depends upon transmission of the same general character as nervous transmission, the functional dominance of the nervous system is the natural and necessary consequence of the primary relations.

As regards the role of the nervous system in development and reconstitution, there has been much difference of opinion. Many biologists have maintained that the nervous system exerts a specific formative influence on various parts and so determines their course of development and differentiation, while others deny the existence of any such influence. In the case of certain organs and parts, e.g., striated muscle, it has been definitely demonstrated that embryonic development may occur without nervous connection, but in the mature condition frequent nervous stimulation is necessary for maintenance of structure and function. And as regards reconstitution, some investigators have found that certain parts, such as the amphibian leg, regenerate incompletely or not at all in the absence of nerves, while others have maintained that connection with nerves is unnecessary for complete regeneration of these parts. These apparently contradictory and confusing results can, I believe, be very simply inter-