Bot. Gaz. 8(6): 286-295 (June, 1921)
CERTAIN ASPECTS OF THE PROBLEM OF PHYSIOLOGICAL CORRELATION1

C. M. CHILD
(Received for publication January 14, 1921)

1Invitation paper read before the Physiological Section of the Botanical Society of America, in the symposium on biophysics, at Chicago, December 28, 1920.

The existence in the growth and development of axiate plants of a relation of dominance and subordination, of control and being controlled, has long been recognized. This relation is very evidently associated in some way with at least certain fundamental physiological activities of plant protoplasm and apparently particularly those which have to do with growth. The active vegetative tips are the chief regions of dominance, but other growing regions may exercise a similar dominance to a greater or less degree. That this relation is a real physiological relation and dependent on the dynamic activity of the dominant part has been demonstrated repeatedly by experiments in which the dominance is abolished by inhibition of the fundamental metabolism and growth of the dominant region, but reappears when the inhibiting factor is removed and the dominant region returns to or approaches its original condition.

My work on so-called physiological polarity, correlation, and integration in animals has shown very clearly that in axiate animals as well as in plants a relation of dominance and subordination exists, not only as regards the functional activities of the fully developed individual, but also in growth and development. The evidence indicates that the functional relations of later stages as expressed in the nervous system and in the chemical interrelation of parts are the consequence and outgrowth of a more general and primitive relation which exists before the nervous system appears and before the various parts differentiate. Since this relation in its more general form as it appears in the simpler animals and the earlier stages of development seems to be very similar to the relation in plants, I have very naturally been much interested in attempting by means of work along various lines with plants to discover whether, or to what extent, such similarity exists. It is because of some of this work on plants that I have been asked to take part in this program. My objection that the work cannot properly be regarded as biophysics any more than biochemics, and that it has not yet attained the exact quantitative character and formulation which would warrant its inclusion in either of these special fields of physiology, was overruled by those in charge of the program, so that responsibility rests upon them.

A very brief survey of certain lines of work is necessary by way of introduction to the experiments of which I wish particularly to speak. Study of several hundred species, including animals from all the chief groups and many algae and some other plants, have shown that physiological polarity and symmetry in their simplest terms consist of gradients in physiological condition and activity of the protoplasm or cells composing the organism. These gradients have been called axial, metabolic, or physiological gradients. That they have to do with the fundamental physiological condition of the protoplasm is clearly shown by the many different lines of evidence which demonstrate their existence. They appear as gradients in susceptibility to certain toxic ranges of concentration or intensity of external agents, e.g., cyanides, heavy metal salts, anesthetics, acids, alkalies, other neutral salts, CO2 various dyes, extremes of temperature, and the negative condition, lack of oxygen. Within certain limits of concentration or intensity these susceptibility gradients are non-specific, i.e., essentially identical in their larger features with all external agents tested, at least in the simpler animals and plants and in the earlier stages of development of higher forms. It has been shown that these differences in susceptibility are indicators of differences in rate of fundamental metabolism, particularly oxidation. The physiological gradients can be demonstrated as gradients in the rate of penetration of non-toxic or only slightly toxic vital dyes. Again, in dilute solutions of the oxidizing agent KMnO4 they appear as gradients in rate and amount of reduction of the salt. In certain cases the indophenol reaction has been used, and a gradient in the rate of appearance of the indophenol suggests a gradient in oxidizing enzymes. A gradient in electric potential is a characteristic feature of the physiological gradient in all forms thus far examined, though in some plants the electrical situation is apparently complicated by the occurrence of reactions which give rise to opposite potential differences, viz., the oxidations and photosynthesis. And finally, in animals in which it has been found possible to determine the oxygen consumption and CO2 production of different regions along the axis, differences corresponding to those indicated by other methods have been found. It has not yet been possible to apply all these methods to each species examined. These physiological gradients also very commonly appear in differences in structure of the protoplasm along the axis, as in many plant and animal embryos, and they are definitely related to differences in rate of development and differentiation.

It has also been possible to show that the localization and differentiation of organs and parts occur in a definite relation to the physiological gradient, in fact are determined by it. The most active region as finally determined, i.e., the region of highest susceptibility, of greatest permeability, of greatest reducing capacity, of highest external electro-negativity, and of highest rate of respiration in the polar axial gradient, becomes the apical end of the axis, or in animals the head, and the other organs develop at different levels of the gradient. The question how the primarily quantitative differences in such a gradient can give rise to the qualitative differences characteristic of differentiation of cells presents no fundamental difficulties. Differences in the relation between available nutritive substance and the rate of oxidation at different levels of a gradient undoubtedly determine the appearance of certain substances in the cells at one level and their absence at another. Differences in concentration of certain substances at different levels may also determine the formation of different products, and various other factors in the complex protoplasmic system doubtless play a part in determining the origin of qualitative differences from the quantitative differences of the gradient. But whatever the local factors involved in each particular case, the physiological gradient constitutes the primary factor in determining localization and differentiation of parts along an axis.

The important point for present purposes is that in such a gradient a relation of dominance and subordination exists, the high end, the most active region, of the gradient being the dominant region and determining to a greater or less extent conditions at other levels within a certain distance, which differs with the stage of development, the condition and differentiation of the protoplasm, and the degree of activity of the dominant region. Since this dominance is effective only within a certain range or distance, the possibility of physiological isolation exists, that is, either in consequence of increase in length of the organism, of decrease in activity of the dominant region, or through a blocking in some way of the passage of the controlling influence, certain parts may become isolated from the action of the dominant region, even though still in physical continuity with it. In the simpler animals and plants such physiological isolation results, like physical isolation, in dedifferentiation and development of new axes, or parts already present but previously inhibited, such as. latent buds in plants, become active and develop.

The question of the nature and origin of physiological gradients is obviously of fundamental importance. The gradients, so far as can be determined, represent primarily quantitative rather than qualitative differences, and if this is true, as all the evidence indicates, the relation of dominance and subordination cannot be fundamentally a matter of chemical or transportative correlation, that is, of mass transportation and action upon one part of specific substances or hormones produced in another. In order that such correlation may exist the parts concerned must already be qualitatively different, and the evidence indicates that dominance and subordination exist in the absence of such differences. Unquestionably chemical correlation is of great importance as soon as qualitative differentiation begins, but it cannot be the primary factor in correlation and in determining such differentiation in the organism.

The only other possibility appears to be the transmission of dynamic change of some sort, that is, of excitation. We must therefore inquire whether the physiological gradient shows any similarity to an excitation gradient. All living protoplasm is excitable and to some degree capable of transmitting excitation. Excitation in its most primitive form appears to be an acceleration in the rate of living, particularly as regards the energy-liberating aspects of life. 'Where specialized conducting paths are not present transmission of excitation occurs with a decrement, that is, the transmitted change becomes weaker and finally disappears at a greater or less distance from the point of origin. Such a process of excitation and transmission gives rise to an excitation-transmission gradient. We usually think of such gradients as temporary or reversible, but the physiological gradients show all the characteristics of excitation gradients which have become more or less permanent.

Moreover, it has been shown experimentally that these gradients can he produced in cells or cell masses by subjecting them to a quantitative differential in the action of external factors. For example, new gradients and so new polarities can be determined experimentally in the simpler animals by a sufficient difference in oxygen supply, by an electric differential, perhaps in some forms by a light differential, and probably also by various other differentials. A differential inhibition may have the same effect as a differential excitation or acceleration. Turning to the plants, the polarity of the Fucus egg, which is primarily a gradient, is determined by the differential action of light, and the polarity of the Equisetum spore has a similar origin. The relation of dorsiventrality and symmetry to light in the plants is a familiar fact. In order to establish a gradient sufficiently persistent to serve as a physiological axis, the differential action of the external factor must persist for a certain length of time dependent on the nature and intensity of the factor and the character of the protoplasm. The fixation of such a gradient in protoplasm must depend upon the occurrence at the different levels of changes which are more or less irreversible under the existing conditions and which differ in degree according to level. Once established in a cell or cell mass, such a gradient may persist through division or other reproductive processes and become the basis of the axis of the new individual or individuals. In other cases the original gradient may disappear in reproduction and a new gradient arise. In many eggs, both animal and plant, the gradient is apparently determined by a differential in relation to the parent body, such, for example, as the difference in conditions at the attached and the free end of the egg; but in some eggs it may perhaps persist from earlier cell generations, while in others, as in Fucus, it is determined by a factor external to the organism.

We come now to the question of the nature of the dominance or control, and this involves the question of the nature of transmission. Many hypotheses have been advanced concerning the process of transmission, and most of them connect it in one way or another with the electric changes which are a characteristic feature of excitation. On the basis of extensive experimental investigation R. S. Lillie has developed an electro-chemical conception of excitation and transmission which seems to interpret and account for the various phenomena more satisfactorily than others previously advanced. The unexcited surface layer of the cell behaves as if more permeable to positive than to negative, or to certain negative, ions, and is therefore electrically polarized. Excitation increases its permeability to the negative ions and depolarization results, with an increase in electronegativity of the external surface. In this change a chemical reaction, an oxidation, is involved, whether as the primary or as a secondary factor is not at present known. The electric current arising at any point of excitation becomes the factor determining depolarization and excitation at all points within a certain distance, beyond which it is too weak to be effective, and each new region of excitation becomes the source of current which may, if strong enough, excite further points. At the same time the current tends to restore the polarization at the point of original excitation and so to reverse the excitation process at that point. By means of this current, then, according to Lillie, transmission occurs. With simple inorganic models he has been able to demonstrate the occurrence of transmission in this way, both with and without decrement and at different speeds, as well as the development of fixed gradients. The speed of transmission has no relation to the speed of electrical transmission, but depends on the velocity of the changes at each point of excitation which give rise to the current. The development of fixed gradients occurs when conditions determine the persistence of the region of high potential which in turn determines a potential gradient extending over a greater or less distance and so a. gradient in the conditions determined by the electric current.

Whether this theory of excitation is in all respects correct or not, it enables us to see how a region of excitation in undifferentiated protoplasm may determine the origin of a physiological gradient. The facts indicate that these gradients do arise in this way, and if we admit this, it follows that the primary factor in dominance and subordination is transmission. The establishment of a region of high activity must affect adjoining regions within a certain distance as a region of excitation affects them, and it is difficult to believe that the electric potential characteristic of such a region is not a factor and probably the primary factor in such a relation. If the degree or intensity of excitation at each level is in any degree proportional to the strength of current, a gradient must result, and if the conditions determining the gradient persist for a certain length of time, changes in the protoplasm at the different levels may determine the more or less permanent fixation of the gradient. In most protoplasms this relation between stimulus and excitation does exist, and a gradient results from local excitation. In the nerve fibers of the higher animals, however, the excitation process is specialized so that any stimulus above the threshold gives rise to maximal excitation and there is therefore theoretically no decrement in transmission.

The relation of dominance and subordination does not necessarily persist in its primitive form throughout life. In animals, for example, the development of the nervous system, with highly specialized paths capable of transmitting excitation so much greater distances than embryonic protoplasm, makes possible a much more complete and extensive dominance, which nevertheless is built up on the basis of the primitive relation. On the other hand, the qualitative differentiation of different organs affords a basis for complex chemical or transportative correlation. In plants, buds which are inhibited for a time may sooner or later become incapable of development, even when physiologically isolated, either because they have riot been able to develop channels for the passage to them of water and nutrition, or because of changes in the cells in consequence of the action of the dominant region upon them. On the other hand, even in plants the conductivity of certain tissues may increase with differentiation and dominance be possible over greater distances than at first. In short, the primitive transmissive relation may develop and attain greater importance in certain types of relation, or it may be supplemented or even replaced by chemical correlation, or, finally, with advance in differentiation of parts the correlative factors may be chiefly nutritive. In any case the situation in the plant remains much simpler than in the higher animals, in which both the transmissive and the transportative relations are extremely complex.

My work on plants, in which Dr. A. W. Bellamy assisted me, was undertaken with the hope of being able to throw some light on the question of the nature of dominance and subordination as it exists in the growing plant. Thus far I have merely succeeded in blocking by means of low temperature the correlative factor on its way without interfering to any marked degree with the flow of fluids in the plant. As I have pointed out elsewhere, the results favor the view that correlation is accomplished by a transmission rather than by mass transportation of special substances, but much remains to be done before positive conclusions are permissible.

My experiments thus far have been chiefly with three plants, Bryophyllum calycinum, Phaseolus multiflorus, and Saxifraga sarmentosa. The method of experiment and the results obtained with Bryophyllum have already appeared in the Botanical Gazette. As regards method, it may be said here that the low temperature is applied by surrounding the zone to be cooled with a coil or loop of small block-tin piping which can readily be bent and adjusted and through which water of controlled temperature flows. The region to be cooled is first wrapped in tinfoil and direct contact with the piping is avoided, the space between the coil and the plant being loosely packed with slightly moistened absorbent cotton, and the whole region of the plant and the coil after adjustment well wrapped in order to reduce temperature change from external sources to a minimum. All cases in which visible external injury due to pressure or to too low temperature occurs are discarded. Temperatures ranging from 3 to 8 C. are used according to the plant and the particular object in view. In the bean seedling cell turgor is somewhat reduced in the cooled region after some days, particularly in the lower temperatures. On removal of the coil, however, normal turgor is reestablished within a few hours, and if the temperature is raised gradually at the end of the experiment the turgor is normal when the wrapping is removed.

2In the original presentation of these and other experimental data lantern slides were used.

In the experiments on Bryophyllum2 the low temperature is applied to a zone of the petiole 2-3 cm. long, and the leaf is immersed in water so far as its position on the plant will permit. The opposite leaf of the same node and usually leaves of other nodes above and below are also immersed. In the experimental leaf all or nearly all the immersed buds develop, in the opposite leaf there is as much or almost as much development in most cases, and more or less development occurs in leaves of nodes above and below the experimental node. Controls with leaves immersed but without the low temperature often show development of a bud here and there, but the effect of the low temperature is clear and unmistakable in the experimental leaf, in the opposite leaf, and to a less extent in leaves of neighboring nodes.

The case of the scarlet runner bean is of greater interest than that of Bryophyllum, for here the low temperature is applied to the main stem of the seedling, and all water and nutrition passing to parts above must pass this zone. If the low temperature interferes appreciably with the flow of fluids, this should be evident in the retardation of growth above the zone, or in extreme cases in wilting. There is in some cases slight retardation of growth of the tip, particularly when the low temperature is applied to the upper part of an internode of the young seedling in which elongation is still going on and the vascular bundles are still developing. Except when the temperature is very low, however, this retardation is only temporary and the rate of growth of the tip increases even before the low temperature is removed, and in no case is the effect on the tip sufficient to decrease its dominance to such an extent that axillary buds above the low temperature zone develop. Moreover, that this retardation has nothing to do with isolation of buds below the cooled zone is shown by the fact that a temperature of 5-6 C., applied to the upper end of an internode, produces at first marked retardation of growth of the tip but no growth of buds below, while the same temperature applied near the lower end of the internode produces no appreciable retardation of the tip, but the buds in the axil below develop. In general, the farther away from the axils to be isolated the low temperature is applied, the less effective it is in producing growth of the buds and vice versa. These facts suggest that the inhibiting factor, if it passes at all through a cooled zone, undergoes a gradual return or approach to its original effectiveness in its further course, so that when the cooled zone is farther away from the buds to be isolated it is less effective. Moreover, with temperatures which are not too low, a long cooled zone is more effective as a block than a short one, but with sufficiently low temperatures the short zone is effective. In these respects the correlative factor apparently behaves to some extent like the nerve impulse.

Growth of the buds isolated from the tip by low temperature is usually evident within one to two days. If the temperature is near the upper limit of effectiveness, the growth of the buds below the zone usually ceases after a few days in spite of the presence of the cooled zone. This is undoubtedly due to the occurrence of some degree of acclimation in the cooled zone with consequently more effective passage of the block by the inhibiting factor. If the buds are allowed to grow for ten days or more before the removal of the low temperature, they usually continue to grow more or less rapidly afterward, and in this way plants with three or more stems can be produced. Earlier removal of the low temperature usually results in renewed inhibition.

In my experiments with the saxifrage, the low temperature is applied to a zone of a runner which has not attained its full length, with the result that the runner soon ceases to elongate and begins to develop a new plant at the tip, even when suspended in air. Here also water and salts reach the runner tip only by passing the cooled zone, and the rapid development of the new plant shows that this flow is not seriously affected. According to Loeb's hypothesis, it seems that substances inhibiting the development of the new plant at the runner-tip must be transported by this current, but as a matter of fact the low temperature isolates the tip without stopping the current.

Whatever the nature of the correlative factor may prove to be, these experiments, particularly those on the bean seedling, seem to me to offer difficulties to the hypothesis that this factor consists of an inhibiting substance or substances transported in mass through the plant. Since the correlative factor can be blocked by a zone of low temperature, we must assume, if it consists of a substance or substances, either that it is transported through the living protoplasm and that its passage is dependent upon the physiological condition of the cells, or that it is of such a nature that it is precipitated out of, or otherwise removed from, the fluids of the plant as they pass the cooled zone.

On the basis of the first assumption, we should expect a substance which inhibits the growth of vegetative tips to inhibit the growth of the cells through which it passes. Below the chief growing tip, for example, such a substance must pass through the region of most active growth in the axis, but it does not inhibit this region. In fact, if such a substance passes through the living cells of the plant, most complex and remarkable relations of immunity and susceptibility must exist. Each growing tip, for example, or any other part producing such a substance, must be immune to the substance which it produces, but other growing tips are susceptible to it.

The alternative assumption, that the substance is transported in the fluids of the plant and removed in some way from them in the cooled zone, does not serve any better than the first for the interpretation of certain experimental results. In fact, the hypothesis of inhibiting substances and their transportation, in whatever form we state it, does not account for the fact that within certain limits of temperature the effectiveness of a cooled zone of certain length in the stem of the bean seedling decreases with increasing distance from the buds to be isolated, even though the more distant zone maybe more effective in inhibiting the growth of the chief tip. In other words, a cooled zone which has a marked inhibiting effect upon the movement of water and salts in the stem is not necessarily effective in isolating buds below, while a zone which has no appreciable inhibiting affect on regions above it may be effective in isolating buds below.

Assumptions concerning the transportation of nutritive substances are not, I believe, any more satisfactory than the hypothesis of inhibiting substances in aiding us to account for the facts of physiological correlation in the plant. As already noted, physiological isolation of buds below a cooled zone may be brought about without retarding the movement of water and salts to any marked degree. In the case of Bryophyllum, the leaf itself is able to produce starch, and in the bean seedling the reserves of the cotyledons are available for the buds in the axils of the cotyledons and those produced by the first pair of leaves are available for the buds of the second node. In the saxifrage runner the cooled zone may retard to some extent the passage of nutrition to the runner tip, but the results with the other species show clearly enough that this is not the primary factor in the physiological isolation of parts in plants. In the case of the Bryophyllum leaf with cooled zone about the petiole, the passage of water and salts to the leaf may be somewhat retarded, but the leaf contains plenty of starch. In the bean seedling the passage of water and salts to the buds to be isolated is not interfered with, since the cooled zone is above them, and here also plenty of carbohydrate is available. In both these cases, as well as in the saxifrage runner, physiological isolation and growth of buds occur. Again, if we cut off the stem of the bean seedling below the first foliage leaves and remove the cotyledons, the buds in the axils of the cotyledons, which are now the only buds on the plant, will develop, although in the absence of the more apical parts of the plants the movement of water and salts must be greatly decreased and the removal of leaves and cotyledons must have decreased the amount of carbohydrate available. In short, the result as regards development of the buds is essentially the same under experimental conditions which must determine very different internal conditions as regards the movement of nutritive substances and the amount available in a particular region.

Finally it may be noted that in the bean seedling the rate of reaction of the buds of the different axils to physiological isolation by a cooled zone differs according to the level of the plant. With a cooled zone of given length and given temperature at a given distance above the node concerned, the buds of a more apical node begin to grow earlier and grow more rapidly than those of a more basal node. This fact also seems to me to offer difficulties to the hypotheses of dominance by means of inhibiting or by means of nutritive substances. On the other hand, it is apparently an expression of the physiological gradient, the primarily quantitative gradation in physiological condition along the plant axis, which, as I believe, is the basis of physiological correlation in the plant. In other words, the relation of dominance and subordination is also an expression of the physiological gradient, and the movements of substances in the plant are not the primary factors in physiological correlation, but rather the consequences of the differences which constitute the physiological axial gradient.