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Notes: Summary The term tropism has been applied to almost every sort of reaction. It has been defined in some twenty different senses. There is so much diversity concerning its meaning that every one who wishes to convey anything but the vaguest sort of an idea finds it necessary to state in which sense he proposes to use it. This term has by some, also, been endowed with mystical causal powers. It would consequently be advisable to drop it altogether and substitute terms with more precise meanings. The following are suggested: negative or positive orientation or reaction, to light, gravity etc.; photo-, geo etc. negative or positive; or merely negative or positive reactions to light, gravity, chemicals etc.

Notes: Summary 1. Amoeba proteus orients very definitely in a galvanic current and moves toward the cathode. 2. If the anterior end is directed toward the cathode when the circuit is made there is increase in rate of streaming beginning at the anterior end and progressing toward the posterior end. If the current is weak, this increase is slight, if it is strong the anterior end spreads out and the posterior end contracts, if it is still stronger the amoebae disintegrate, beginning at the posterior (anodal) end. 3. If the posterior end is directed toward the cathode when the current is made the rate of streaming decreases beginning at the posterior (cathodal) end. If the current is not too weak retardation in streaming is followed by reversal in the direction of streaming at the posterior end while it continues in the opposite direction at the anterior end. If the current is strong the final effect is the same as it is with the anterior end directed toward the cathode, i. e. the amoebae disintegrate, beginning at the anodal surface. 4. In reactions to electricity, as in reactions to light, there is no specific threshold and the “all or none law” does not hold. The responses are due primarily to the solating action of the current on the plasmagel on the side directed toward the cathode and contraction of the plasmagel in adjoining regions. The nature of the response depends upon the extent of solation which in turn depends upon the density of the current and the time it acts. 5. Contraction at the anodal side accompanied by expansion at the cathodal side is due to gelation at the anodal side and electroendosmotic transfer of water from this side to the opposite side. 6. Rupture at the anodal side is due to irreversible gelation of the plasmagel with loss of water, making it so friable and weak that it readily breaks. This may be augmented by pressure against it, owing cataphoretic movement toward the anode of negatively charged granules. 7. Solation is due to accumulation of positive ions at the cathodal side, union of these ions with hydroxyl-ions in the presence of water and migration toward the anode of the hydrogen-ions liberated, resulting in increase in alkalinity at the cathodal side. 8. Gelation is due to accumulation of negative ions at the anodal side, union of these ions with hydrogen-ions in the presence of water and migration toward the cathode of the hydroxyl-ions liberated, resulting in increase in acidity at the anodal side. 9. In the epidermal cells of the leaves of Tradescantia which contain a natural indicator it can be clearly seen that the galvanic current causes increase in alkalinity at the cathodal end and increase in acidity at the anodal end. 10. In alternating current Amoeba proteus orients and moves at right angles to the direction of the current. 11. Viscosity increases at the two sides facing the poles and decreases in the central portion. This results in marked contraction of the two sides, forcing the central portion out in the form of pseudopods in either direction at right angles to the direction of the current. Later one of these pseudopods is retracted and the organism moves in the direction of the other. 12. If the current is strong the amoeba breaks at one or both surfaces facing the poles, after which the plasmasol usually flows out but the granules do not move toward the poles, cataphoresis being neutralized. 13. Increase in viscosity is due to localized accumulation of negative ions and union of these with hydrogen-ions resulting in increase in acidity. Decrease in viscosity is due to localized accumulation of positive ions and union of these with hydroxyl-ions resulting in increase in alkalinity. The process of accumulation depends upon difference in the rate of movement of positive and negative ions in a viscous medium and chemical union or electrostatic stabilization. 14. In both direct and alternating current the amoebae do not disintegrate if the surrounding medium is acid, no matter how strong the current may be. 15. The conclusion reached by various authors that the electric current causes increase in viscosity of cytoplasm is misleading. It probably always causes decrease in viscosity in some portions of cells and simultaneous increase in others. 16. Amoeba dofleini does not respond to the electric current. It does not disintegrate and no marked changes in viscosity were observed in it.

Notes: Summary 1. The direction of locomotion of Euglena in a field of light consisting of two horizontal beams crossing at right angles is in close accord with that demanded by the “Resultantengesetz”, that is, the tangent of the angle between the direction of locomotion and the rays in the stronger beam is approximately equal to the intensity of the weaker divided by that of the stronger beam. 2. This can be explained on the assumption that the eyespot is a photoreceptor, that the photosensitive substance in it is a layer in the concavity of the pigmented portion, that this layer is practically parallel with the direction of locomotion and that the light absorbed by it varies from a maximum when the incident rays are perpendicular to it, to zero when they are parallel with it, for if all this obtains and the “Resultan-tengesetz” holds the amount of light absorbed in all symmetrically opposite positions assumed in the process of rotation is equal when the organism is oriented, no matter what the ratio of intensities in the two beams may be. 3. The fact that the direction of locomotion in Euglena is in accord with the “Resultantengesetz” merely indicates that if the organism goes in any other direction it is unequally stimulated in different positions assumed in the process of rotation and that this results in responses which change its direction of locomotion until it is no longer unequally stimulated. It has no bearing on the problem concerning the quantitative relation between the stimulus and the response. 4. In Volvox and Gonium the direction of locomotion in light from two sources is not in accord with the “Resultantengesetz”. 5. In these forms the light is brought to a focus in the eyespots. The position of the focal point varies with the direction of the incident light and the stimulating effect of a given amount of light absorbed at this point varies with its location. 6. In Gonium the stimulating effect of a given amount of light absorbed by the photosensitive substance in the central part of the eyespot is about 9 times as great as that of the same amount of light absorbed by this substance in the posterior part. The photosensitive substance in the central part of the eyespot is therefore about 9 times as sensitive as that in the posterior part.

Notes: Summary 1. Under certain conditions Volvox is photopositive, under others, photonegative, and under still others it is neutral. If it is positive, a shadow on the photosensitive substance in the eyespots in the zooids causes change in the direction of the stroke of the flagella of the zooids from diagonal to backward and a flash of light on this substance causes change from diagonal to sidewise. If it is negative the reverse obtains and if it is neutral there is no response unless the changes in luminous intensity are great. 2. The nature of the response to light in Volvox depends upon the state of adaptation and the intensity of the illumination. If it is fully adapted it becomes positive if the intensity is increased and negative if it is decreased. If it is not fully adapted, it becomes negative if the intensity is increased and positive if it is decreased. 3. The time required for colonies of Volvox to become photopositive (reaction time) in illumination of a given intensity, after having been in strong light 1 to 2 hr., followed by darkness, increases to a maximum and then decreases as the time in darkness increases. If the colonies are kept longer in strong light and then subjected to darkness the reaction time decreases to a minimum and then increases as the time in darkness increases. The time required for dark adaptation depends upon preceding illumination. 4. The reaction time of dark adapted colonies is closely correlated with luminous intensity. As the intensity increases it decreases from 29 min. in 5/24 m. c. to a minimum of .098 min. in 7.5 m. c., and then increases to .358 min. in 62222 m. c. The energy required to make the colonies positive varies directly with the luminous intensity over the whole range tested and over most of the range the variation is nearly proportional to the variation in intensity. 5. If colonies are kept in a given intensity or in darkness they become adapted, i. e. they lose the ability to respond to light and if the intensity is now changed they regain it. If dark adapted colonies are exposed to light of 22400 m. c. for .05 min., then returned to darkness, it takes about 20 min. in darkness to eliminate the effect of the light. If they are left in the light .15 min. it takes about 30 min. The processes associated with adaptation and those induced by change in illumination are antagonistic. 6. To account for the responses to light in Volvox, it is necessary to postulate at least three groups of interacting substances.

Notes: Summary 1. Volvox rotates on the longitudinal axis as it swims through the water. This is due to the fact that the flagella usually beat diagonally backward. 2. If the luminous intensity is suddenly decreased without changing the direction of the rays photopositive colonies which are oriented stop rotating and spurt directly forward. This is due to a change in the direction of the stroke of the flagella from diagonal to more nearly directly backward. 3. If the intensity is suddenly increased locomotion stops and the rate of rotation increases. This is due to a change in the direction of the stroke of the flagella from diagonal to more nearly sidewise. 4. Photonegative colonies react precisely the same as photopositive colonies except that the effects of increase and decrease in luminous intensity on the direction of the stroke of the flagella is reversed. 5. If, after either of these changes in the direction of the stroke o f the flagella has been initiated, there is no further change in the intensity of the light, the flagella continue to strike in the new direction for only 2.5 + seconds, after which they again strike in the direction which obtained before the intensity was changed. 6. If the intensity is gradually changed, there is no observable response unless the new intensity is maintained for a considerable period of time, which may result in increase or decrease in activity. 7. The eye-spots in Volvox consist of a pigment-cup, a lens which is located at the opening of the cup, and photosensitive substance which is located in the cup. There is one in each zooid, and all face outward. They are primitive eyes. 8. Orientation in photopositive colonies is due to an increase in the backward component of the stroke of the flagella on the shaded side and a decrease in this component on the opposite side. The former is due to reduction of light caused by the shadow of the pigment-cup on the photosensitive substance as the zooids are transferred from the illuminated side of the colony to the shaded side; the latter to increase of light caused by exposure of this substance as the zooids are transferred from the shaded to the illuminated side. 9. In negative colonies precisely the opposite obtains in the process of orientation. 10. Photic orientation is due to a change in the direction of the stroke of the flagella and not to change in activity. Galvanic orientation is due to decrease in activity on the side toward which the colonies turn. The processes involved are consequently not the same. 11. The orienting stimulus and the changes in the direction of the stroke of the flagella which result in orientation cease after the colonies are oriented. They remain oriented because, in the absence of external stimulation, they tend to take a straight course and because, as soon as they swerve from this course, opposite sides become unequally illuminated, resulting in change of intensity on the photosensitive substance in the eyes which leads to reorientation.

Notes: Summary 1. Volvox usually is photopositive in low and photonegative in high illumination, but the opposite obtains under certain conditions. 2. If it is dark adapted, it is always positive when exposed to light, no matter how intense, but it soon becomes negative if the light is sufficiently intense and the higher the intensity the shorter the time required. 3. If Volvox is fully adapted to any given illumination it becomes positive if the intensity is increased and negative if it is decreased, provided the change in intensity is sufficiently great and sufficiently rapid. 4. Colonies which have thus become positive remain so for a time, then they become neutral and later negative if the change of intensity is sufficiently great. The greater the change in intensity the sooner they become negative. The reverse obtains in colonies which have become negative. 5. If colonies are negative in a given luminous intensity and the intensity is momentarily reduced they become positive, remain so for a time, then become negative again. The time they remain positive varies directly with the magnitude of the reduction of the light and the time it remains reduced. The opposite obtains for colonies which are positive in a given luminous intensity. 6. Volvox may be either photopositive or photonegative in every condition of luminous intensity in which orientation occurs. 7. Darkness or reduction in luminous intensity induces changes which result in reversal from photonegative orientation in a given illumination to photopositive orientation in the same illumination, and light or increase in luminous intensity induces changes which result in the opposite. 8. If colonies are subjected to any given constant illumination, they become neutral, i. e. they become adapted to that illumination and orientation ceases. 9. Colonies which in any given illumination are not fully adapted, usually become negative if the intensity is increased and positive if it is decreased, precisely the opposite from that which obtains in colonies which are fully adapted. 10. Photopositive orientation is brought about primarily by a change in the direction of the stroke of the flagella on the shaded side of the colonies from diagonally backward to more nearly directly backward and photonegative orientation by the same change on the illuminated side. The one is due to decrease, the other to increase in the illumination of the sensitive tissue in the eyes. 11. Decrease in the illumination of the photoreceptors in colonies in the photopositive state has the same effect as increase in the illumination of the same structure in colonies in the photonegative state. 12. This difference is probably associated with the difference in the electric charge on colonies in the two states, indicated in the results presented in a subsquent paper. 13. It is assumed that response to light in Volvox is associated with photochemical changes, and an attempt has been made to account for reversal in the direction of photic orientation on the basis of postulated photochemical processes.

Notes: Summary 1. Amoeba proteus orients in directive illumination and is negative in strong light and probably positive in very weak light. 2. Negative orientation is due to inhibition in the formation of pseudopods on the more highly illuminated surface; and this is due to increase in the elastic strength of the plasmagel on this side. 3. If the luminous intensity is rapidly and greatly increased, all movement ceases, but if it is only slightly increased there is only a slight momentary retardation in the rate of streaming and this is confined to the tip of advancing pseudopods. Between these extremes there are innumerable types of responses depending upon the magnitude of increase in luminous intensity. 4. Cessation in streaming is due to gelation produced by the light, resulting in increase in the thickness and the elastic strength of the plasmagel sheet at the tip of advancing pseudopods. 5. The magnitude of gelation is correlated with the extent of increase in luminous intensity. This accounts in part for the relation between the nature of the response and the extent of increase in luminous intensity. 6. The smaller the increase in luminous intensity, the less the gelation and the less the retardation in rate of flow. This relation continues theoretically until the increase in intensity is reduced to zero. 7. There is no threshold and the “all or none law” does not apply.

Notes: Summary 1. Volvox orients fairly precisely in a constant galvanic current. 2. It is positive to the cathode whenever it is photopositive and positive to the anode whenever it is photonegative. Anything which causes reversal in the direction of orientation in light causes reversal in the direction of orientation in a galvanic current, e. g. light, temperature, chemicals, hydrogen-ion concentration. 3. Galvanic orientation is due to cessation or diminution in the activity of the flagella on the side of the colony toward which it turns. Photic orientation is brought about largely, if not entirely, by change in the direction of the stroke of the flagella due to change in the amount of light received by the photosensitive tissue in the eyes, owing to rotation on the longitudinal axis. 4. When the circuit is closed the activity of the flagella in colonies which are not rotating, diminishes or ceases on the surface facing the pole toward which the colonies are positive, but the decreased activity continues only a few seconds after which the activity increases to that which obtained before the circuit was closed. If the circuit is now opened a similar response is obtained on the opposite side. 5. If the colonies are positive to the cathode the flagella on the cathode side respond after the circuit is closed and those on the anode side after it is opened. If they are positive to the anode the reverse obtains. 6. The galvanic response in the individual zooids in Volvox is only momentary. But in colonies which are swimming freely and rotating on the longitudinal axis the galvanic response on the side toward which they turn is continuous, owing to the continuous transfer of zooids to this side, from the opposite side, i. e. from the side where they are not affected by the current to the side where they are affected. 7. Reversal in the direction of galvanic orientation is probably accompanied by a change in the electric charge of the colonies. Colonies which are positive to the cathode usually drift cataphoretically toward the anode, indicating a negative charge; and those which are positive to the anode usually drift toward the cathode, indicating a positive charge. This has, however, not been unequivocally established. 8. A galvanic current induces in the lower organisms various chemical and physical changes at the anode and at the cathode surfaces but the changes induced at these two surfaces differ greatly. In some the changes at the surface toward the cathode are followed by certain conspicuous phenomena, e. g. reversal in the direction of the stroke of the cilia in Paramecium; in others those at the surface toward the anode are followed by equally conspicuous but different phenomena, e. g. contraction in Amoeba and bioluminescence in ctenophores; and in still others those on one side are followed by the same conspicuous phenomena if the organisms are in a given state, as those on the other side, if the organisms are in different state, e. g. decrease in the effectiveness of the stroke of the flagella in Volvox. 9. The reversal in galvanic response observed in Volvox is not in accord with Pflüger's law. Whether or not the responses observed on the anode side in amoeba and ctenophores also violate this law, depends upon its interpretation. 10. Galvanic stimulation in Volvox is probably associated with decrease in surface polarization and decrease in water-content.