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In the directed reactions of lower organisms we find repeated attempts by various authors to distinguish in one way or another two great classes of phenomena, and for these two classes different terms have been used. Thus ROTHERT' distinguishes two sorts of taxis, strophic and apobatic, depending on the manner in which they are brought about. Strophic taxis corresponds to the typical taxis or tropism of Verworn or Loes. It is conceived as brought about by a turning of the organism toward or froin the source of stimulation, due to a difference in the intensity of action of the stimulus on the two sides or ends of the organism. Apobatic taxis is defined as produced by a backward movement at passing from one concentration to another.2 The typical case for this is the reaction method of bacteria. RothErt includes under apobatic taxis the reaction of Paramecium and other infusoria to chemicals, etc., though for this his definition is inadequate, since the backward movement is only a comparatively unimportant detail of the behavior, and is often omitted. 3

PFEFFER' rejects Rothert's terms, substituting for strophic taxis the word topotaxis, for apobatic taxis the word phobotaxis. MASSART applies the term taxis only to the phenomenon called by Rothert strophic taxis, while the word phobism is proposed for the backward movement called by ROTHERT apobatic taxis.

DAVENPORT makes in his discussion of the reactions to light a distinction corresponding to a certain extent with those already mentioned. He distinguishes reactions whose direction is determined by the direction of the rays of light, from re..

* Flora, 88, 1901, p. 393. i Rothert, l. 6. p. 393. 3See the account by the present author, this Journal, 14, 1904, pp. 458-460. *Pflanzenphysiologie, Bd. 2, 1904, p. 755. 3 Annales de l'Institute Pasteur, 1901, p. 25. 6 Experimental Morphology, vol. I, 1897, p. 180.

actions whose direction is determined by differences in intensity of illumination. The former he calls phototaxis, the latter photopathy. Photopathy is held to be due to the fact that increased brightness causes a movement forwards, that a diminution of brightness causes a movement backwards, or vice versa, thus resulting in an accumulation of the organisms in the darker or lighter parts of the field.” (1. c. p. 210.) Phototaxis on the other hand includes that active migration of organisms whose direction is determined by that of the rays of light” (p. 180). In elongated organisms DAVENPORT thinks that phototaxis is due to the difference in intensity of illumination of the two sides of the organisms (p. 209), while in Ameba he thinks it possible that the direction taken by the ray in passing through the organisms is the determining factor (p. 210). YERKES! holds, with Holt and LEE, that both phenomena are due in some way to the intensity of the light, and classes as taxis those reactions in which the direction of movement is determined by orientation, while -pathy includes reactions in which the movement is not definitely directed through the orientation of the organism. This distinction doubtless indicates the real observational source of the attempt to separate two great classes of reactions, and does not attempt to make the definition outrun the known facts.

Kinesis is a term which seems to have been first used by ENGELMANNfor the increase or decrease of activity produced by certain agencies. The fact that certain bacteria increase or de . crease movement in the light he called photokinesis. ROTHERT (l. c., p. 374) accepted the term kinesis for such changes in the amount of activity produced by chemicals, calling this chemokinesis. LOEB' had observed similar phenomena and applied to them the German word Unterschiedsempfindlichkeit. GARREY, working with LOEB, later substituted the term kinesis for the German word. The term Unterschiedsempfindlichkeit, signifying literally sensitiveness to differences, might well include much more than a mere increase or decrease of activity; it could properly

Mark Anniversary Volume, 1903, p. 361.
* Arch. f. d. ges. Physiol., 30, 1882.
* Arch. f. d. ges. Physiol., 54, 1893, p. 81.
4 Am. Journ. Physiol., 3, 1900, p. 29.

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embrace all reactions due to a change in the intensity of action of a stimulating agent. GARREY's use of the term kinesis seems a little uncertain, since he seems in one place (p. 292) inclined to consider it equivalent to DAVENPORT'S -pathy, thus including directed reactions to changes in intensity, while in general he seems to limit it to increase or decrease of activity, in the sense in which it was employed by ENGELMANN and ROTHERT. GARREY uses the term in contradistinction to tropism, which is defined in accordance with the well known views of LOEB.

Terms ending in -metry have been contrasted with taxis, especially by STRASBURGER' and OLTMANNS, to distinguish those features of the reactions that are determined by differences in the intensity of the stimulating agent. Photometry is employed by STRASBURGER to signify especially the phenomenon that some organisms move toward the source of light in a certain intensity of illumination, while they move away from it at a higher intensity. By OLTMANNS photometry is used of any reactions due to differences in the intensity of light; “movements produced by light of different intensity could fittingly receive the name photometric movement” (OLTMANNS, I. c., p. 190).

Thus we find various attempts to distinguish by one criterion or another two great classes of reactions. On the one hand we have apobatic taxis (Rothert), phobotaxis (PFEFFER), phobism (MASSART), kinesis (ENGELMANN, GARRY and others), -pathy (DAVENPORT), -metry (STRASBURGER and OlTMANNS); in these orientation is not a marked feature. On the other hand we have tropism or taxis proper (LOEB, DAVENPORT, MASSART, VERWORN and others), strophic taxis (ROTHERT), topotaxis (PFEFFER); in these orientation is a marked feature. What is the precise basis for this distinction into two classes ?

In the behavior of infusoria (ciliates and flagellates) it is possible to distinguish clearly the two classes of phenomena on which this distinction is based, and to determine the real nature of the difference. 3 In both classes the cause of reaction is, in

Jena. Zeitschr. f. Naturw., 12, 1878. ? Flora, 75, 1892, p. 183.

3 In the effects of the electric current the usual reaction method is in the Ci. liata interfered with by the forced cathodic reversal of the cilia,-a factor not par.

the infusoria, some change in the relation of the organism to the environment. It is usually, or perhaps universally, a change in the intensity of action of a stimulating agent on the body as a whole or on its most sensitive portion. The organism, having been in one condition, passes to another, and it is the transition that acts as the effective stimulation. In one of the two classes of reactions to be distinguished, the effective stimulation is due to the fact that as the organism progresses from one region to another it meets different conditions, and to the changes thus caused it reacts.

This is the case under natural conditions with the reactions to mechanical stimuli, to heat and cold, to chemicals, to osmotic pressure, and to light when passing to a region of greater or less intensity of illumination.

The cause of reaction is analogous to that in our own case in passing from a region of moderate temperature to a hot or cold region. The differences determining reaction are here arranged along the axis of the course, so that it is the backward or forward movement that brings them into action. The reaction is a change of movement such as to carry the organism successively in many directions—a series of trial movements. As soon as one of these movements carries the organism away from the stimulating agent—that is, in such a direction that the changes to which it subjects the organism lead toward the optimum instead of away from it—the reaction ceases, since the cause for it has ceased. The organism therefore continues in that direction. The position of the body has little or no effect on the production of the change that causes stimulation, or on the release from stimulation. The organism might retreat from the stimulating agent backward or forward or sideways; in the one case as in the other it would be relieved of the stimulating changes. The different individuals may swim away directly or obliquely, their

alleled under any other conditions. As a result of this interference the reaction of the ciliates to the electric current takes in many respects a different character from the rest of their behavior. I wish therefore to emphasize the fact that the general relations set forth in the text do not apply to the reactions of the ciliates to this agent. Save for the forced cathodic reversal of cilia the response to the electric current would fall in our second class of reactions, as it actually does in the flagellates and rotifers.

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various paths crossing at all sorts of angles; the only requirement is that the path shall on the whole lead away from the greatest (or least) intensity of the stimulating agent. Hence when many specimens react in this way their paths need not be parallel, and as a rule no marked orientation results. When a common orientation of many individuals does occur, it is produced through "exclusion,”-through the fact that under the given conditions movement in any direction but one causes the changes which act as effective stimuli, so that all are forced to move in that direction. The behavior of Oxytricha in react. ing to heat, as shown in Fig. 7 of the author's paper on Reaction to Heat and Cold' will serve as a type for this reaction method. It is, so far as the infusoria are concerned, the basis for apobatic taxis (ROTHERT), phobotaxis (PFEFFER), and -pathy (DAVENPORT), and might farther be classed, from certain points of view, as kinesis or -metry.

The second class of reactions includes those in which the changes that act as stimuli are brought about by a swerving toward one side or the other, while the movements in the axis of progression have no such effect. In the infusoria movements from side to side are of course a regular part of the locomotion. In the reactions to the effects of water currents, of centrifugal force, of gravity, and of light rays coming from one side, the lateral movements of unoriented animals induce marked changes. In a water current or under the action of a centrifugal force, or of gravity, lateral movements meet with less resistance in one direction, greater resistance in another direction, and these changes of resistance act as stimuli. In light coming from one side the sensitive anterior end is more illuminated as the organism swerves towards one side; less illuminated as it swerves to. ward the other, and these changes act as stimuli. The reaction is the same as in the first class; the organism changes its course by a series of trial movements. It continues these movements till it comes into a position in which it is no longer subjected to the changes that act as stimuli. Such a position is found only when the axis of the course coincides with the direction of ac

Carnegie Institution of Washington, Publication 16, 1904, p. 16.

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