STIMULI AND THEIR ACTIONS 447 



cumulation of Paramcecia can take place, he increased by rotation 

 the pressure at the peripheral end in comparison with the 

 central end, and thus artificially imitated the conditions which, 

 according to the laws of the earth's gravity, prevail in a vertical 

 tube. The result was that with not too rapid rotation of the disc 

 the Paramcecia collected at the places of lower pressure, i.e., at 

 the central end of the tube, a phenomenon which Jensen puts 

 beside geotaxis as centrotaxis. With a proper rate of rotation they 

 frequently accumulated with greater certainty than in the upright 

 tube. If they were centrifugalised too rapidly, naturally they 

 were thrown out passively toward the periphery like heavy bodies. 

 Accordingly, geotaxis, which has occupied a peculiar position so 

 long in botany, must be regarded as a special case of barotaxis. 



3. Phototaxis 



A ray of light extends through space from a source of light in a 

 straight direction, and diminishes in intensity with the distance. 

 Hence, any two points in the line of the ray possess different in- 

 tensities ; the point that is nearer the source has the greater, that 

 which is farther away has the less intensity. A ray of light, there- 

 fore, fulfils very completely the conditions that are necessary to the 

 appearance of unilateral stimulation in fact, it is extremely 

 difficult to establish conditions under which an organism is 

 stimulated by light uniformly upon all sides. As a result of this, 

 stimulation by light calls out very pronounced directive effects, 

 which have been termed phenomena of phototaxis 1 and form a 

 complete analogy to those of chemotaxis and barotaxis. 



The phenomena of phototaxis have been known longest in 

 plants ; as a matter of fact, plant physiology, on account of the less 

 complexity of its objects of study, was able to develop in general 

 into systematic completeness much earlier than animal physiology. 

 Every one who cultivates plants in a room has the fact of positive 

 phototaxis daily before his eyes. He sees that the growing parts 

 turn constantly toward the light ; and, in order to make a plant 

 grow straight upward, he must turn the pot about from time to 

 time so that any phototactic curving may be compensated. Many 

 plants are so extremely phototactic that in bright sunshine in a 

 garden they follow the course of the sun in their curving. For 

 example, in a bed of blue gentians, all the plants turn the broad 



1 Formerly a distinction was made between heliotropism and phototaxis, 

 the former word signifying the attitude, bending, and turning of fixed organisms, 

 the latter the movement of motile organisms, with reference to the source of the 

 light. This distinction is not only superfluous, but it introduces the false idea 

 that the phenomena in the two cases are dependent upon different causes. A 

 double terminology for processes that are based upon the same principle should 

 be avoided. The processes are now understood better than at first, and the old 

 distinction, which arose from purely external points of view, should be discarded 

 as unscientific. Many authors have already done this. 



