446 



GENERAL PHYSIOLOaY 



the centre of the earth. In this case the stimulus is afforded by 

 the minimal differences of pressure that exist at points of different 

 height, both in the water and in the air. 



These phenomena have been known longest in botany, for all 

 plants are geotactic in a pronounced manner. The roots grow 

 toward the centre of the earth and, therefore, are positively geo- 

 tactic ; the branches and the stems grow away from the centre of 

 the earth and, therefore, are negatively geotactic. Further, in the 

 behaviour of the leaves and in many cases the branches, which grow 

 essentially tangential to the earth's surface, a third sort, transverse 

 geotaxis, is seen. 



In free-living cells geotactic properties have been recognised, 

 especially by Schwarz ('84, 1), Aderhold ('88), Massart ('91), and 

 Jensen ('93, 1), who have found that of Infusoria and Bacteria in 

 closed vessels containing water, some rise 

 upward and collect upon the surface, while 

 others seek the depths and crowd together 

 upon the bottom. If, e.g., water containing 

 numerous Paramcecia be put into a vertical 

 glass tube, the Infusoria, as Jensen found, in 

 a short time rise and collect at the upper end 

 of the tube (Fig. 221), whether the latter be 

 open or closed. Paramcecia are, therefore, 

 negatively geotactic. Many Bacteria, as 

 Massart observed, behave conversely ; with 

 a similar arrangement of the experiment 

 they accumulate at the lower end of the 

 tube. These are, accordinglj-, positively 

 geotactic. 



Until very recently either very mystical 

 ideas or none at all had been formed con- 

 cerning the manner in which gravity calls 

 out geotactic phenomena ; but Jensen has 

 now shown that the effects are due to differ- 

 ences in pressure at different heights. As is well known, the 

 hydrostatic pressure in a column of water is considerably less 

 at the top than at the bottom. The higher pressure operates 

 as a stimulus and causes the organisms to leave the place 

 where it is present and seek the places of lowest pressure. 

 As all consideration at once shows, no other differences exist 

 between the upper and the lower portions of the column of 

 liquid in the vertical glass tube. An unprejudiced observer 

 must, therefore, recognise in geotactic phenomena a pressure- 

 effect. But that they are this actualh-, Jensen was able to 

 show by experiment upon the disc of a centrifuge. In tubes 

 placed horizontal and hence in the line of the radius of the 

 disc, in which under ordinary circumstances no geotactic ac- 



FiG. 221. — Glass tube con- 

 taining Paranuecia., which 

 as a result of negative 

 geotaxis have collected at 

 the upper end. (After 

 Jensen.) 



