Ill 



LIVING MATTEE 



A third form of barotaxis is geotaxis, or the well-known 

 property of plants to place themselves with their median axis in a 

 definite direction toward the centre of the earth. The stimulus in 

 this case is afforded by minimal differences of pressure acting on 

 points at different heights of the organism. The stems of trees 

 grow away from the centre of the earth, and are, therefore, 

 negatively geotactic ; the roots grow toward the centre of the 

 earth, and are, therefore, positively geotactic ; further, the leaves, 

 and not seldom the branches, grow in a direction tangential to the 

 earth's surface, and thus exhibit transverse geotaxis. 



Loeb (1888) discovered that geotaxis is a phenomenon widely 

 diffused among animals also. It is possible to convert animals 

 that exhibit negative, into animals ex- 

 hibiting positive, geotaxis, and vice versa. 



Many infusoria and 'bacteria exhibit 

 geotactic phenomena. They frequently 

 collect on the surface of the water in 

 which they live (negative geotaxis, Fig. 

 20) ; at other times they sink down and 

 crowd together at the bottom (positive 

 geotaxis). 



Knight (1809) showed that geotactic 

 phenomena are determined by differences 

 in pressure acting like gravity on the 

 different points of the vegetable organ- 

 ism. He employed wheels turning in 

 a vertical plane, to which he attached 

 plants in various positions, as well as 



r , . .. FIG. -20. Glass tube containing Para- 



germmatmg seeds. He found that all 



the stems grew in towards the centre 



of the wheel, while the roots grew away 



from it. Jensen practically repeated the 



same experiments on infusoria living at the surface of the water, 



by rotating the test tubes which contained them in the centrifuge. 



Provided this were not driven too quickly, so as to make the 



infusoria, which are specifically heavier bodies, drop to the bottom 



of the test tube, they remained at the top, where pressure is lowest 



during rotation. 



IX. Heat rarely exerts any direct stimulating action on living- 

 matter. In the higher animals, however, the special terminal 

 organs of certain centripetal nerve-fibres are excited by heat. 

 Kiihne was the first to observe thermal tetanus in Amoebae 

 when the temperature was raised to 35 C. On cooling the 

 atmosphere again, the amoeboid movements were slowly restored ; 

 heating to 40-45 C. kills the animal by coagulation of its proto- 

 plasm. 



When the heat acts on one part only of the amoeba, the 



moecia. (Jensen.) Inconsequence 

 of negative geotaxis, the infusoria 

 have collected at the top of the 

 tube, although they are specifically 



heavier than the fluid. 



