GEOTROPISM 459 



driving motors and hence their speed is determined by the intensity 

 of the Hght falling on the lenses. The steering magnets arc opposed, 

 i.e. if both seleniinn cells are illuminated equally, both magnets 

 will receive the same current and the steering wheel will lie parallel 

 to the driving wheels. If more light falls on one selenium " retina " 

 than on the other, the former has its power to conduct electricity 

 increased in proportion to the relative increase in intensity ; 

 consequently the magnets controlling the position of the rear 

 wheel are activated asynmietrically. The wheel is pulled over 

 to make an angle with the previous line of traction of the " dog." 

 The mechanism is so arranged that this steering movement turns 

 the machine towards the light. It will continue to turn till both 

 lenses are equally illuminated. As soon and as long as both 

 " eyes " are equally illuminated in sufficient intensity, the machine 

 moves in a straight line towards the source of the light. The 

 apparatus is fitted with a reversing switch which will convert it 

 from a positively to a negatively heliotropic machine. 



If, say, a portable electric light be turned on in front of the 

 machine it will immediately start to follow (or run from) the light 

 at a speed which may attain to 100 yds. per minute. On reducing 

 the intensity of the light, the " dog " will slow down, and on 

 switching off, it will stop. In this way the machine follows a 

 lantern in a dark room just like a positively heliotropic animal. 

 By reversing the direction of the current one may make the 

 machine negatively heliotropic. 



II. Geotropism. Most animals so orientate themselves that 

 their plane of symmetry passes through the centre of the earth. 

 They, therefore, either move towards the earth and are positively 

 geotropic like the tap roots of plants or, more commonly, are 

 negatively geotropic and climb. Rodless mice or other blind 

 animals if placed in any position on a steeply inclined plane, after a 

 few orientating movements, move straight up the plane. The 

 steeper the plane, the fewer are the initial exploratory movements. 



III. Stereotropism is the term applied to the tendency of certain 

 organisms to bring their bodies as much as possible on all sides 

 in contact wdth solid bodies. " The butterfly Amjjhipijra, which 

 is a fast runner, will come to rest under a glass plate when the 

 plate is put high enough above the ground so that it touches the 

 back of the butterfly." Man orientates himself partly by apprecia- 

 tion of the tactile influences on the soles of the feet. When these 

 are weakened as in locomotor ataxia, and when the orientating 

 influence of the eyes is removed, the patient finds difliculty in 

 standing and in walking (Romberg's sign). 



IV. Chemotropism plays an important part in the life of the 



