38 BIOLOGY 



for a time, the roots gradually bent downwards towards 

 the earth and the shoots upwards away from it. This 

 is, the roots were positively geotropic, the shoots nega- 

 tively so. 



Another experiment which can be carried out without 

 much trouble and gives excellent results, is as follows : 

 Moisten some mustard-seeds and throw them against the 

 inside of a damp, empty flower-pot, where they v/ill 

 adhere and germinate. The pot is then turned upside 

 down over some damp sawdust and covered with a wet 

 cloth. If the pot be examined after a few days, it will 

 be seen that the roots have turned downwards, adhering 

 to the sides of the pot, while the shoots have grown 

 upwards but not in contact with the sides of the pot. 

 Now place the pot in its normal position, cover it over 

 with a dark cloth and leave it for a couple of days ; the 

 roots and shoots will then be seen to have curved round 

 through an angle of 180 degrees and to have regained 

 the original position, shoots upwards, roots downwards. 



Among animals geotropic reactions are not so easily 

 distinguished. Among the plant-like forms of the 

 Coelenterates numerous cases of marked geotropism are 

 recorded. Loeb has found that some of the Holothurians 

 are also negatively geotropic. But while definite positions 

 with relation to gravity are assumed by all the higher 

 forms, the mechanism connected therewith is so complex 

 that it is impossible to say in how far they are geotropic. 



Heliotropism, or the response to photic stimulation, 

 is common among animals and plants alike. Everyone 

 who has attempted to keep pot plants in a window 

 knows how the shoots and leaves grow towards the 

 window, thus necessitating constant turning if the 



