494 MANUAL OF BOTANY 



checked. This explanation is directly contradicted by the 

 phenomenon of apheliotropism as exhibited by a root. It is 

 moreover proved to be an insufficient explanation by the fact 

 that the part which is sensitive to the stimulus is not the part 

 which actually bends. Darwin showed this by amputating 

 a small region about *! inch in length from the tip of the 

 seedling, or by lightly covering the same part with an opaque 

 cap, when he found that the heliotropic curvature did not take 

 place. Further, when the part normally curving under the 

 influence of the stimulus is mechanically hindered from bending, 

 the curvature takes place at a part a little lower down, which 

 normally remains straight. 



When the lateral light is fairly intense, the resulting move- 

 ment takes place uninterruptedly ; when it is only weak, the 

 position is assumed by a series of zigzag movements, indicating 

 that the new movement is an exaggeration of the ordinary 

 circumnutation of the part. When the final position is reached 

 the organ is found to circumnutate about the new direction of 

 the axis. 



A somewhat similar response to a lateral light is exhibited by 

 many unicellular organisms. When these are exposed to oblique 

 illumination thej^ take up a definite position with regard to the 

 incident rays, placing their long axis parallel to them if the light 

 is weak, and at right angles to them if it is intense. This 

 behaviour is known as Phototaxis ; it is exhibited by the 

 zoospores of many of the Algae and by certain Desmids. 



Geavity. — The force of gravity exerts an influence upon 

 plants somewhat resembling that of lateral illumination. Thus 

 most stems grow vertically upwards into the air ; primary roots 

 grow vertically downwards into the soil. A few organs, such as 

 certain rhizomes, the stolons of many plants, and most secondary 

 roots, grow at right angles to the direction of gravity. When one 

 of these is placed at an angle from the position it usually takes, 

 a curvature of the growing region results, which lasts till the 

 normal attitude is regained. Thus when a young seedling is 

 detached from the earth and laid upon its side, the stem 

 gradually curves through an angle of 90^ and becomes erect, 

 while the young root curves in the opposite direction till it 

 points vertically downwards. Similarly when a stolon is placed 

 vertically, its apex is slowly deflected until it is parallel with the 

 soil. These movements are termed apogeotropism, geotrojpisni, 

 and diageotrojpism respectively. 



To prove these movements to be responses to the stimulus 



