GEOTROPISM 125 



ivy (Hedera helix). This is negative phototropism. 

 Too great intensity of light may cause a positively 

 phototropic organ to become negatively phototropic. 

 A very small amount of light scarcely perceptible to 

 the human eye is sufficient to induce phototropic cur- 

 vature in some plants. The effective rays of light 

 are those of the blue and violet portion of the spec- 

 trum. The perceptive region may be some distance 

 from the region of curvature. Thus in the seedlings of 

 oats the tip of the first leaf is the perceptive region while 

 the curvature takes place at a point near the ground. 



186. Geotropism. If a seedling that usually grows 

 upright be placed in a horizontal position for a few hours 

 the tip of the stem will be found to be curved so as to be 

 directed upward, while the tip of the root will have 

 assumed a position directed downward, the remainder 

 of the stem and roots being horizontal. If the root tip 

 and stem tip have been previously marked with cross 

 lines at equal distances it will be found that the curvature 

 begins and is carried out by those regions of stem and root 

 respectively where the growth is usually most rapid and 

 the curvature has taken place by the more rapid growth 

 on one side than on the other. The main root, then, is 

 positively geotropic and the stem negatively so. 



187. If the plant has been allowed to grow until 

 horizontal lateral roots have been produced and then is 

 placed with the main stem horizontal it will be found 

 that not only does the main stem curve upward and the 

 main root downward, but that the lateral roots, which 

 are now of course some of them directed upward and some 

 downward, will curve so as to occupy a horizontal position 

 again. Thus it is apparent that some stimulus causes 

 certain plant parts to grow toward, other parts away from 

 and still others parallel to the surface of the earth. Care- 



