CHEMOTROPISM 



111 



eflfective stimulus. Second, the turning is brought about 

 by growth and not by contraction, as in the muscles of 

 animals. Such turnings by growth in response to the 

 direction of gravity are technically known as geotropism 

 (Fig. 22). 



Chemotropism. — Next to directing roots into the soil 

 is the need to direct them to that part of it where water 

 and the necessary soil nutrients are to be found in the 

 most suitable quantity. It is a familiar fact that roots 

 are likely to find sewers in a dry soil and clog them 



Fig. 22. — Geotropism of Roots. Fig. l.a, shows a bean with a 

 young root marked into lengths of 2 mm. each and then placed in a 

 horizontal position. 1.6, shows the same root a day later. A com- 

 parison of the two figures shows that the greatest amount of elonga- 

 tion has taken place in the 6 mm. region immediately behind the root 

 cap. In Fig. II. a, the tip was cut off along the Hne ab about 30 minutes 

 after the root was placed in position. II.6, shows that the root never- 

 theless elongated and bent down toward the earth. Fig. Ill.a, shows 

 a root from which the tip was removed before being placed in posi- 

 tion. It elongated, Fig. III.6, but it did not bend. The three experi- 

 ments together show that the stimulus of gravity is received at the 

 tip but that the response by growth occurs in the slightly older part. 



with a great mass of rootlets. That they are sensi- 

 tive to water and dissolved salts can also be readily 

 shown by experiment in the laboratory. If seeds are 

 planted in a vessel so that the young roots grow out 

 through the bottom into dry air, they turn back, in spite 



