CHAPTER XIV. 



IRRITABILITY. 



196. We should now examine more carefully certain move- 

 ments which the members of the plants exhibit. By this 

 time we have probably observed that the direction which the 

 root and stem take upon germination of the seed is not due to 

 the position in which the seed happens to lie. Under normal 

 conditions we have seen that the root grows downward and the 

 stem upward. 



197. Influence of the earth on the direction of growth. 

 When the stem and root have been growing in these directions 

 for a short time let us place the seedling in a horizontal position, 

 so that the end of the root extends over an object of support in 

 such a way that it will be free to go in any direction. It should 

 be placed under a bell jar so as to prevent drying, or a germi- 

 nated pea may be pinned to the lower side of a cork, which is 

 then placed in the mouth of a bottle containing a little water. 

 In the course of twelve to twenty- four hours the root which was 

 formerly horizontal has turned the tip downward again. If we 

 should mark off millimeter spaces beginning at the tip of the 

 root, we should find that the motor zone, or region of curvature, 

 lies in the same region as that of the elongation of the root. 



It was found by Knight, as a result of experiments, that the 

 force which causes the roots to take the downward direction is 

 gravity. This force is geotropism, which means a turning in- 

 fluenced by the earth, and is applied to the growth movements 

 of plants influenced by the earth, with regard to the direction 

 of growth. Growth toward the earth is also termed progeotro 



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