Growth Movements 497 



normal position of a sensitive structure is shifted, or when 

 this member comes under the influence of new or intensified 

 forces, or of forces acting from a new direction. The 

 curvature of horizontally placed roots toward the earth 

 and the bending of the hypocotyl of a seedling exposed to 

 one-sided illumination are familiar illustrations. 



The importance of a means of orientation in order to 

 assume or restore the normal is obvious. If the reaction 

 of the growing organ results in orientation parallel to the 

 direction of the exciting force, the organ is parallelotropic, 

 while one assuming a position at an angle to the direction 

 of the stimulus is plagiotropic, diatropic being at right 

 angles to the path of the stimulus. The tropic movements 

 here discussed are effected in growing structures, or those 

 in which growth may be initiated, and they cease with 

 incapacity for growth. Moreover, the stimulus remaining 

 the same, the rapidity of growth determines the prompt- 

 ness of the response, or reaction time. 



Attention has been directed to the tropic responses of 

 plants to light (phototropism) and to heat (thermotropism) . 

 Other stimuli inducing reactions are such as gravity (ge- 

 otropism), contact (thigmotropism), moisture (hydrotro- 

 pism), electricity (electrotropism), and certain chemical 

 agents (chemotropism). 



302. Geotropism. The vertical position of the main 

 axis of most plants is as apparent as the erect posture of 

 man. A seed may be planted in any position in the soil ; 

 but as soon as sufficient growth is made the parallelotropic 

 position of the axis is assumed, with the root directed 

 straight downwards (positively geotropic) and the stem 

 directly upwards (negatively geotropic). Germinating 



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