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PLANT PHYSIOLOGY AND ECOLOGY 



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possesses a specific sensibility to gravity, just as it does to light. 

 When a change in the position of the plant occurs, the normal rela- 

 tion of the cytoplasm to gravity is modified, and changes are set 

 up in it that tend to restore the normal. The perception of the 

 stimulus of gravity by the root and the stem takes place largely 

 in the meristem of the tip. In the root the sensory zone is scarcely 

 more than a millimeter wide, comprising the tip alone. This zone 

 in the stem is less restricted, the entire meristem of the apex being 

 sensitive. In addition, the cells of the cortical parenchyma, and 

 sometimes those of the pith, are capable of perceiving the stimulus. 

 In the case of leaves and flowers, it is probable that the power of 

 perception resides in all the living cells. 



The length of time for which a plant must be changed from its 

 normal position before a response to gravity becomes evident is 



termed the reaction time. It is a curious 

 fact that only a part of this time is neces- 

 sary for the perception of the stimulus. If a 

 plant is put in a horizontal position for a 

 quarter of an hour, and then placed upright, 

 geotropic curvature will still occur. Con- 

 sequently, reaction time consists of two 

 periods, one necessary for perception, the 

 other for reaction. 



159. Region of curvature. The geotropic 

 curvature of the root is effected in a zone 

 scarcely wider than the sensory zone and 

 lying just behind it. The manner in which 

 the stimulus is transmitted from the one to 

 the other is unknown. The actual bending 

 is due to the elongation of the cells upon 

 the upper or converse side, and is accom- 

 panied by the compression of those upon the 

 concave surface. The region of curvature in 

 the stem is much more extensive than that 

 of the root. The reason for this difference is 

 apparently to be found in the fact that the 

 major portion of the root is held firmly by 

 the soil, while the stem is free to curve. The curvature of the 

 stem is first apparent in the region of rapid growth just below 

 the tip. It travels downward in such a manner that, as it ap- 



FiG. 41 . The region of 

 curvature in the pri- 

 mary root of the 

 horse-bean, Vicia 

 faba. In A, the root 

 was marked into five 

 areas of 2 mm. each; 

 B shows the growth 

 and curvature at the 

 end of two hours, and 

 C, at the end of 

 twenty-three hours. 

 (After Sachs.) 



