200 GROWTH HORMONES IN PLANTS 



shown by the fact that when reheaded and placed vertically, the 

 root curved toward the side that had been lowermost when the 

 root was horizontal (Keeble, Nelson, and Snow, 1929). Although 

 it has been shown that the growth of the root is in no way retarded 

 by decapitation, the possibility still exists that the ability of the 

 growing zone to react is lessened by decapitation. It would be 

 far more satisfactory to have the localization of sensitivity in the 

 tip demonstrated by experiments with intact plants, but such a 

 requirement is much harder to fulfill for geotropic than for photo- 

 tropic stimulation. Czapek (1895) tried to obtain data along this 

 line with the use of his well-known tube experiments. Haber- 

 landt (1908), Jost (1912), and Dewers (1914), using the method of 

 Piccard, showed that if the tip and basal regions of a root were 

 stimulated in opposite directions on a centrifuge, the whole root 

 reacted uniformly in response to the stimulation of the tip, pro- 

 vided the length of this tip was 1.5 to 2 mm. It may be con- 

 cluded that the tip of the root is more sensitive to the effect of 

 gravity than the proximal regions and that the stimulus must be 

 transmitted from the tip into the zone of reaction. 



Stimulus Transmission. — Snow" (1923, 1924a) gave the first 

 direct proof for transmission of stimulus in the root. He showed 

 that when the root tips of Vicia Faba were removed after geotropic 

 stimulation and then replaced on the cut surfaces with gelatin, 

 geotropic curvatures took place. Of 76 roots treated in this way, 

 45 showed positively geotropic curvatures after 15 to 24 hours, the 

 mean size of the curvature amounting to about 30 deg. Of 32 

 decapitated roots, on which the tip was not replaced, 27 remained 

 straight, 4 showed a weak positive curvature, and only 1 exhibited 

 a strong curvature. Snow concluded that the geotropic stimulus 

 can be transmitted over a wound surface, and his work was 

 confirmed by Boysen Jensen (1933c). In decapitated roots of 

 Vicia Faha (variety Windsor White), the d value of the geotropic 

 curvature which took place in 24 hours amounted to 0.04 mm. 

 (average of 36 plants) ; in decapitated roots with replaced tips, it 

 amounted to 0.44 mm. (average of 39 plants). 



Snow determined upon which side of the root the stimulus was 

 transmitted. A transverse incision was made either upon the 

 upper or on the under side of roots of Vicia Faha, and a mica plate 

 was inserted in the wound in order to check the transmission of 

 the stimulus over the wound. The roots were placed in a hori- 



