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volved at a speed that made the centrifugal force equal that of gravity, 

 both roots and stems grew obliquely, taking the position of a resultant of 

 the two forces, that is, of forty-five degrees to the vertical. 



But this discovery of Knight's was not very fruitful, for no one could 

 tell how gravity could produce the effect ascribed to it. If it pulled the 

 root down, why did it push the stem up? The stem is as heavy as the 

 root, why are not both attracted toward the center of the earth ? It was 

 a curious paradox to say that the same force acted now one way and now 

 exactly the reverse on different parts of the same plant; as if pulling and 

 pushing were the same thing. It was supposed that gravity acted upon 

 the root as it does upon a mass of taffy candy, drawing it downward. 

 But Sachs showed in 1873 that the root of a bean fixed horizontally over 

 mercury could penetrate the mercury in assuming a vertical position. As 

 mercury is thirteen and a half times as heavy as water, or the tissue of a 

 young root, it is evident that far more force was expended in penetrating 

 the mercury than could have been derived from the physical action of 

 gravity, that, is, from the simple weight of the root. The experiment has 

 since been tried in another and more obvious way by harnessing a root 

 tip lying horizontal to a weight suspended over a pulley, the weight being 

 raised as the root bends downward in response to gravity. From these 

 experiments we must conclude that gravity does not act physically but 

 physiologically to induce the curvature, that is, it acts as a stimulus. It is a 

 small spark that fires the gun. The same spark will fire a pistol or a can- 

 non, the result depending solely upon the amount and arrangement of 

 the explosive material. So in the root, if there is the proper mechanism 

 and storage of force, gravity will release this force and cause the bending, 

 the amount of work done being enormously out of proportion to the initial 

 expenditure of energy. But when the bending takes place, will it be up- 

 ward or downward? If it were a purely mechanical device, it is evident 

 that by knowing the structure of the organ, one could predict the direc- 

 tion of movement under stimulation. But we shall have to look beyond 

 and above simply mechanical laws for an explanation. The wooden horse 

 could not have destroyed Troy without a guiding principle within more 

 intelligent and effective than mechanical force. 



But in attempting to solve the problem, do not let us attempt too much. 

 Let us accept such an explanation as we would consider satisfactory in 

 case of a similar problem regarding the behavior of an animal. To see 

 with our eyes and not with our fingers, to hear only with our ears, taste 



