396 The Movements of Plants 



and roots curve in the same direction. The seedling is in the lower 

 world, but its tip containing the supposed sense-organ is in the 

 strange world where roots curve upwards. By observing whether 

 the root bends up or down we can decide whether the impulse to 

 bend originates in the tip or in the motile region. 



Piccard's results showed that both curvatures occurred and he 

 concluded that the sensitive region is not confined to the tip 1 . 



Haberlandt 2 has recently repeated the experiment with the 

 advantage of better apparatus and more experience in dealing with 

 plants, and has found as Piccard did that both the tip and the 

 curving region are sensitive to gravity, but with the important 

 addition that the sensitiveness of the tip is much greater than that 

 of the motile region. The case is in fact similar to that of the oat 

 and canary-grass. In both instances my father and I were wrong 

 in assuming that the sensitiveness is confined to the tip, yet 

 there is a concentration of irritability in that region and transmission 

 of stimulus is as true for geotropism as it is for heliotropism. Thus 

 after nearly thirty years the controversy of the root-tip has ap- 

 parently ended somewhat after the fashion of the quarrels at the 

 Rainbow in Silas Marner "you're both right and you're both 

 wrong." But the " brain-function " of the root-tip at which eminent 

 people laughed in early days turns out to be an important part 

 of the truth 3 . 



Another observation of Darwin's has given rise to much con- 

 troversy 4 . If a minute piece of card is fixed obliquely to the tip of 

 a root some influence is transmitted to the region of curvature and 

 the root bends away from the side to which the card was attached. 

 It was thought at the time that this proved the root-tip to be 

 sensitive to contact, but this is not necessarily the case. It seems 

 possible that the curvature is a reaction to the injury caused by the 

 alcoholic solution of shellac with which the cards were cemented to 

 the tip. This agrees with the fact given in the Power of Movement 

 that injuring the root-tip on one side, by cutting or burning it, 

 induced a similar curvature. On the other hand it was shown that 

 curvature could be produced in roots by cementing cards, not to the 

 naked surface of the root-tip, but to pieces of gold-beaters skin 



1 Czapek (Pringsheim's Jahrb. xxxv. 1900, p. 362) had previously given reasons for 

 believing that, in the root, there is uo sharp line of separation between the regions of 

 perception and movement. 



2 Pringsheim's Jahrb. XLV. 1908, p. 575. 



3 By using Piccard's method I have succeeded in showing that the gravitational sensi- 

 tiveness of the cotyledon of Sorghum is certainly much greater than the sensitiveness of 

 the hypocotyl if indeed any such sensitiveness exists. See Wiesner's Festschrift, Vienna, 

 1908. 



4 Power of Movement, p. 133. 



