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 1s Mm 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’. 
Haberlandt? 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’. 
Another observation of Darwin’s has given rise to much con- 
troversy*. 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 no sharp line of separation between the regions of 
perception and movement. 
2 Pringsheim’s Jahrb. xiv. 1908, p. 575. 
8 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. 
* Power of Movement, p. 183. 
