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. (Czapek
(Pringsheim's "Jahrb." XXXV. 1900, page 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.)

Haberlandt (Pringsheim's "Jahrb." XLV. 1908, page 575.) 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 apparently 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. (By using
Piccard's method I have succeeded in showing that the gravitational
sensitiveness 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.)

Another observation of Darwin's has given rise to much controversy.
("Power of Movement", page 133.) 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