GROWTH AND MOVEMENT 
461 
FIG. 690. Successive positions, from photographs, of 
Impatiens glanduligera in erecting itself from the horizon- 
tal. After PFEFFER. 
to a reverse curvature, and this also, by reason of continued stimulation 
during the long reaction time, may again carry the tip past the vertical; 
thus, only by a series 
of pendulum-like 
swings is the position 
of equilibrium at- 
tained. The succes- 
sive positions of the 
stem of Impatiens 
shows the way in which 
such a stem erects it- 
self (fig. 690). It shows 
also that the curvature 
begins in the region 
of most active growth 
and gradually affects 
less active regions, becoming permanent finally as the tissues of the 
growing region most remote from the apex cease to grow. 
That the curvature appears in the region of most active 
elongation is clearly shown by the behavior of certain roots. 
If a suitable one be marked at intervals of i mm. and then 
fixed in a horizontal position, it will be found after some hours 
.^~ ' ' ' that curvature is taking place in the third and fourth of these 
//k*' 692 divisions; after twenty-four hours it is easy to see that the 
**/ \. second and third divisions have grown most, though the chief 
curvature still persists in the fourth division that was grow- 
n __ ing most rapidly (figs. 691-693). 
r693 
Presentation time. It is not necessary to con- 
tinue stimulation until the reaction appears. In 
other words reaction time is longer, than presenta- 
tion time. These periods are, of course, very vari- 
able. The shortest presentation time recorded for 
FIGS. 691-693. geotropic curvature is 2-3 minutes (cut shoots of 
Geotropic curvature of C apsdla, hypocotyls of Hdiemthus, and peduncles 
a root of VICM Faba : r J r J 
691, placed horizon- of Plantago). In many plants it is 15-25 minutes; 
tai; 692, seven hours j n i ess sensitive plants it is double or treble this, or 
later ; 693, twenty- 
three hours later. _ even extends to several hours. Both periods are 
After SACHS. greatly influenced by temperature. Thus, a seedling 
of View Faba, having at 14 C. a presentation time of 70 minutes and a 
reaction time of 120 minutes, had these periods at 30 C. respectively 
