GROWTH AND MOVEMENT 



461 



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 



. . FIG. 690. Successive positions, from photographs, of 



01 most active growth I m p a tiens glanduligera in erecting itself from the horizon- 

 and gradually affects tai. After PFEFFER. 



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 

 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- 

 ing most rapidly (figs. 691-693). 



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 

 691-693. geotropic curvature is 2-3 minutes (cut shoots of 

 Capsella, hypocotyls of Helianthus, and peduncles 

 of Plantago). In many plants it is 15-25 minutes; 

 tai ; 692, seven hours j n i ess sen sitive 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 Vicia 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 



FIGS. 



691, placed horizon- 



