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 growtn impatiens glanduligera in erecting itself from the horizon- 



and gradually affects tal. — After Pfeffer. 



less active regions, becoming permanent finally as the tissues of the 



growing region most remote from the apex cease to grow. 



^ II'- 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 



/jr T^ divisions; after twenty-four hours it is easy to see that the 



iy \ 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). 



/\^ 693 .„ . . 



/ A Presentation tune. — 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 

 geotropic curvature is 2-3 minutes (cut shoots of 

 Geotropic curvature of Qapsella, hypocotyls of Helianthus, and peduncles 



a root of Vicia Faba .• -r ' .' f -f ' ^ 



691, placed horizon- of Plafitago). In many plants it is 15-25 minutes; 

 tal ; 692, seven hours j^ iggg sensitive plants it is double or treble this, or 



later ; 693, twenty- t> 1 • j 



three hours later. even extends to several hours. Both penods 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 



U 



Figs. 691-693. 



