GROWTH AND MOVEMENT 



461 



Fig. 690. — Successive positions, from photographs, of 

 Impaiiens gland uligera in erecting itself from the horizon- 

 tal. — After Pfeffer. 



to a reverse curvature, ami iliis 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 Impaiiens 

 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. 



■>p ■ ■ ■ — That the curvature appears in the region of most active 



IT ' elongation is clearly shown by the behavior of certain roots. 



If a suitable one be marked at intervals of 1 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 



f/C 692 divisions; after twenty-four hours it is easy to see that the 



t/ \ second and third divisions have grown most, though the chief 



curvature still persists in the fourth division that was grow- 



y^\_ ing most rapidly (figs. 691-693). 



/ V' 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 

 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; 

 in less sensitive plants it is double or treble this, or 

 even extends to several hours. Both periods are 

 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. 691-693. — 

 Geotropic curvaturrof 

 a root of Vicia Faba : 

 691, placed horizon- 

 tal; 692, seven hours 

 later ; 693, twenty- 

 three hours later. — 

 After Sachs. 



