GR< 'Will AND M< <\ i Mi N T 



461 



to a reverse curvature, ami tliis also, by rea on of continued stimulation 

 during the long reaction time, may again carry the tip past tin- vertical; 



thus, only by a series 

 of pendulum-like 



swings is tlu' 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 affe< ts 



1 h.. 6 ( )o. — Successive positions, from photographs, <>f 

 Impatiens glanduligera in erecting itself from tin- horizon- 

 tal. — 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 1 mm. ami then 



fixed in a horizontal position, it will be found aftersome 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 



i/ V. sci ond and third divisions have grown most, though the c hief 



urvature still persists in the fourth division that was grow- 



ing most rapidly (figs. 691-603). 



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 sari- 

 able. The shortest presentation time recorded for 

 geotropic curvature is 2-3 minutes (cut shoots of 

 Capsella, hypocotyls of Helianthus, and peduncles 

 placed horizon- <>f PlatUago). In many plants it is 15 25 minutes; 



t.d; 69a, seven hours j n | t ._ sensitive plants it is double or treble this, or 

 later ; 6,,;,, twenty- ' 



three hours later.— even extends to several hours. Moth periods are 

 After S.miis. greatly inlhicnced by temperature. Thus, a seedling 



of Vicia Faba, having at 14 C. a presentation time of 70 minute- and a 

 reaction time of 120 minutes, hail these periods at 30 C. respectively 



Figs. 601-60 3. — 

 ( reol ropii 1 urvatureof 

 a root of Vit iii Faba 

 60 



