Chap, IX. SUMMAEY OF CHAPTEll. 485 



lately such movements \\'ere believed to result simply 

 from increased growth on the shaded side. At preseni 

 it is commonly admitted * that diminished light in- 

 creases the turgescence of the cells, or the extensibility 

 of the cell-walls, or of both together, on the shaded 

 side, and that this is followed by increased growth, 

 Jiut Pfeffer has shown that a difference in the tur- 

 gescence on the two sides of a pulvinus, — that is, an 

 aggregate of small cells which have ceased to grow at 

 an early age, — is excited by a difference in the amount 

 of light received bv the two sides ; and that move- 

 ment is thus caused without being followed by in- 

 creased growth on the more tiu-gescent side.t All 

 observers apparently believe that light acts directly 

 on the part which bends, but we have seen with the 

 above described seedlings that this is not the case. 

 Their lower halves were brightly illuminated for hours, 

 and yet did not bend in the least towards the light, 

 though this is the part which under ordinary circum- 

 stances bends the most. It is a still more striking 

 fact, that the faint illumination of a narrow stripe on 

 one side of the upper part of the cotyledons of Phalaris 

 determined the direction of the curvature of the lower 

 part ; so that this latter part did not bend towards the 

 bright light by which it had been fully illuminated. 



* Emil Goillewsld has given GS, 12;-!, &o. Fiiiiik hns also 



t'Bot. Ziituiig,' 1879, Nos. 6-'J) inoisti d ('Die I\atiirliche wii- 



au excellent account (p. 120) of gireulite Eichtung von Pflnu- 



• the present state of the question. zc-ntlieikn,' 1870, p. 53) on the 



Si'e also Vines in 'Arbeiteii des important part which the pulvini 



Hot. Inst, in Wiirz'nir';,' ls7'^. B. of tlic leattets of compound h avca 



ii. pp. 114-147. Hugo du Vries play in placing tie icatlits in a 



hns recsntly published a still proper po.-ition wih respect t" the 



more important article on tliis lif^ht. This holds gooil, especially 



fculiject : ' Bot. Zeitung,' Dec. 19tli with the leave- of climbing plants, 



and 26th, 1K79. ■which are carried into all sorts 



t ' Uio Periodischen Bewegun- of positions, ili-adapted for tha 



^en der Blattorgaue,' Jb75, pp. 7, action of the hght. 



