CHAP. IX. SUMMARY OF CHAPTER. 485 



lately such movements were believed to result simply 

 from increased growth on the shaded side. At present 

 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. 

 But 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 by the two sides; and that move- 

 ment is thus caused without being followed by in- 

 creased growth on the more turgescent 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 GodlewsM has given 63, 123, &c. Frank has also 



('Bot. Zeitung,' 1879, Nos. 6-9) insisted ('Die Naturliche wa- 



an excellent account (p. 120) of gerechte Riclitung von Pflan- 



the present state of the question. zentheilen,' 1870, p. 53) on the 



See also Vines in 'Arbeiten des important part which the pulvini 



Bot. Inst. in Wiirzburg,' 1878, B. of the leaflets of compound leaves 



ii. pp. 114-147. Hugo de Vries play in placing the leaflets in a 



has recently published a still proper position with respect to the 



more important article on this light. This holds good, especially 



subject : 4 Bot. Zeitung,' Dec. 19th. with the leaves of climbing plants, 



and 26th, 1879. which are carried into all sorts 



t ' Die Periodischen Bewegun- of positions, ill-adapted for the 



gen der Blattoryane,' 1875, pp. 7, action of the light. 



