CHAP. IX. SUMMARY OF CHAPTER. 485 



lately such movements were believed to result simply 

 from increased growth on the shaded side. At presenl 

 it is commonly admitted * that diminished light in- 

 creases the turgesceiice 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, 

 ]>ut 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.f 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 Godlewski has given G3, 123, &c. Frank htis also 



('Bot. Z.-ifung,' 1879, Nos. G-'J) insisted ('Die J^atnrliche \vii- 



.TII excellent account (p. 120) of gcrechte Riclitnng von Pfl-i'i- 



the present state of the qm-stion. zentheik-n,' 1870, p. 53) on the 



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



Hot. List, in Wurznurg,' l:s7-\ B. of'tlie leaflets of compound leaves 



U. pp. 114-147. Hugo de Vries play in placing the ,'eaflets in a 



JIM recently published a still proper po.-ition with respect to tho 



ir.ore important articlo on this light. This holds good, especially 



.-uhject : ' Bot. Zeitung,' Dec. 19th with the leave- of climbing plants, 



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



t ' Die Poriodischen Bewegun- of positions, ill-adapted for th 



gen der Blattoruaac,' J875, pp. 7, nctiun of the light. 



