Chap. IX. SUMMAEY OP CHAPTEE. 485 



lately such movements were 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. 

 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 Pbalaris 

 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 Godlewsti has given 63, 123, &c. Frank has also 



('Bot. Ztitung,' 1879; Nos. 6-9) insist< d ('Die ^'atu^liche wa- 



an excellent account (p. 120) of gi reulite Eichtung von Pflaii- 



the present state of Ihu question. zentheilin,' 1870, p. 5.S) on the 



See also Vines in ' Arbeiten des impoi bmt part which the pulvinl 



Bot. Inst, in WUrzburg,' ls7-'. B. of tlie leaflets of compouiid leaves 



ii. pp. 114-147. Hugo do Vries play in placing ti.e leaflets in a 



h*3 recently published a still proper position wirh respect to tho 



ir.ore important article on this liKht. This holds good, especially 



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



and 26th, 1H79. which are carried into nil sorts 



t ' Die Pcriodischen Bewegun- of positions, ill-adapted for tha 



gen der Blattorgane,' 1875, jip. 7, aiti.m of the ight. 



