2 INTRODUCTION. 



axes directed to different points of the compass. 

 Whilst describing such figures, the apex often travels 

 in a zigzag line, or makes small subordinate loops oi 

 triangles. In the case of leaves the ellipses are 

 generally narrow. 



Until recently the cause of all such bending move- 

 ments was believed to be due to the increased growth 

 of the side which becomes for a time convex ; that this 

 side does temporarily grow more quickly than the 

 concave side has been well established ; but De Yries 

 has lately shown that such increased growth follows 

 a previously increased state of turgescence on the 

 convex side.* In the case of parts provided with a 

 so-called joint, cushion or pulvinus, which consists of 

 an aggregate of small cells that have ceased to 

 increase in size from a very early age, we meet with 

 similar movements; and here, as Pfeffer has shown f 

 and as we shall see in the course of this work, 

 the increased turgescence of the cells on opposite 

 sides is not followed by increased growth. Wiesner 

 denies in certain cases the accuracy of De Vries' con- 

 clusion about turgescence, and maintains} that the 

 increased extensibility of the cell- walls is the more 

 important element. That such extensibility must 

 accompany increased turgescence in order that the part 

 may bend is manifest, and this has been insisted on by 

 several botanists ; but in the case of unicellular plants 

 it can hardly fail to be the more important element. 

 Or. the whole we may at present conclude that in- 



• Sachs first showed CLchr- 19, 1879. p. 830. 



buoh,' &o., 4th edit. p. 452) the t ' Die Periodisclien Bewegun- 



intimate connection between tur- gen der Blattorgane,' 1875. 



gescenoe and growth. For De | ' Untersuchungen uber den 



Vries' interesting essay, ' Wachs- Heliotropismus,' Sitzb. der K. 



thumskrummungen mehrzelliger Akad. derWissenschaft. (Vienna), 



Organe,' see ' Bot. Zeitung,' Deo. Jan. 1880. 



