1G4 REVIEWS. 



the motion at the point of contact is necessarily arrested, but 

 the free projecting part goes on revolving. Almost immedi- 

 ately another and upper point of the shoot is brought into 

 contact with the support and is arrested ; and so onward to the 

 extremity of the shoot ; and thus it winds round its support. 

 When the shoot follows the sun in its revolving course, it 

 winds itself round the support from right to left, the support 

 being supposed to stand in front of the beholder ; when the 

 shoot revolves in an opposite direction, the line of winding is 

 reversed. As each internode loses from age its power of 

 revolving, it loses its power of spirally twining round a sup- 

 port. If a man swings a rope round his head, and the end 

 hits a stick, it will coil round the stick according to the direc- 

 tion of the swinging rope ; so it is with twining plants, the 

 continued contraction or turgescence of the cells along the 

 free part of the shoot replacing the momentum of each atom 

 of the free end of the rope. 



" All the authors, except Von Mold, who have discussed the 

 spiral twining of plants maintain that such plants have a nat- 

 ural tendency to grow spirally. Mohl believes (S. 112) that 

 twining stems have a dull kind of irritability, so that they 

 bend toward any object which they touch. Even before 

 reading Mohl's interesting treatise, this view seemed to me 

 so probable that I tested it in every way that I could, but 

 always with negative results. I rubbed many shoots much 

 harder than is necessary to excite movement in any tendril or 

 in any foot-stalk of a leaf-climber, but without result. I then 

 tied a very light forked twig to a shoot of a Hop, a Ceropegia, 

 Sphaerostema, and Adhatoda, so that the fork pressed on one 

 side alone of the shoot and revolved with it ; I purposely 

 selected some very slow revolvers, as it seemed most likely 

 that these would profit from possessing irritability ; but in no 

 case was any effect produced. Moreover, when a shoot winds 

 round a support, the movement is always slower, as we shall 

 immediately see, than whilst it revolves freely and touches 

 nothing. Hence I conclude that twining stems are not irri- 

 table ; and indeed it is not probable that it should be so, as 

 nature always economizes her means, and irritability would bo 



