1 66 Nettle ( U rticacecE ) . [No. 24. 



A vigorous, herbaceous climber, with stems six to ten 

 feet long, or more, very rough downward — almost prickly. 

 It is both ornamental and of great value commercially. 

 The bitter and aromatic principle for which it is noted is 

 due to small, reddish grains with which the calyx, fruit, 

 and other parts are more or less dotted. 



Darwin describes the motion of the Hop Vine in 

 twining : 



" When the shoot of a Hop rises from the ground, the 

 two or three first-formed joints are straight and remain 

 stationary, but the next-formed, whilst very young . . . 

 bend to one side and travel slowly round all points of the 

 compass, moving, like the hands of a watch, with the sun. 

 The average time for one revolution is two hours and 

 ei^ht minutes." 



& 



He suggests an explanation of this motion : 



" Let us suppose that the cells along the northern sur- 

 face [of the revolving part] from the base to the tip were 

 to grow much more rapidly than on the other sides, the 

 whole shoot would then be bowed to the south. Let the 

 longitudinal growing surface creep round the shoot, 

 deserting by slow degrees the northern side, and en- 

 croaching on the western side, and so round by the south, 

 by the east, again to the north. In this case the shoot 

 would remain always bowed and with the point successively 

 directed to each point of the compass." 



The movement is not a twisting of the shoot, but a con- 

 stant bowing, front, sideways, backwards, round and round 

 until contact occurs or it stiffens with age. 



