Chap. U HYPOCOTYLS, ETC., WHILST ARCHED. 09 



height upwards, the basal part ceases to cireumnutate, 

 whilst the upper part continues to do so. 



That an arched hypocotyl or epicotyl, with the two 

 legs fixed in the ground, should be able to cir- 

 eumnutate, seemed to us, until we had read Prof. 

 Wiesner's observations, an inexplicable fact. He has 

 shown* in the case of certain seedlings, whose tips 

 are bent downwards (or which nutate), that whilst the 

 posterior side of the upper or dependent portion grows 

 quickest, the anterior and opposite side of the basal 

 portion of the same internode grows quickest ; these 

 two portions being separated by an indifferent zone, 

 where the growth is equal on all sides. There may 

 even be more than one indifferent zone in the same 

 internode ; and the opposite sides of the parts above 

 and below each such zone grow quickest. This pecu- 

 liar manner of growth is called by Wiesner "un- 

 dulatory nutation." Circumnutation depends on one 

 side of an organ growing quickest (probably preceded 

 by increased turgesoence), and then another side, 

 generally almost the opposite one, growing quickest. 

 Now if we look at an arch like this f) and suppose 

 the whole of one side — we will say the whole convex 

 side of both legs — to increase in length, this would 

 not cause the arch to bend to either side. But if the 

 outer side or surface of the left leg were to increase 

 in length the arch would be pushed over to the right, 

 and this would be aided by the inner side of the 

 right leg increasing in length. If afterwards the 

 process were reversed, the arch would be pushed over 

 to the opposite or left side, and so on alternately, — 

 that is, it would cireumnutate. As an arched hypo- 



* 'Die unduUrende Nutation , Also published separately ae« 

 dor Internodien,' Ahad. der Wis- p. 32. 

 tench. (Vienna), Jan. 17th, 1878. 



