MOVEMENTS OF IRRITATION'. 485 



power of movement. Easily recognisable points or angles of the 

 ink-dots serve as points of reference for the measurements, which 

 are made by means of the horizontal microscope described in 153. 

 If, e.g., dots are made on the perianth segments of the closed 

 flowers, and we determine the distance between the marks again 

 after the flower has been induced by rise of temperature to open, 

 then we find hardly any alteration in the distance between the 

 marks on the outside of the segment, while on the inside the 

 distance between the marks has increased considerably, perhaps 

 by O08 mm., if the original distance between them was 3 mm. 

 The lengthening of the now convex side of the segments is 

 permanent, and therefore fixed by growth. When the flowers 

 close, the outer side, which becomes convex, lengthens much more 

 conspicuously than the opposite side. 



J Pfeffer, PJanzenphysiologische Unters., 18TB. 



185. The Darwinian Curvature, etc. 



We germinate seeds of Vicia Faba in sawdust, and leave the 

 seedlings in the sawdust until the roots have attained a length of 

 a few cm. It is now required to injure the tip of the root on one 

 side, arid, according to my experience, this is most conveniently 

 effected by touching the root on one side with a -small fragment of 

 lunar caustic (Silver nitrate). Care must be taken that only a 

 short length of the root tip, say 1'5 mm., is brought into contact 

 with the lunar caustic. From what was said in 157, it appears 

 not immaterial which side of the root receives the stimulus, and it 

 is best to stimulate neither the anterior nor the posterior surface 

 of the root, but one of the flanks. The plants are now placed in 

 glass cylinders as described in 154, and laid aside in the dark. 

 After twelve to twenty-four hours we find that the side of the root 

 touched with the caustic has become convex. On the side stimu- 

 lated, the growth, in the region of the root capable of growth, is 

 considerably more rapid than on the opposite side, so that uni- 

 lateral injury to the root tip has induced a characteristic nutation, 

 which we term the " Darwinian curvature." Nutations likewise 

 take place if the injury to the tip of the root is effected not by 

 means of Silver nitrate, but in some other manner ; it is, however, 



