448 MB. F. DAIiWIW ON THE POSITION OF 



upper and lower halves of the petiole ; so that the circumnutation 

 of a leaf consists chiefly of alternate hypo- and epinastic move- 

 ments ; a leaf, therefore, which appears to be stationary in the hori- 

 zontal position is in reality constantly oscillating up and down. 

 Circumnutation is regulated by stimuli ; it is only when an organ is 

 m its normal position with regard to external forces (light, gravita- 

 tion, &c), that circumnutation remainsin a state of equilibrium, and 

 the oscillations are so equalized that the mean position of the organ 

 remains the same. If the organ is displaced, the altered stimuli of 

 the external forces act on the circumnutation and bring back the 

 organ to the position of equilibrium. If a seedling plant is 

 lighted from above, and therefore growing vertically upwards, 

 its apex will be oscillating about the vertical line. If the illu- 

 mination becomes lateral, the plant will bend heliotropically, owing 

 to the exaggeration of the oscillations towards the light, and the 

 diminution of those away from it. In the same way, if the hypo- 

 cotyl is prevented from bending, the cotyledons adjust themselves 

 to the lateral light by a similar kind of modified circumnutation. 

 ihe change of position which occurs under these circumstances is 

 a rising of one cotyledon and a sinking of the other. The rising is 



curva ture . This is not necessarily the case. Both rise and fall are 

 more properly to be defined as exaggerations of circumnutating 

 movements in these two directions, due to the stimulus of light 



Ihe above experiments (especially those with Ranunculus Fi- 

 cana) support this view, since they show that there are powers 



of Z rT \ re81 t ng ^ the Peti ° le Which are under the control 

 frnl ,l! g f' 8 ° that the leaves can be * d either towards or 



ZZ VTVI ]jght ' iD ° rder t0 reach the diaheliotropic 

 plane Besides light, other stimuli can alter the circumnutation 



thP in!! 3 ' "^ents are affected by gravitation and by 



WemlvT lm r e r hlch ^ ri -toe P inaSyandhyponast^ 

 ImXs nT.l , WS ***** ° n ^e leaves-the elastic 



stimulus, and those of gravitation and light. If the sensitiveness 



Pi e of d ft' G leafwiU be able t0 <*ey this stimulus in 

 Th s i« dl f U f rbanCe9 ' 8ucn a8 the loss of the gravitation-stimulus, 

 vettabl. ° CCU ^ Wlth * Ficaria > ™* *!*»tago, and the 



Iwv h P T r r. (CU r bita "&"»' But in theise of the 

 erowTJ ' * i "T^" " DOt 8tron g enough ; the plant, when 

 r o r o L / J ' trU f t0 the °PP° 8i ^ force* of epinasty and 



apogeotropism to produce an approximate balance, the final 



