532 



PHYSIOLOGY OF GROWTH. 



been inverted for several hours, we find above all things that the 

 blades have risen considerably, and a slow subsequent elevation is 

 also exhibited in the leaf- stalks dependent on the negative geo- 

 tropic irritability of the joints. 1 



If we now bring the plants back again to the upright position, 

 the leaves regain their normal position in the course of a day. 

 If the plants have been kept inverted for several days, and are 

 then at last placed upright again, the leaves resume their original 

 position very slowly or not at all, since the geotropic curvature of 

 the pulvini has now been fixed by growth. 



1 For further particulars see Pfeffer, Periodische Bewegungen, 1875, arid 

 A. Fischer, Botan. Zeitung, 1890. 



200. Experiments with the Lever Dynamometer. 



To obtain an approximate value for 

 the force which is developed in the ex- 

 ecution of movements by pulvini, the 

 lever dynamometer may be employed. 1 



On the brass column s is movable the 

 sleeve e, which can be fixed by means 

 of the thumb screw r. This carries a 

 graduated arc, and a three-legged lever 

 resting on a knife edge. Of the two 

 legs h and ti which are in the same 

 straight line, the longer serves as the 

 pointer, while the object under investi- 

 gation is laid on the shorter ; the third 

 leg, p, is at right angles with the other 

 two. If it is displaced, e.g., by pressure 

 brought to bear on the shorter leg, the 

 force with which it endeavours, pendu- 

 lum-wise, to get back to its position of 

 equilibrium, increases with the sine of 

 the angle of deflection. By weighting 

 it we can, of course, vary the force re- 

 quired to produce a given 



r deflection, and so also regu- 



~j late the deflection of the 

 pointer effected by applying 



FIG. 182. Pfeffer's Dynamometer. 



