IRRITABLE RESPONSE 



229 



Observation of the rate of response raises the query whether 

 the bending is a passive and easily resisted process, or one which 

 proceeds with some force, and we ask : 



Against what resistance can geotropic response take place? 



Experiment. Prepare a flotation dynamometer as shown by 

 the accompanying figure 65, the essential part of which is a vial 

 float resting upon mercury and sliding 

 in a cylindrical vessel, the float being 

 guided in the vessel with a minimum 

 of friction by pins attached by sealing- 

 wax to the float. Firmly support a 

 seed above the vial, with the root, 

 germinated to about 1 cm. length, 

 pressing into a sealed glass tube set 

 in the cork; give water-supply, place 

 in a flower-pot moist-chamber, and 

 observe how deeply the root can push 

 the float before beginning to bend, a 

 point which can be registered by 

 placing on the mercury a little mois- 

 tened cork dust, which will leave 

 traces at the highest point reached. 

 Then remove the seed and add weights 

 to find the pressure in grams needed 

 to depress the float to the same depth. 



Other Methods of Study. A 

 method of measuring the force of geo- 

 tropic response was devised by Sachs, 

 who made the growing roots press against 

 tiny scale-pans connected over a pulley 

 with small weights. A modification of 

 this, with a special wire cage for the root 

 tip, is described by Stone in the Botanical 

 Gazette, 22, 1896, 293. For demonstration 

 purposes Sachs' familiar method of allowing a horizontally extended root 

 to turn into mercury (covered with a film of water) is rather effective, the 

 upward bend of the root behind the tip, as well as the depth of penetration, 

 giving some qualitative idea of the resistance. This experiment may very 

 conveniently be applied by aid of a hollowed wooden block used as shown 

 by the accompanying figure (Fig. 66); or it may simply be hollowed by two 

 auger holes of differing size and depth: A dynamometer for such meas- 

 urements is described and figured by Detmer, 447, 532, by MacDotjgal, 

 25, by Stone, Botanical Gazette, 22, 1896, 251, while a very exact instru- 

 ment adaptable to this purpose has been described by Pfeffer (figured 

 in Darwin and Acton, 132). 



Fig. 65. — Flotation dynamom- 

 eter, FOR MEASUREMENT OF 

 THE GEOTROPIC GROWTH POWER 

 OF ROOTS; X J. 



Explanation in text. The size of the 

 float must be adjusted to the power of 

 the root. 



