. A laboratory course in plant physiology. Plant physiology; Plant ecology. 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 sealin
. A laboratory course in plant physiology. Plant physiology; Plant ecology. 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
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