ELECTROTROPISM 1 07 



the seedling in each, using ten for each funnel. Measure the 

 roots each day and compare the average growth in length of each 

 series. For longer experiments radishes can be grown in boxes 

 of earth provided with copper electrodes, and the weights of the 

 plants determined afterwards. By using copper and zinc elec- 

 trodes a battery will not be necessary in this experiment, as a 

 current will be generated sufficient to accelerate growth by the 

 action of the soil moisture on the electrodes. 



164. Effects of Alternating Secondary Currents. Place one cc. 

 of seeds into the glass tube as in 162 and attach the electrodes 

 to a secondary coil of a DuBois Raymond inductorium as in 161. 

 Attach two cells to the induction coil with a key in circuit and 

 place the secondary coil at 2—4 centimeters from the primary coil. 

 Stimulate the seeds for one minute and note their germination 

 and growth as before. If the same strength of current is used 

 in this experiment as in 163, it will be found that the electrical 

 excitation of an alternating secondary current is greater than that 

 of the primary direct current. 



165. Influence of Static Electricity. Construct a small Ley- 

 den jar out of a glass cylinder of about 100 cc. capacity by cov- 

 ering the outside with tinfoil. Place a cover on the top, and pass 

 a wire through it. Coil the wire at one end and let it rest on 

 the bottom of the jar, and on the outer end braze a metal 

 bulb. Place some soaked mustard seed in the bottom of the jar 

 and charge from a frictional machine. Remove the seed to a 

 germinator and note the result as before. Compare anodic with 

 the cathodic electrified seeds, and determine whether there is any 

 difference in their germination and rate of growth. 



166. Electrotropism. The roots of a number of species are 

 influenced by an electric current in such manner that they tend 

 to direct their apices toward the cathode, within a certain range 

 of strength of the current. The technical unit of strength is a 

 milliampere. The physiological unit used in experimental work 

 is one-millionth as great and is designated by the symbol d. Ac- 

 cording to Brunchorst's observations the maximum strength at 



