METHODS OF EXPERIMENTATION WITH COCKLEBUR 83 



pigment phytochrome (see Chapter 7) is more sensitive to the red 

 than to the far-red. Thus it is quite possible to demonstrate the 

 red-far-red phenomenon with only an incandescent source, a red 

 cellophane filter, and a far-red filter. 



3. The Use of Chemicals 



Chemicals to be tested for their effects on the flowering process 

 are usually dissolved in water to which a few drops of wetting agent 

 is then added. Six drops per liter of liquid Tween-20 detergent will 

 usually allow the solution to wet the leaves uniformly and give a 

 good uptake of the compound in question. Other wetting agents will 

 work equally well, and the amount used is not very important. 



Often the investigator is faced with the rather serious problem of 

 how to get the compound into solution. Acids and bases are often 

 used, but one must avoid destruction of the compound, and the 

 acidity (pH) of the solution will probably influence its eff'ectiveness. 

 Some compounds are soluble in alcohol, and fortunately cocklebur 

 will tolerate solutions of 50 % ethyl alcohol with neither damage to 

 the plants nor effects upon flowering. As standard practice, control 

 plants are treated with solvent containing no chemicals. 



In initial experiments with a new compound it is desirable to apply 

 it over a range of concentrations. Usually a logarithmic concentra- 

 tion series of solutions is prepared. An aliquot of an initial solution 

 is diluted to a predetermined volume. The same sized aliquot is then 

 taken from the diluted solution and again diluted to the predeter- 

 mined volume. We use series in which the numerical concentration 

 values are repeated with a tenfold dilution at regular intervals in the 

 series. The quantities to be used in preparing such series are listed 

 in Table 5-2, and the results of an experiment of this type are shown 

 in Fig. 5-7. 



How should solutions be applied? Many investigators spray the 

 solution on the plants. If the experiment is subsequently to be 

 compared with experiments to be carried out in the field where 

 chemicals are applied by spraying, this is probably a good method 

 to use. Furthermore, certain plants such as rosette or cushion plants 

 are difficult to treat with chemicals in any other way. But if the plant 

 is caulescent (has a stem with leaves) such as a cocklebur, it is much 

 easier to apply the chemical by simply dipping the leaf in the solution. 

 This probably provides a more uniform application (the excess is 



