IRRITABLE RESPONSE 235 



Experiment. In a simple water-culture vessel (page 116) ger- 

 minate radish or other convenient small seeds, and keep in dark until 

 the roots and stems are 1 to 2 cm. long. Then place the vessel in 

 a chamber with light coming wholly from one side through a space 

 about its own size, and note the responses of the roots. 



It will be noticed at once that the effects observed in this 



experiment are complicated by the action of geotropism, thus, 



incidentally, giving an excellent opportunity to observe the effect 



where two stimuli are acting at the same time, but from different 



directions. But for our present purpose we prefer the light 



alone, and gravity may be eliminated by the following : 



Suggested Experiment. Germinate 4 or 5 seeds (Corn, Oats, Barley, 

 Beans) upon corks on the margin of a klinostat disc revolving in a vertical 

 plane (Fig. 60) ; let light fall upon them parallel with the axis of the instru- 

 ment, and note positions of stems and roots. 



Turning now to the location of the response in relation to 

 the reception of the stimulus, we inquire : 



Is the phototropic stimulus received at the place of actual 



response? 



Experiment. Fill a small flower-pot with earth to the brim (so 

 this will cast no shadow) ; sow in it a dozen Oats, and keep in dark- 

 ness until these appear about 1 cm. above the surface. Prepare from 

 tin-foil six very tiny cylindro-conical caps 5 mm. long and just large 

 enough to fit snugly over the tips of the young shoots (they may be 

 modelled on the lead of a lead-pencil); fit them upon half of the 

 shoots (where they will cover the tips, but not the growth zone), leav- 

 ing the others exposed; then place the pot in one-sided light, and note 

 results. 



In this connection the student should inform himself as to 

 our knowledge of the place and method 0} reception of the light 

 stimulus in other parts, notably in leaves, on which he should 

 note Haberlandt's recent theory of the function of the swollen 

 epidermal cells. Also, he should inquire as to the quantitative 

 relation existing between stimulus and response, the application 

 of Weber's law, and the relations of the energy supplied by the 

 stimulus to that causing the response. 



Every student, in working with light effects upon plants, 

 will recall that sunlight is a very composite source of energy, 

 and will wish to determine: 



