IRRITABILITY. 57 



circle, the ends brought together through a piece of cork and united. 

 Four brass wires serve as spokes, while the hub is made from a 

 heavy cork. The axis is also made from a piece of wire, about 15 

 cm. in length, in order that the seedlings may have enough space 

 for their curvatures. The axis rests in bearings made of the ends 

 of sealed glass tubes (Fig. 51, -5), which are fastened in stationary 

 corks by means of sealing wax. If now a sufficient stream is al- 

 lowed to fall perpendicularly on the cork plates on one side, ai 

 rapidity of revolution will be secured that will in five or six hours, 

 effect a noticeable change in position of the seedlings, which have 

 been placed with their roots toward the center. If large casks are- 

 at hand, the experiment may be carried on without a water system, 

 A glass tube of an internal diameter of 4 mm. with a fall of i meter 

 of the water will furnish a stream that will revolve the wheel more- 

 than twice per second, which is entirely sufficient for the experi- 

 ment. The replacement of the water is necessary for the continua- 

 tion of the experiment. 



46. Heliotropism, Thermotropism, etc. — Radiant energy m. 

 the form of heat, light, and electricity exercises a very marked 

 directive influence on the position of plant-organs. The effect 

 of sunlight is much better known than that of the other stimuli 

 acting on the plant. It is a matter of general observation 

 that the shoots of a large number of plants bend toward the 

 light. A close inspection shows that various organs respond 

 in a different manner to light, as also to gravity. Thus 

 many roots direct themselves away from the source of light 

 {apheliotropisni), trailing shoots, leaves, etc., at right angles tO' 

 the rays {diaheliotropisni), while, as noted above, others, such 

 as stems, bend toward the light {proheliotropism). It will be 

 seen that while the force of gravity acts always in the same 

 direction, the line of light-rays from the sun moves through 

 an angle of 180 degrees daily. This change of the position of 

 the source of light causes corresponding movements in helio- 

 tropic organs. Sunlight affords two separate stimuli to the 

 plant : one from the blue-violet rays which causes helio- 



