XHI. PLANT IRRITABILITY 



155. PHOTOTROPISM, OR RESPONSE TO LIGHT 



Object. To study the effect of light on the daily 

 movements cf plants. 



Apparatus. A vigorous young sunflower or sweet clover plant. 



Method. Make observations on the plant early in the 

 morning of a bright, sunny day, noting the position of the 

 stem as well as that of its crown of leaves. 



From time to time during the day observe it and take 

 note of any change of direction. At evening, just before 

 sunset, again make observations and again note the direction 

 taken by leaves and stem. How does its morning position 

 compare with that at evening? How may this be ex- 

 plained ? 



After sunset again make observations. Any new change ? 



On a cloudy day make similar observations on the same 

 plant. Result ? 



Conclusion. What may be given as the probable cause 

 of these movements of plants ? 



Note. The term phototropism has been used here because it 

 is broader in its meaning than heliotropism. The former indicates 

 the response to light of any sort. The latter implies response only 

 to sunlight. 



Where possible, experiments should be made with plants under 

 artificial light. 



173 



