346 PLANT PHYSIOLOGY 



response is commonly seen in some form of growth and this requires 

 time. The reaction time is thus the sum total of the presentation 

 time, the transmission time, and the growth time. Many organs are 

 more sensitive at certain times and under certain conditions than 

 at others. When Osmunda cinnamomea fronds are maturing, they 

 will respond to gravity after a presentation time of only 20-40 

 seconds. 



If the stimulus is increased in intensity, the time required for 

 presentation is correspondingly shortened so that the presentation 

 time varies inversely with the intensity of the stimulus. A strong 

 light acting for a short time is just as efficient in evoking a re- 

 sponse as a weak light acting for a long time. The law of summa- 

 tion then follows: A stimulus too slight to produce the end reac- 

 tion, if repeated at proper intervals, will ultimately suffice to 

 cause the end reaction. 



Weber's Law. — If a change in the environment is great enough 

 to produce a response, it has been found that the necessary 

 change bears a certain relation to the original stimulus. Let 

 us suppose that the plant is receiving 100 units of light on all 

 sides and it is desired to increase the light from one side so as to 

 cause a bending. It may require a difference of ten units, i. e., 

 one side may have to receive ten units more than the others be- 

 fore any bending is observed. If this is the case, and it is desired 

 to bend a plant which is receiving 200 units, then one side would 

 have to receive twenty more than the others. This is expressed 

 by Weber's Law: "The smallest change in the magnitude of a 

 stimulus which will call forth a response always bears the same 

 proportion to the whole stimulus." In the example given the 

 proportion was 10%. 



Two Stimuli from Different Directions. — When two stimuli act 

 upon a plant tending to cause a response in different directions, 

 the organ affected will assume a position intermediate, according 

 to the laws of forces. If the force exerted by each stimulus is 

 represented by one side of a parallelogram, the force actually 

 acting upon the organ will be represented by the diagonal of the 

 parallelogram, as described in any elementary textbook on phys- 

 ics. In the same way, if two forces are acting in directly opposite 

 directions, the ultimate reaction will correspond to the difference 

 of the forces acting; if operating in the same direction, the response 

 corresponds to their sum. 



