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PART IV. — THE PHYSIOLOGY OF PLANTS. 



former condition tends to accelerate growth, but the latter to 

 retard it. The actual rate of growth is the resultant of the 

 acceleration due to diminished light, and of the retardation due to 

 a falling temperature. Similarly, in the morning, the increasing 

 intensity of the light tends to retard growth, whereas the rising 

 tempei-ature tends to accelerate it. The actual rate of growth is 

 the resultant of the retardation due to increased intensity of light, 

 and of the acceleration due to a rising temperature. 



Fjg. 480.— (After Sacht). Illubtrating the daily periodicity of growth in length. 



In Fig. 480 there is given a graphic representation of two complete dailij 

 periods of growth as observed in a Dahlia-stem. The abscisste of the base-line 

 represent periods of two hours, and the divisions of the ordiuates represent units 

 of growth in length. The tracing marked 1/t is the result of hourly measure 

 ments of the growing stem ; the tracing marked 3/t is the result of measure- 

 ments made every three hours. The variations of temperature have been also 

 taken into account, and are represented by the tracing marked f^ : the base-line 

 represents 12° R., and five of the divisions of the ordinates represent 1° R. : it 

 -will be seen that the temperature varied between about 12.5° R. and 15.8° 11. 

 The shaded spaces represent periods of darkness. 



The paratonic action of light varies with its intensity : the more 



