530 LIFE MOVEMENTS IN PLANTS 



of movement, whereas light acting from below retarded the 

 movement. Here we have instances of photo-hyponastic 

 modification of natural movement. Similarly epinastic 

 organs will, normally speaking, have their natural down 

 movement retarded by light from above, and accelerated 

 by light from below. If the periodicity of the autonomous 

 movements coincides with the periodicity of the external 

 stimulus, the resulting movement will be determined by 

 algebraical summation ; it will be very pronounced when 

 the two effects are concordant. If the two periodicities do 

 not agree, the interference effecis will become extremely 

 complicated. 



Positive thermonasty. — Rise of temperature inducing differ- 

 ential growth brings about the closure of the flower. Fall 

 of temperature on the other hand induces a movement of 

 opening. Example of this has already been given in the 

 responsive movement of Nymphoea. 



Negative thermonasty. — The opposite type of movement 

 is exhibited by Crocus and Tulip. Pfeffer has shown that 

 a rise of temperature induces in these flowers, a quicker 

 rate of growth of the inner side of the perianth. Rise of 

 temperature thus induces a movement of opening, and a 

 fall of temperature brings about the opposite movement of 

 closure. I shall presently describe the effects of both 

 positive and negative thermonasty, in diurnal movements 

 of flowers. 



Thermo-geotropism.. — I have already described the accen- 

 tuation of geotropic curvature during the fall, and a 

 flattening of curvature during the rise of temperature 

 (p. 519). The influence of this factor on diurnal move- 

 ment will presently be treated in fuller detail. 



There are thus more than ten variables, and the result- 

 ing effect due to their combinations will exceed a thousand. 

 This will explain why attempts at explanation of the 



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