392 LIFE MOVEMENTS IN PLANTS 



takes place at an intermediate temperature, and the organ 

 thus appears insensitive to light. 



SEASONAL VARIATION OF PHOTOTROPIC CURVATURE. 



Reference has been made of the phototropic curvature 

 of Tropceolum and of Ivy undergoing a change from 



Fig. 145. — (a) Positive curvature of tendril of Passiflora ai 15"C. ; (6) negative 

 phototropic curvature at 30^C. 



positive in autumn to negative in summer. The experi- 

 ment described above shows that rise of temperature, by 

 enhancing transverse conductivity, transforms the positive 

 inio negative heliotropic curvature. The reversal of the 

 phototropic curvature of Tropceolum and Ivy, from positive 

 in autumn to negative in summer, finds a probable explana- 

 tion in the higher temperature condition of the latter 

 season. This inference finds independent support from the 

 fact previously described (p. 100) that while the velocity of 



