Chap, xi Sensitivity and Stimulation 497 



Sachs showed also, in 1863, that deprivation of light causes 

 the disappearance of the power of appreciating stimulation, 

 the plant passing into a state of rigor. He gave the name 

 phototonus to the normal condition depending on normal 

 illumination. Bert, in 1870, confirmed the observation 

 that various organs which contain chlorophyll lose their 

 sensitiveness in darkness. In 1870 Frank proved that cold 

 stops geotropic curvature if the temperature is low enough 

 to check growth. Conversely, Sachs showed in 1874 that 

 high temperatures cause protoplasm to pass into rigor. 

 Loss of motility was brought about in Mimosa by heating 

 it to 50 C. in moist air. Darwin found the leaves of 

 Drosera paralysed at 55 C. It seems probable that the 

 temperature in these cases affects the motor rather than 

 the sensitive mechanism. In 1873 Pfeffer showed that the 

 irritability of Mimosa is diminished by keeping it at a low 

 temperature. In 1878 Wiesner noticed that stems of Galium 

 and radial shoots of Cornus are heliotropically positive to 

 feeble illumination but react negatively when the light is 

 strong. Vochting made the same observation in 1882 with 

 regard to the peduncles of Erodium and Taraxacum. The 

 manifestation of response to stimulation varies during the 

 life of the plant as its needs change. 



Sachs showed that age influences the response of the 

 shoots of Tropaeolum and the roots of Hedera, both of which 

 are positively heliotropic when young and negatively so 

 when older. Oltmanns' researches on the relation of inten- 

 sity of light to the direction of heliotropic response and 

 its change as the illumination increases have already been 

 referred to (p. 466). Goebel, in 1883, suggested that the 

 geotropic change in rhizomes when they put out long 

 upright shoots is caused by age, and he correlated it with 

 the change from a bare rhizome to a leafy shoot. Wiesner 

 showed in 1882 that there is a change in heliotropic 

 reaction from positive to negative in the peduncle of 

 Helianthemum after fertilization has been effected. Hoi- 

 green 1 i 



