HISTORICAL REVIEW 3 1 



ness of the extreme tip of the stem, although I am not aware 

 that this is an established fact." 



It is strange that such a theory should have been sug- 

 gested to explain heliotropic curvatures in plants twenty- 

 six years after Darwin (see p. 18) proved that only the tips 

 of certain radicles and plumules are sensitive to light and 

 that the region where the curvature takes place is fre- 

 quently not at all sensitive, and several years after Pollock 

 (1900) had shown that traumatic stimuli are in many 

 instances transmitted from the tip of radicles to the motory 

 zone 5 to 8 mm. distant and produce curvatures toward 

 the uninjured side even if the cortex, the conducting tissue, 

 is cut on the side between the point of stimulation and the 

 motory zone. Moreover Loeb's theory fails utterly to 

 account for curvatures in structures having but a single cell 

 cavity as, for example, Vaucheria, the rhizoids of liver- 

 worts, and the hyphae of molds, all of which were known 

 to respond to light long before his theory was formulated. 



Loeb's idea that the movements in plants and anim.als are 

 unequivocally controlled by external agents is emphasized 

 in the following quotations: (1905, p. 107), "By the help 

 of these causes it is possible to control the ' voluntary ' 

 movements of a living animal just as securely and une- 

 quivocally as the engineer has been able to control the 

 movements in inanimate nature"; (1906, p. 128), "It 

 should be observed that the essential feature in these re- 

 actions is the compulsory turning of the head by the light, 

 which leaves the animal no choice, making all the cater- 

 pillars of Porthesia or all the plant lice of the same culture 

 behave exactly alike, just as in the case of a magnet all the 

 pieces of iron are compelled to behave ahke"; (1906, 

 p. 124), " The light would turn them automatically until 

 their axis of symmetry was in the direction of the rays of 

 light, and theanimal could then move only in this direction." 



Thus we have seen that in 1906 Loeb asserts that orien- 

 tation in light is unequivocally controlled by the relative 

 intensity on symmetrically located sensitive parts of the 



