INTRODUCTION AND HISTORICAL SKETCH 7 



determine whether or not these play an essential role in stimulus 

 conduction, Rothert proceeded as follows: Both bundles were 

 severed; the tip was exposed to light; and the lower wounded 

 portion was darkened. A phototropic curvature occurred in this 

 basal portion, and from this Rothert concluded that "it is 

 proved that the hehotropic (phototropic) stimulus is conducted 

 in the parenchyma of the fundamental tissue." 



Fitting. — Carrying the work of Rothert further, Fitting (1905- 

 1906, 1907) studied the fundamental processes of stimulus 

 conduction. The question that he set out to investigate was 

 stated as follows: "In tropism, how is the organ of perception so 

 linked with the zone of reaction that the externally applied 

 stimulus can indirectly determine the direction of the curvature?" 

 To solve this problem, Fitting tried to ascertain whether the 

 conduction of a stimulus on a certain side of the coleoptile is in 

 any w^ay oriented with respect to the direction of light. The 

 experiments, on Avena coleoptiles for the most part, were carried 

 out in a room saturated with moisture at a temperature of 30°C. 

 The influence of incisions upon growth and curvature of the 

 Avena coleoptile was studied first. It was found that the growth 

 rate remained practically unchanged by unilateral wounding. 

 Weak curvatures of the coleoptiles were observed, first away from 

 the wound and then toward it. The influence of unilateral 

 transverse incisions upon longitudinal conduction of a stimulus in 

 the Avena coleoptile was then investigated. Incisions were made 

 midway between the base and the tip in coleoptiles measuring 

 1 to 1.5 cm. in length. The coleoptiles were darkened past the 

 point of incision with tinfoil tubes or with collars made of black 

 paper. Then, if the tip of the coleoptile was illuminated uni- 

 laterally, a decided positive phototropic curvature resulted in the 

 darkened basal portion under the incision, no matter what the 

 orientation of the incision was with respect to the direction of 

 light, i.e., whether the incision was on the illuminated or on the 

 shaded side (Fig. 1). These experiments were modified in 

 various ways, but the result was always the same. Even when 

 tw^o incisions were made, one above the other and on opposite 

 sides, the conduction of the phototropic stimulus met with no 

 interference. 



Fitting concluded that the stimulus was conducted around an 

 incision and transmitted exclusively through the Hving material. 



