GROWTH SUBSTANCES FOR PHOTOTROPISM 139 



An important question is whether or not the phototropic 

 stimulus can be transmitted in an acropetal direction as well as in 

 a basipetal one. Rothert (1894) and van der Wolk (1911) found 

 that it could not, but von Guttenberg (1913) demonstrated that, 

 when coleoptile tips of plants whose basal portions had been 

 previously illuminated unilaterally were lighted from the opposite 

 side, the tip curvature which might have been expected was 

 either absent or exhibited feebly. The author concluded that a 

 stimulus is conducted from the unilaterally illuminated basal 

 portion to the tip. This is not consistent with the in^-estigations 

 on growth-substance transport which ha^'e shown that its trans- 

 mission takes place only in a basipetal direction. Arisz (1915) 

 criticized the work of von Guttenberg on other grounds. Lange 

 (1927) suggested that during unilateral illumination of the base, 

 some of the light might fall upon the tip and in this way influence 

 the transmission of stimulus. Reinhard and Bro (1933) have 

 pointed out still other possibilities. 



Quaniity-of-stimulus Principle. — It has been held for a long 

 time that in certain photochemical reactions the amount of 

 applied energy is of importance in bringing about a constant 

 effect. Bunsen and Roscoe (1862) proposed a quantitative rule 

 for the effect of light upon a sensitive photographic plate, where 

 the product of the exposure time X intensity of light = a con- 

 stant value. 



Froschel (1908, 1909) and Blaauw (1909) showed that a definite 

 amount of light must be applied to a plant organ in order to 

 produce a threshold phototropic response. The amount of light 

 is the product of two factors: the intensity of light and the dura- 

 tion of illumination. That the quantity-of-stimulus principle 

 holds over a wide range for phototropic curvature in the Avena 

 coleoptile may be seen in Table 6, wliich is taken from the work 

 of Blaauw. The fact that such a short illumination as 

 1/1,000 second can call forth a phototropic curvature is of 

 importance for the comprehension of the induction process. 

 Certainly the matter of photic stimulation is an excellent example 

 of the general rule regarding the excitation of irrital)le protoplasm, 

 i.e., that a relatively small amount of applied energy is capable 

 of setting into action a chain of processes which leads eventually 

 to a comparatively large response. The significance of the 

 Blaauw theory will be discussed in greater detail later on. 



