MOTOR RESPONSES 303 



tensity or the location of the Hght in the photosensitive substance in the 

 eyespot, by virtue of colony rotation on the longitudinal axis. It should 

 be noted that the responses observed are not the result of quantitative 

 differences due to continuous action of the light. The explanation offered 

 is therefore not in accord with the Ray-Verworn theory. 



Wave length and response. — The distribution of stimulating efficiency 

 in the spectrum for Volvox (Laurens and Hooker, 1920) and Gonium 

 (Mast, 1917) is essentially the same as it is for Euglena; but for the 

 closely related forms Pandorina and Spondylororum (Mast, 1917) the 

 maximum is at 535 ni\\ in place of 485 wp, and the effective region ex- 

 tends from this wave length much farther in either direction than it does 

 for Euglena, Gomu7n, and Volvox (Fig. 102). The orange, pigmented 

 portion of the eyespot in these forms is opaque in reference to the light 

 of all those regions of the spectrum which have the highest stimulating 

 efficiency. The distribution of stimulating efficiency for these forms con- 

 sequently supports the conclusions reached concerning the structure of 

 the eyespots, the distribution of photosensitive substance in them, and 

 their function in the process of orientation. 



Threshold. — Mast (1907), on the basis of quantitative results, con- 

 cludes that the minimum difference in light intensity on opposite sides 

 of a colony which is necessary to induce a response varies greatly with 

 the physiological state of the colony; but that with colonies in a given 

 physiological state, the response varies directly with the intensity, and 

 the ratio is nearly constant, i.e.. nearly in accord with the Weber-Fechner 

 law. His observations, however, covered such a small range (2-27 meter- 

 candles) and the probable error in the results is so large that further 

 observations concerning this relation are highly desirable. 



Reversal in response. — Volvox is usually positive in weak, and nega- 

 tive in strong light. However, the reverse obtains under some conditions. 

 It may be positive, negative, or neutral in every condition of illumination 

 in which orientations occurs. If it is positive, a shadow on the photo- 

 sensitive substance in the eyespots causes a change in the direction of the 

 stroke of the flagella of the zooids from diagonal to backward. A flash of 

 light on this substance causes a change of stroke from diagonal to side- 

 wise. If the colony is photonegative, the reverse obtains; and if it is 

 neutral, there is no response unless the changes in luminous intensity are 

 great. 



