290 MOTOR RESPONSES 



out the visible spectrum. He found that as the wave length increases, 

 the stimulating efficiency also increases very rapidly from zero at about 

 410 wp to a maximum of 21 arbitrary units at 485 wp, and then de- 

 creases equally rapidly to zero at about 540 wp (Fig. 102). He holds, 

 however, that the limits of the stimulating region depend upon the 

 luminous intensity. 



Kinetic responses. — If Euglena is subjected for long periods to low 

 illumination or to darkness, it gradually becomes less active; and if the 

 illumination is then increased, it gradually becomes more active. The 

 rate of change in activity varies with the magnitude of the change in 

 intensity. But this response is never so sudden and abrupt as the shock 

 reaction. There are therefore two types of responses to light in Euglena, 

 one depending primarily upon the rate of change in luminous intensity, 

 the other primarily upon change in the amount of light received. The one 

 results in orientation and aggregation, the other in change in activity. 



Mast and Gover (1922) measured the rate of locomotion in several 

 different flagellates in different intensities of light and found very little 

 correlation between rate and intensity. The environmental factors were, 

 however, not accurately controlled, and adaptation was not considered. 

 The measurements should therefore be repeated, with the methods used 

 by Mast and Stabler (1937) in their observations on Amoeba. 



Reversal in response. — Euglena is ordinarily photopositive in weak 

 light and photonegative in strong light. The orienting response therefore 

 tends to keep it in light of moderate intensity, indicating that these re- 

 sponses are fundamentally adaptive. This has not been demonstrated, 

 however, because the direction of orientation is not specifically correlated 

 with luminous intensity. For example, euglenae which are strongly photo- 

 positive in a given intensity of light at room temperature may become 

 equally strongly photonegative if the temperature is rapidly decreased 

 10 to 15 degrees, the extent of the requisite decrease depending upon 

 the state of adaptation (Mast, 1911). 



This problem is much in need of thorough investigation. It is a very 

 important problem because it concerns the biological significance of 

 response to light in these organisms. 



II. Peranema tricophorum. — Peranema is a colorless flagellate with- 

 out an eyespot. It is usually in contact with the substratum and moves 

 slowly with the flagellum extending forward (Fig. 103). If the luminous 



