176 LIGHT AND PROTOPLASM [Ch. VII 



If this plant is kept for several days in darkness, the usual 

 response to touch does not occur. From some observations of 

 Bert (70, p. 338), it appears that it is the absence of the 

 blue-violet and orange-red rays which brings about this dark- 

 rigor ; for it occurs nearly as rapidly in green light as in the 

 dark. In these cases the absence of movement in the dark 

 might seem to be the result of diminished assimilation. 



But dark-rigor occurs under conditions which destroy the 

 general validity of this conclusion ; for example, in the reddish- 

 purple bacteria * whose reactions have been studied chiefly by 

 ExGELMAXN ('83 and '88). It appears that in these organisms 

 light is essential to movement ; for, after having been kept 

 over night in the dark, they are found in the morning at first 

 motionless ; only later, after 5 to 10 minutes of illumination, 

 do they awaken to activity. If now, after keeping for a time 

 in the light, the organisms are brought again into the dark, 

 their movements gradually diminish until, in a few hours, they 

 have ceased. We have seen above (p. 51) that oxygen is nec- 

 essary to movement, and we know that many plants excrete 

 oxygen in the light. We might expect that the quiescence of 

 these organisms in the dark is a consequence of their failure to 

 produce the oxygen necessary to locomotion, and indeed they 

 do produce in the light a slight quantity of oxygen, by virtue 

 of their chromophyll (bacterio-purpurin, Lankestee). But 

 that it is not merely oxygen which induces movement is shown 

 by the fact that when an abundant oxygen supply is artificially 

 furnished, no movement occurs in the dark. Thus light, in the 

 presence of oxygen, is essential to movement ; it seems to be 

 necessary to the irritable condition upon which locomotion 

 depends. This irritable state of the protoplasm conditioned 

 upon a certain intensity of light Engelmann calls phototonus.-f 



The analysis of this matter has been carried further. It has 

 been found that a perceptible time (latent period) elapses 



* This term includes "bacteria known as Bacterium photometrioum, Bacterium 

 roseopersicinum, rubesoens, etc., Monas okeni, Spirillum violaceum, and by other 

 names. 



t The term was applied to this phenomenon hy Engelmann on account of its 

 resemblance to that already described for the higher plants, and to which Sachs 

 had previously given this name. 



