THE INFLUENCE OF ILLUMINATION 321 



illumination or darkening exercises no apparent effect upon protoplasmic 

 streaming. If, however, preparations which have been kept in darkness for 

 some time are suddenly exposed to concentrated sunlight, a temporary 

 stoppage lasting from a few seconds to a minute or a distinct retardation 

 may often be seen. The latter may be followed by a slight acceleration, after 

 which streaming rapidly decreases and ultimately ceases if the exposure 

 is continued l . A shock-effect may possibly always be exercised when 

 a sudden change is made from prolonged darkness to sufficiently intense light, 

 but the reverse does not hold good, since intense light rapidly proves fatal. 

 Plants may, however, exist in which both sudden darkening and sudden 

 illumination produce the same shock-effect. 



SECTION 69. The Tropic Action of Light on Freely 

 Motile Organisms. 



As in the case of rooted plants, varying degrees and forms of irritability 

 are shown. More especially the sensitive and actively motile zoospores 

 place their long axes immediately parallel to the direction of illumination 

 and swim in a definite direction instead of all ways as they do in uniform 

 diffuse light. The anterior end is turned towards the source of illumination 

 when this is of moderate strength, but away from it when intense, and the 

 direction of movement follows suit. The velocity is little if at all altered, 

 and if the organism comes into contact with a glass plate or adheres by 

 its hinder end, it may still show the same tropic orientation as before. In 

 fact, under these circumstances. Stahl 2 found that the positively phototropic 

 orientation of Euglena viridis became negative in intense light. Oscillarias 

 and Diatoms also place their long axes parallel to the direction of the 

 light falling from one side only, and move towards or away from it according 

 to its intensity. Various Desmids behave similarly, and some forms are 

 plagio-phototropic in light of medium intensity. 



An accumulation of Bacterium photometricum is, however, also possible 

 by means of the backward shock-movement experienced every time the 

 organism passes to a dark region. Engelmann 3 was unable to produce 

 any distinct local accumulation by unilateral illumination, so that a gradual 

 decrease in the intensity of the illumination does not appear to act as 

 a phobic stimulus. It is, therefore, uncertain whether the attraction 

 observed by Winogradsky, and the repellent action of light on Beggiatoa, 

 are phobic or tropic in origin 4 . 



1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 71. 



3 Stahl, Bot. Ztg., 1880, p. 410. 



3 Engelmann, Pfliiger's Archiv f. Physiol., 1882, Bd. xxx, p. 121. 



* Winogradsky, Beitrage z. Morphol. u. Physiol. d. Bact., 1888, Heft i, p. 94; Bot. Ztg., 1887, 



PFEFFKR. Ill 



