184 LIGHT AND PEOTOPLASM [Ch. VII 



substratum and becomes attached, and the former attached end 

 becomes free. Stahl explains this on the ground that the 

 ends of Closterium periodically exchange their tendency to 

 point towards the light. The appearances just described are 

 found in diffuse daylight. In stronger light the azimuthal 

 position is 90° from the infalling light. If direct sunlight 

 falls upon desmids, they move from the light (negative pho- 

 totaxis). 



Diatoms have been studied by Stahl ('80) and Veeworn 

 ('89, p. 47). Locomotion is effected in these organisms as in 

 desmids by the secretion of mucous threads. The movement 

 towards diffuse daylight (Navicula, Stauroneis) takes place 

 slowly, but it often affects nearly all the individuals. The 

 long axis does not seem to be clearly oriented in the direction 

 of the infalling rays, which may be partly accounted for by 

 the normal zigzag method of locomotion. Under strong sun- 

 light diatoms appear negatively phototactic. Occasionally a 

 culture will be found whose individuals are separated into two 

 groups — one next the positive side, the other next the nega- 

 tive side of the vessel. 



Osaillaria. — Vbewoen ('89, p. 50) has made experiments 

 on the reaction of these organisms, whose method of locomotion 

 is probably similar to that of desmids. They are markedly 

 positively phototactic from half darkness to direct sunlight ; 

 only in intense sunlight do they fail to accumulate at the posi- 

 tive end of the vessel. The aggregation at the positive pole 

 takes place by the threads assuming a direction parallel to the 

 rays of light and creeping forward thus, side by side. Vee- 

 WOEN states that after all have attained the + edge, rotation 

 of the slide or vessel through 180° does not cause a prompt 

 transfer of all individuals towards the light side — at least 

 during the time of his observation only a few had crawled 

 towards the light in its new position. According to Wino- 

 GEADSKY ('87), Beggiatoa is generally negatively phototactic. 



Myxomycetes. — In its amoeboid form and when subjected to 

 strong sunlight jEthalium septicum retreats into the substratum, 

 but while in the dark it comes to the surface (Hofmeistee, '67, 

 p. 625 ; Steasbtjrgee, '78, p. 620). Also, when the Plasmo- 

 dium is partially illuminated, the protoplasm tends to flow from 



