§4] PHOTOTAXIS AND PHOTOPATHY 183 



spores which moved towards a light of a certain intensity would 

 move from light of a certain greater intensity. That was the 

 condition of knowledge on this subject when Stbasbuegbb's 

 ('78) epoch-making paper appeared. 



Strasbxxegeb worked with swarm-spores of various species 

 of algse, and with the flagellate Chilomonas and Euglena. He 

 observed again the phenomenon that the sense (4- or — ) of 

 response depends upon the intensity of the light. He also 

 showed that the rate of movement is quicker in stronger light 

 on account of the fact that the path taken by the organism is 

 straighter ; and (p. 586) that phototaxis is the result of the 

 organism putting its long axis in the axis of the infalling rays. 

 Steasbueger found also that, in general, the smaller species 

 of swarm-spores and the smaller individuals are more respon- 

 sive than the larger ones. 



Later studies have extended our knowledge of the distri- 

 bution of phototaxis in this group. Swarm-spores have been 

 studied by Stahl (78, '80) ; Euglena, by Engelmann ('82% 

 p. 396) ; and Volvox, by Cienkovtski ('56), Veewoen 

 ('89, p. 45), and Oltmanns ('92). Especially interesting 

 is the fact that colorless swarm-spores, like those of Chytri- 

 dium, which are parasitic upon chlorophjdlaceous forms, respond 

 like the green organisms. (Steasbuegee, '78, p. 568.) 



Desmids, especially Closterium, have been experimented with 

 by Stahl ('78 and '79), Klebs ('85), and Adekhold ('88). 

 All are markedly phototactic in moderate, diffuse daylight. 

 This phototaxis is the more striking since the method of loco- 

 motion of these forms is peculiar. The crescentic Closterium 

 moniliferum, for example, stands inclined and glides along, one 

 extremity touching the substratum, the free extremity in ad- 

 vance. The gliding seems to result from the secretion of a 

 stream of mucus along the substratum. Now Stahl believed 

 that the angle of inclination of the Closterium is dependent 

 upon the direction of the infalling rays of light, being parallel 

 thereto. This relation has been denied by Klebs, but Adee- 

 HOLD, by varying the direction of the infalling rays, has shown 

 that the azimuthal position is determined by light. Under 

 certain conditions Closterium moniliferum moves by a sort of 

 head-over-heels motion, since the free end bends down to the 



