14 STUDIES IN GENEKAL PHYSIOLOGY 



Finally, I have to mention the heliotropic investigations 

 on Infusoria which were made along the lines mapped out 

 by Sachs. To bring these investigations before the reader 

 I shall describe the more important observations which have 

 been made on Euglena. The influence of the direction of 

 the rays of light on these Infusoria was first demonstrated 

 by Stahl: 1 



Those individuals which did not swim about freely remained 

 with their pointed posterior ends attached to the cover-glass or to 

 other objects, while their free anterior ends were, according to con- 

 ditions, either turned toward or away from the source of light. The 

 longitudinal axes of both the motile and sessile Euglenge coincided 

 as nearly as possible with the direction of the rays of light. The 

 motionless ones behaved like the free-swimming ones whenever the 

 direction or intensity of the light was suddenly changed, except 

 that they reacted more slowly. If, for example, the glass slip was 

 suddenly rotated through an angle of 180, the position which the 

 animals occupied originally with reference to the source of light 

 was slowly reassumed, while the swimming individuals left their 

 former path and moved in the original direction toward the light 

 immediately after a change in its direction. 



Engelmann studied in Euglena the relation between the 

 effect of the rays of light and their refrangibility. 2 After 

 he had established the fact that when a drop of Euglenae is 

 only partially illuminated the animals gradually accumulate 

 in the lighted area, he brought the animals into a micro- 

 spectrum. Here they collected on the more refrangible side 

 of the spectrum. The orientation of Euglena therefore 

 depends on the direction of the rays, and especially on 

 that of the more refrangible ones. It must finally be men- 

 tioned that the anterior ends of the Infusorise are most sen- 

 sitive to light; yet the pigment spot is not, as might be 

 supposed, the most sensitive, but the colorless protoplasm in 

 front of this. 



Besides these direct effects of light in phenomena of 



1 Botanische Zeitung, 1880. 2 Pflugers Archiv, Vol. XXIX (1882). 



