450 GENERAL PHYSIOLOGY 



phototactic movements in the Desm.idiacem, and showed that these 

 alga-cells place themselves with their long axis parallel to the 

 light-rays, and in this position, by the extrusion of their secretion, 

 move along the bottom in their peculiar manner toward the 

 source of light, or with greater intensity away from the source 

 (Fig. 224). In a preparation containing living Closteria ^ or Pleuro- 

 tcenice all individuals place themselves with their long axis parallel 

 to one another and to the direction of the incident light-rays. 

 Thus, we find that among unicellular organisms, so far as they are 

 irritable at all to light, phototaxis is a wide-spread phenomenon. 



After phototactic phenomena had been discovered, the question 

 necessarily arose as to whether the different rays of the spectrum 

 are phototactically effective in an equal degree ; this was decided 

 very easily by the introduction of coloured glasses and solutions 

 between the source of light and the object. The media employed 

 were so chosen that they let through only rays of a certain portion 

 of the spectrum, so that only rays of certain wave-lengths were 



Fia. 224.— Phototaxis of Cloaterimn. The light comes from the right side. The arrow indicates 

 the direction of movement of the Cloaterium. 



allowed to fall upon the organism (Fig. 225). In this way Cohn, 

 and later Strasburger, established the fact that in general the 

 rays possessing a short wave-length, in other words, the blue and 

 the violet especially, are more effective than those having a 

 greater wave-length, viz., the red ; with not too high degrees of 

 intensity the latter act like complete darkness. 



One point more deserves mention in the discussion of photo- 

 tactic phenomena. From the preceding consideration and by 

 analogy with the directive effects of other stimuli it is evident 

 that only the difference in the intensity of the light upon different 

 parts of the body can produce a directive effect; where the 

 stimulus acts upon the surface of the body from all sides with 

 equal intensity, the reason for a definite axial position disappears, 

 as is to be observed most clearly in the action of chemical stimuli 

 upon all sides. Although this is obvious, some investigators, 

 such as Sachs and Loeb, have believed that the direction of the 

 rays is more responsible for the manifestation of phototactic 

 phenomena than are differences in intensity. It is difiicult to 



1 Of. p. 231. 



