G2G RADIATION BIOLOGY 



sensitivity of amebas even more difficult. He found tfiat amebas detect 

 vertical liglit beams 20 n in diameter at a distance of 100-150 yu (perhaps 

 from scattered light), usually move toward the beam, do not react if it is 

 extinguished before they reach it, and may proceed through the beam or 

 react negatively on entering it, with no obvious difference in the effect of 

 white or spectral colors. Food (globulin grains) laid over a beam of 

 intense white or blue light was "sought" and ingested, but over green or 

 dark pencils in a general field of vertical illumination it was frequently 

 avoided. Dark pencils in the light field were detected at a distance, 

 as were light pencils in a dark field, though scattering could scarcely be 

 invoked as an explanation for the former and no mechanism allowing this 

 was found. In a 1929 paper Schaeffer reported that, when amebas mov- 

 ing around glass rods are stimulated by light, they significantly alter the 

 ratio of right turns to left turns. He postulated two kinds of proto- 

 plasmic constituents, similar to isomers, a right-turning and a left-turning, 

 which are produced or destroyed at different rates depending on the 

 species of ameba and rate of growth, are stable for hours under ordinary 

 conditions, determine the ratio of right turns to left turns in locomotion, 

 and are affected differentially by light on a temporary basis in so far as 

 their power to determine the direction of the ameba's apparently ran- 

 dom path is concerned. This too seems a far cry from conventional 

 photochemistry. 



The orientation and free movements of Paramecium and Stentor sub- 

 jected to sudden changes in illumination have been studied without 

 reaching much understanding of the photosensory mechanisms. Most 

 responses found can be laid to "general irritabihty" rather than to spe- 

 cific reactions to light or directional effects. Fox (1925) reported upward 

 swimming by Paramecium in darkness, downward in light, with the short 

 wave lengths most effective in altering the direction. 



Among flagellates, definite pigment organelles ("stigmata") are known 

 in many forms. Mast (1928) summarized previous work with the gener- 

 ahzation that in unicellular flagellates the stigmata consisted of a "spoon- 

 shaped pigmented structure and a hyaline mass which contains photo- 

 sensitive substance." Furthermore these appear to be connected directly 

 or indirectly to the flagellar mechanism. Because of its position the pig- 

 ment mass may cast a shadow in directed light over the "hyaline mass," 

 stimulating greater or lesser activity toward the hght, so that the sum of 

 apparently random movements shifts the organisms toward or away from 

 a light source (Fig. 14-3). This is the situation in Euglena, Trachelo- 

 monas, and some others. 



In Chlamydomonas, however. Mast found (1916, 1928) that the pig- 

 mented region is eUiptical and dishlike in being concave, with a maximum 

 diameter of over 2.5 n in some specimens. This stigma is located at the 

 surface of the body under a slightly projecting hyaline biconvex lens 



