§4] 



PHOTOTAXIS AND PHOTOPATHY 



185 



(Baeanetzki, '76, p. 328, and Stahl, 



the illuminated region. 

 '84, p. 167.) 



Baranetzki proceeded as follows: a glass plate was placed in a saucer so 

 that its surface was 2 or 3 mm. below the rim. The plate was covered by 

 filter paper which extended 

 over the rim and here dipped 

 into water, by which means 

 it was kept moist. Over the 

 saucer was laid an opaque 

 cover, blackened below and 

 provided with a narrow slit. 

 The Plasmodium was placed 

 on the filter paper and diffuse 

 daylight was thrown upon the 

 slit by means of a plane mir- 

 ror. In less than half an hour 

 the illuminated threads of the 

 Plasmodium had become very 

 tliin, owing to the retreat of the 

 protoplasm from under the slit 

 to the darker region (Fig. 52). 



Rhizopoda.- — Although, 

 as we have seen, Pelo- 

 myxa is irritated by a 

 sudden illumination, a 

 phototactic or photo- 

 pathic response has not 

 hitherto been certainly 

 observed in this group. Verwoen ('89, pp. 40, 41), indeed, 

 experimented, but with negative results, upon Amcfiba Umax, 

 Amceba princeps, Actinosphaerium, and Actinophrys. Only 

 in Polystomella crispa did he notice a slow wandering 

 towards the source of light ; but he was uncertain whether 

 this was due to light. 



Verworn's method was not well devised, however, for bringing out 

 phototactic response. The Protista were placed on the slide, and, after cut- 

 ting out heat rays by means of a plate of ice, were subjected to the light or 

 to the Engblmann" microspectrum, and illuminated at difEereiit intensities 

 either over the whole body or over only a part. All disturbing influences, 

 he says, were as far as possible eliminated : gravity, by an exact horizontal 

 position of the microscope on a table with three screw-feet ; the action of 

 the edge of the drop, by using a very broad drop ; and, iinally, the laterally 



Fig. 52. — Plasmodium of Jithalium septicum, 

 after having been kept in tlie dark for some 

 time and tlien illmninated, for lialf an liour, 

 over a cross-shaped area, only. The illumi- 

 nated area is on the upper part of the figure. 

 The protoplasm has retracted from it, leaving 

 a partially clear region in the form of a cross. 

 (From Baranetzki, '75.) 



