426 PHYSIOLOGY OF GROWTH. 



behaviour of the s warmers in the hanging drop we do not always 

 find that in diffused light they seek the illuminated side. If the 

 microscope is put close up to the window, they often collect on 

 the side of the drop towards the room. The swarmers are then 

 attuned to light of lower intensity, and in order to bring them 

 to the illuminated edge of the drop, the microscope must be 

 removed more or less from the window. 



We make a circular hole 5-7 cm. in diameter in the window 

 shutter of a dark room. In front of this opening may be placed 

 glass flasks with parallel walls, containing either a solution of 

 Potassium bichromate or an ammoniacal solution of Copper oxide 

 The former solution is so prepared as to let through only the less 

 refrangible rays (red, orange, yellow, and some green), the latter 

 absorbs these rays and transmits only the more refrangible rays. 

 If a dark room is not available a large box may be used, the 

 inside of which is covered with dull black paper. We now expose 

 swarmers in the hanging drop to light of different wave lengths. 

 Under the influence of the less, refrangible rays the organisms 

 behave as in darkness ; the rays transmitted by the ammoniacal 

 Copper oxide solution however, like mixed white light, materially 

 influence the direction of motion of the swarmers. 



For the method to be employed in investigating the behaviour 

 of the swarmers in the individual groups of rays in the objective 

 spectrum, see Strasburger's cited work ; also Pfeffer in Botan. 

 Zeitung, 1872, No. 23. Here and in many other cases the spec- 

 _-._., / trum obtained by means of Rauland's grating might be recom- 

 mended for investigations in plant physiology. 



Even under the influence of gas-light the swarmers exhibit 

 their phototactic p -culiarities. If between the gas-light and the 

 microscope we interpose in a suitable manner a concentrated solu- 

 tion of alum, the organisms react in the usual way to the stimulus 

 of the light. Their reaction is, however, arrested when a solution 

 of Iodine in Carbon disulphide is interposed. Since the alum 

 solution is highly athermanous, while the Iodine solution is 

 strongly diathermanous, we may conclude from the above experi- 

 ment that dark heat rays are without influence on the direction of 

 movement of the swarmers. 



Also the swarmers of algee, as can easily be determined by 

 employing strips of paper according to the method described in 

 171, possess no rheotropic properties. But these swarmers 

 exhibit geotropic irritability, as the following experiment seems 



