H ELIOT ROP ISM OF ANIMALS 31 



the tiilir. If at the beginning <>f the experiment (he animals 

 were mi Ilu> room side of the test-tube, (hey move until they 

 reach (liis boundary. 



IIV therefore </et f/ie name rexitlls !>// usiiit/ red </l(isx 

 that ire tjot hi/ IIXIIKJ o/>/ni/ie jHixlehoanl in a />rerioiis 

 e.rjtertiiteiil. Taken together with the preceding ones, this 

 experiment proves that pre-eminently the more refrangible 

 ra\ s of mixed day light are heliotropically effective. Although, 

 as we have just seen, the rays passing through red glass or a 

 red solution are not absolutely ineffective, yet the weak light 

 which is reflected from the walls of the room, and which 

 contains some blue rays, is more effective than the diffused 

 light reflected from the sky after it is filtered through red 

 glass. It is for this reason that the animals on the window 

 side under the red cover migrate to the boundary of the red 

 screen where they are held by the rays of diffuse daylight. 



E.rjieriiiient 4. If, as before, we place the test-tube with 

 the longitudinal axis perpendicular to the window, and cover 

 it with red glass on the window side and with blue glass on 

 the room side, the animals collect under the blue glass at its 

 boundary with the red glass. 



Experiment 5. If we place the test-tube with its longi- 

 tudinal axis parallel to the window, the animals scatter over 

 the whole length of that part of the tube which is covered by 

 blue glass. 



From all these experiments it fotloirs flint if /.s eliiejl// flic 

 more refr<in</i/>/e VY///.S ir/iieli (Iclcriiiiiic Hie orientation of 

 flic etiler/n'lldrs of Portlicxid eli ri/xorrlnra loiranl ///////. 



The only difference between the heliotropism of these 

 animals and the heliotropism <>f plants is this, that //// /r.vs 

 i-efrtnii/ililc vf//.s arc not *<> completely ineffective in tin- 

 ciiftc of flte c(ileri>ill(irs of Porthcxid chrysorrhoea as //n-// 

 (il>/>(irnilli/ are in IIKIIII/ fildiilx. This point must, however, 

 be studied more accurately with the aid of a spectrum. 



