166 NEGATIVE HELIOTROPISM. [CH. VII 



(212) lAght of high refrangiUlity most effective^. 



To expose plants to light of different refrangibility we 

 use a box (blackened inside) whose lateral opening can be 

 closed by a flat bottle: the bottles may be filled with 

 various fluids and in this way the efficacy of different 

 parts of the spectrum may be roughly tested''. The 

 bottles must fit into grooves so that no light can enter 

 the box except through the coloured fluid. 



In one box, B, let the bottle contain a solution of 

 potassium-bichromate, and let the bottle in G contain 

 ammoniacal copper-sulphate. In each box place a pot 

 of Mustard seedlings which have been grown in complete 

 darkness and whose vertical hypocotyls are about 20 mm. 

 in length. They may be examined after 4 to 6 hours, 

 when a striking difference should be seen, the curvature 

 in B being frequently hardly perceptible. 



(213) Negative heliotropism. 



Allow, mustard seed to germinate in sawdust which 

 should be thrown lightly into the flower-pot, not pressed 

 down, and should not be too wet. When the radicles are 

 15 — 20 mm. in length the seedlings are pulled out of the 

 sawdust and placed with their roots in water. A disc 

 of thin cork plate is pierced with holes of 5 — 6 mm. 

 in diameter and is supported by 3 bent bits of wire 

 in the mouth of a glass beaker about 1 — 2 mm. above the 

 surface of the water. The mustard seedlings are supported 



' Wiesner, Heliotropische Erseheinungen im Ffianzenreiche. Vienna 

 Denksohr., 1878. 



^ It is far more satisfactory, but not so easy to use a pure spectrum. 



