CH. VIl] HELIOTROPISM. 181 



a dead unvarnished black, and care should be taken that 

 there are no polished objects which might reflect light. 

 The room should, moreover, have double doors separated 

 from each other by a space, so that the observer may 

 enter the room without admitting light. 



The canary grass should be left for 8 or 10 hours, when 

 a distinct heliotropic curvature should be visible. 



(211) After-effect 



After-effect may be observed in the same way mutatis 

 mutandis as has been described for geotropism (exp. 202). 



(212) Light of high refrangihility most effective^. 



To expose plants to light of different refrangihility 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 eflicacy 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 C contain 

 ammoniacal copper-sulphate. In each box place a pot 

 of Sinapis seedlings which have been grown in complete 

 darkness, and whose vertical hypocotyls are about 20 mm. 

 in length, or Phalaris may be used. They may be 

 examined after 4 to 6 hours, when a striking difference 

 should be seen between A and B. 



1 Wiesner, Heliotropische Erscheinungen im Pflanzenreiche. Benk- 

 schr. d. k. Akad. Wien, 1878. 



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



