CH. VIl] HELIOTROPISM. 165 



Section C. Heliotropism. 



(210) Heliotropic Curvature. 



Positive heliotropism may be observed with the 

 hypocotyls of Mustard {Sinapis alba), or with seedlings of 

 Canary Grass (Phalaris canariensis), which latter are 

 extremely sensitive to small differences of illumination^. 



Sow Phalaris in a small pot and let the soil be level 

 with the rim of the pot, which would otherwise shade the 

 plants. Place the pot on a plate of sand and cover it with 

 an inverted cylinder of cardboard or zinc-plate, the edge 

 of which rests in the sand, and keep it in a dark room. 

 When the seedlings are some 10 mm. in height, remove 

 the cylinder and let them be exposed to a small gas-flame 

 at a distance of about 10 feet. The light should be 

 so faint that when the observer stands by the plants 

 he cannot read the figures on his watch, and cannot 

 distinguish any shadow cast by a pencil on white paper. 

 The dark room should' have walls, ceiling and floor of 

 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). 



1 Power of Movement, p. 455. 



