180 HELIOTROPISM. [CH. VII 



probable that these movements are due to flaccidity, 

 because if a branch is fixed so that its axis points vertically 

 downwards the leaves still sink during frost ; in this case 

 the sharp curve of the petiole does not take place and the 

 position of the leaves with regard to the branch is quite 

 different 1 . 



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 2 . 



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 



1 During frost the curious downward curvature of the branches of the 

 lime (Tilia) should be noticed : frozen branches assume their normal 

 form when they are brought into a warm room. See Caspary, Report of 

 Internal. Hort. Exhibition, 1866, also Geleznow, Bull. Acad. Petersburg, 

 1872. 



2 Power of Movement, p. 455. 



