CHAP. VIII. HELIOTROPISM. 425 



manner over nearly the same space, and a figure formerly given 

 is here reproduced (Fig. 172). If the hypocotyl is exposed to 

 a moderately strong lateral light it moves quickly towards this 

 side, travelling in a straight, or nearly straight, line. But when 

 the lateral light is very dim its course is extremely tortuous, and 

 evidently consists of modified circumnutation. Seedlings were 

 placed before a north-east window, protected by a linen and 

 muslin blind and by a towel. The sky was cloudy, and when- 

 ever the clouds grew a little lighter an additional muslin blind 

 was temporarily suspended. The light from the window was 



Fig. 172. 



Brassica oleracea : ordinary circumnutating movement of the hypocotyl of 

 a seedling plant. 



thus so much obscured that, judging by the unassisted eye, the 

 seedlings appeared to receive more light from the interior 

 of the room than from the window; but this was not really 

 the case, as was shown by a very faint shadow cast by a pencil 

 on a card. Nevertheless, this extremely small excess of light 

 on one side caused the hypocotyls, which in the morning had 

 stood upright, to bend at right angles towards the window, 

 so that in the evening (after 4.23 P.M.) their course had to be 

 traced on a vertical glass parallel to the window. It should be 

 stated that at 3.30 P.M., by which time the sky had become 

 darker, the towel was removed and replaced by an additional 

 muslin blind, which itself was removed at 4 P.M., the other twe 



