Ciup. VIII. HELIOTROPISM. 423 



gularly towards a bright lateral light. In order to ascertain 

 how straight a course it would pursue when fairly well illumi- 

 nated on one side, seedlings were first placed before a south-west 

 window on a cloudy and rainy morning ; and the movement of 

 two hypocotyls weie traced for 3 h., during which time they 

 became greatly bowed towards the light. One of these tracings 

 is gi von on p. 422 (Fig. 170), and the course may be seen to be 

 almost straight. But the amount of light on this occasion was 

 superfluous, for two seedlings were placed before a north-east 

 window, protected by an ordinary hnen and two muslin blinds, 

 yet their hypocotyls moved towards this rather dim light in 

 only slightly zigzag lines; but after 4p.m., as the light waned, 

 the lines became distinctly zigzag. One of these seedlings, 

 moreover, described in the afternoon an ellipse of considerable 

 size, with its longer axis directed towards the window. 



We now determined that the light should be made dim 

 enough, so we began by exposing several seedlings before a 

 north-east window, protected by one linen blind, three muslin 

 blinds, and a towel. But so little light entered that a pencil 

 cast no perceptible shadow on a white card, and the hypocotyls 

 did not bend at all towards the window. During this time, 

 from 8.15 to 10.50 a.m., the hypocotyls zigzagged or circum- 

 nutated near the same spot, as may be seen at A, in Fig. 171. 

 The towel, therefore, was removed at 10.50 a.m., and replaced 

 by two muslin blinds, and now the light passed through 

 one ordinary linen and four muslin blinds. When a pencil 

 was held upright on a card close to the seedlings, it cast a 

 shadow (pointing from the window; which could only just 

 be distinguished. Yet this very slight excess of light on 

 one side sufiiced to cause the hypocotyls of all the seedlings 

 immediately to begin bending in zigzag lines towards the 

 window. The course of one is shown at A (Fig. 171): after 

 moving towards the window from 10.50 a.m. to 12.48 p.m. it 

 l)ent from the window, and then returned in a nearly parallel 

 line; that is, it almost completed between 12.48 and 2 p.m. 

 a narrow ellipse. Late in the evening, as the light waned, 

 the hypocotyl ceased to bend towards the window, and circum- 

 nutated on a small scale round the same spot ; during the night 

 it moved considerably backwards, that is, became more upright, 

 through the action of apogeotropism. At B, we have a tracing 

 of the movements of another seedling from the hour (10.50 a.m.) 

 when the towel was removed ; and it is in all essential resppf is 

 28 



