Chap. VIU. 



HELIOTROPISM. 



421 



Fig. 169. 



only slightly zigzag. Although not a single ellipse was even 

 approximately formed, we inferred from the zigzag lines — and, 

 as it proved, correctly — that their hypocotyls were circumnuta- 

 ting, for on the following day these same seedlings wore placed 

 in a completely darkened room, and were observed each time by 

 the aid of a small wax taper held almost 

 directly above them, and their movements 

 were traced on a horizontal glass above ; 

 and now their hypocotyls clearly circum- 

 nutated (Fig. 168, and Fig. 39, formerly 

 given, p. 52); yet they moved a short 

 distance towards the side where the taper 

 was held up. If we look at these diagrams, 

 and suppose that the taper had been held 

 more on one side, and that the hypocotyls, 

 still circumnutating, had bent themselves 

 within the same time much more towards 

 the light, long zigzag lines would ob- 

 viously have been the result. 



Again, two seedlings of Solanum hjco- 

 p-rsicum were illuminated from above, 

 but accidentally a little more light entered 

 on one than on any other side, and their 

 hypocotyls became slightly bowed towards 

 the brighter side ; they moved in a zigzag 

 line and described in their course two little 

 triangles, as seen in Fig. 37 (p. 50), and 

 in another tracing not given. The sheath- 

 like cotyledons of Zea mays behaved, under 

 nearly similar circumstances, in a nearly 

 similar manner, as described in our first 

 chapter (p. 64), for they bowed themselves 

 during the whole day towards one side, 

 making, however, in their course some 

 conspicuous flexures. Before we knew 

 how greatly ordinary circumnutation was 

 modified by a lateral light, some seedling oats, with rather old 

 and therefore not highly sensitive cotyledons, were placed in 

 itont of a north-east window, towards which they bent all day in 

 a strongly zigzag course. On the following day they contimied 

 to bend in the same direction (Fig. 169), but zigzagged much 

 less. The sky, however, became between 12.40 and 2.35 p.s. 



8ta.m. 

 Avena sativa : heliotrcipio 

 movement and circum- 

 nutation of sheath-like 

 cotyledon (1^ inch in 

 height) traced on hori- 

 zontal glass from 8 A.M. 

 to 10.25 P.M. Oct. 16th. 



