CHAP. VIII. APHELIOTKOPISM. 435 



ment, greatly magnified, was traced on a horizontal glass during 

 .67 h. The plant was illuminated during the day from above. A 

 copy of the tracing is given on p. 434 (Fig. 179) ; and there can 

 be no doubt that the descending movement is one of modified 

 circumnutation, but on an extremely small scale. The observa- 

 tion was repeated on another pod, which had partially buried 

 itself in sawdust, and which was lifted up a quarter of an inch 

 above the surface ; it described three very small circles in 24 h. 

 Considering the great length and thinness of the peduncles 

 and the lightness of the pods, we may conclude that they 

 would not be able to excavate saucer-like depressions in sand 

 or sawdust, or bury themselves in moss, &c., unless they were 

 aided by their continued rocking or circumnutating move- 

 ment. 



Relation between Circumnutation and Heliotropism. 

 Any one who will look at the foregoing diagrams, 

 showing the movements of the stems of various plants 

 towards a lateral and more or less dimmed light, will 

 be forced to admit that ordinary circumnutation and 

 heliotropism graduate into one another. When a 

 plant is exposed to a dim lateral light and continues 

 during the whole day bending towards it, receding 

 late in the evening, the movement unquestionably is 

 one of heliotropism. Now, in the case of Tropaeolum 

 (Fig. 175) the stem or epicotyl obviously ciicumnu- 

 tated during the whole day, and yet it continued at 

 the same time to move heliotropically ; this latter 

 movement being effected by the apex of each succes- 

 sive elongated figure or ellipse standing nearer to 

 the light than the previous one. In the case of 

 Cassia (Fig. 177) the comparison of the movement of 

 the hypocotyl, when exposed to a dim lateral light and 

 to darkness, is very instructive ; as is that between 

 the ordinary circumnutating movement of a seedling 

 Brassica (Figs. 172, 173), or that of Phalaris (Figs. 

 49, 174), and their heliotropic movement towards a 

 window protected by blinds. In both these cases, 



