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 circumnutated 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 sufficed 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 bent 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 circumnutated 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 respects [page 424] similar to the previous one.  In these two cases there could be no doubt that the ordinary circumnutating movement of the hypocotyl was modified and rendered heliotropic.

Fig. 171.  Apios graveolens:  heliotropic movement and circumnutation of the hypocotyls of two seedlings towards a dim lateral light, traced on a horizontal glass during the day.  The broken lines show their return nocturnal courses.  Height of hypocotyl of A .5, and of B .55 inch.  Figure reduced to one-half of original scale.

Brassica oleracea.—­The hypocotyl of the cabbage, when not disturbed by a lateral light, circumnutates in a complicated [page 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 whenever 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.