all pointing to the ground, was reversed, being still kept horizontal, so that the pods now pointed directly upwards; it was then placed in a dark cupboard, but the pods still pointed upwards after four days and nights.  The pot, in the same position, was next brought back into the light, and after two days there was some bending downwards of the peduncles, and on the fourth day two of them pointed to the centre of the earth, as did the others after an additional day or two.  Another plant, in a pot which had always stood upright, was left in the dark cupboard for six days; it bore 3 peduncles, and only one became within this

Fig. 179.  Cyclamen Persicum:  downward apheliotropic movement of a flower-peduncle, greatly magnified (about 47 times?), traced on a horizontal glass from 1 P.M.  Feb. 18th to 8 A.M. 21st.

time at all bowed downwards, and that doubtfully.  The weight, therefore, of the pods is not the cause of the bending down.  This pot was then brought back into the light, and after three days the peduncles were considerably bowed downwards.  We are thus led to infer that the downward curvature is due to apheliotropism; though more trials ought to have been made.

In order to observe the nature of this movement, a peduncle bearing a large pod which had reached and rested on the ground, was lifted a little up and secured to a stick.  A filament was fixed across the pod with a mark beneath, and its move-[page 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 observation 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, etc., unless they were aided by their continued rocking or circumnutating movement.]

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 circumnutated during the whole day, and yet it continued at the same time to move heliotropically; this latter movement being effected