Fig. 175.  Tropaeolum majus:  heliotropic movement and circumnutation of the epicotyl of a young seedling towards a dull lateral light, traced on a horizontal glass from 7.48 A.M. to 10.40 P.M.  Figure reduced to one-half of the original scale.

received for a short time less light from the window than from the opposite side (as ascertained by the shadow cast), and then one of the blinds was temporarily removed.  In the evening the blinds were taken away, one by one. the course pursued by an epicotyl under these circumstances is shown in Fig. 175.  During the whole day, until 6.45 P.M., it plainly bowed itself towards the light; and the tip moved over a considerable space.  After 6.45 P.M. it moved backwards, or from the window, till [page 430] 10.40 P.M., when the last dot was made.  Here, then, we have a distinct heliotropic movement, effected by means of six elongated figures (which if dots had been made every few minutes would have been more or less elliptic) directed towards the light, with the apex of each successive ellipse nearer to the window than the previous one.  Now, if the light had been only a little brighter, the epicotyl would have bowed itself more to the light, as we may safely conclude from the previous trials; there would also have been less lateral movement, and the ellipses or other figures would have been drawn out into a strongly marked zigzag line, with probably one or two small loops still formed.  If the light had been much brighter, we should have had a slightly zigzag line, or one quite straight, for there would have been more movement in the direction of the light, and much less from side to side.

Fig. 176.  Tropaeolum majus:  heliotropic movement and circumnutation of an old internode towards a lateral light, traced on a horizontal glass from 8 A.M.  Nov. 2nd to 10.20 A.M.  Nov. 4th.  Broken lines show the nocturnal course.

Sachs states that the older internodes of this Tropaeolum are apheliotropic; we therefore placed a plant, 11 3/4 inches high, in a box, blackened within, but open on one side in front of a north-east window without any blind.  A filament was fixed to the third internode from the summit on one plant, and to the fourth internode of another.  These internodes were either not old enough, or the light was not sufficiently bright, to induce apheliotropism, for both plants bent slowly towards, instead of from the window during four days.  The course, during two days of the first-mentioned internode, is given in Fig. 176; and we see that it either circumnutated on a small scale, or travelled in a zigzag line towards the light.  We have thought this case of feeble heliotropism in one of the older internodes of a plant, [page 431] which, whilst young, is so extremely sensitive to light, worth giving.

Fig. 177.  Cassia tora:  heliotropic movement and circumnutation of a hypocotyl (1 ½ inch in height) traced on a horizontal glass from 8 A.M. to 10.10 P.M.  Oct. 7th.  Also its circumnutation in darkness from 7 A.M.  Oct. 8th to 7.45 A.M.  Oct. 9th.