Cassia tora.—­The cotyledons of this plant are extremely sensitive to light, whilst the hypocotyls are much less sensitive than those of most other seedlings, as we had often observed with surprise.  It seemed therefore worth while to trace their movements.  They were exposed to a lateral light before a north-east window, which was at first covered merely by a muslin blind, but as the sky grew brighter about 11 A.M., an additional linen blind was suspended.  After 4 P.M. one blind and then the other was removed.  The seedlings were protected on each side and above, but were open to the diffused light of the room in the rear.  Upright filaments were fixed to the hypocotyls of two seedlings, which stood vertically in the morning.  The accompanying figure (Fig. 177) shows the course pursued by one of them during two days; but it should be particularly noticed that during the second day the seedlings were kept in darkness, and they then circumnutated round nearly the same small space.  On the first day (Oct. 7th) the hypocotyl moved from 8 A.M. to 12.23 P.M., toward the light in a zigzag line, then turned abruptly to the left and afterwards described a small ellipse.  Another irregular [page 432] ellipse was completed between 3 P.M. and about 5.30 P.M., the hypocotyl still bending towards the light.  The hypocotyl was straight and upright in the morning, but by 6 P.M. its upper half was bowed towards the light, so that the chord of the arc thus formed stood at an angle of 20o with the perpendicular.  After 6 P.M. its course was reversed through the action of apogeotropism, and it continued to bend from the window during the night, as shown by the broken line.  On the next day it was kept in the dark (excepting when each observation was made by the aid of a taper), and the course followed from 7 A.M. on the 8th to 7.45 A.M. on the 9th is here likewise shown.  The difference between the two parts of the figure (177), namely that described during the daytime on the 7th, when exposed to a rather dim lateral light, and that on the 8th in darkness, is striking.  The difference consists in the lines during the first day having been drawn out in the direction of the light.  The movements of the other seedling, traced under the same circumstances, were closely similar.

Apheliotropism.—­We succeeded in observing only two cases of apheliotropism, for these are somewhat rare; and the movements are generally so slow that they would have been very troublesome to trace.

Fig. 178.  Bignonia capreolata:  apheliotropic movement of a tendril, traced on a horizontal glass from 6.45 A.M.  July 19th to 10 A.M. 20th.  Movements as originally traced, little magnified, here reduced to two-thirds of the original scale.