Mustard seedlings, grown in the dark until they were about one or two inches high, were used in all the experiments; they were either placed in a dark room and exposed to flashes of light in the form of electric sparks passed at regular intervals, or they were put in a camera obscura, before which was placed a Swan burner or arc lamp, the light from which was rendered intermittent by the regular opening and shutting of the photographic shutter. The heliotropic effect on the seedlings was found in all cases to be very marked, the most vigorous ones beginning to bend towards the light ten minutes after the flashing began, bending through 45° in as many minutes, and often through another 45° in as many minutes more. By protecting half of the seedlings from the interrupted light, by means of a cardboard cap, then after the experiment uncovering them and exposing that half for the same duration of time to constant sunlight, Mr. Romanes found that the bending was less in this latter case, that is, when the light was continuous. This result was confirmed by placing two sets of plants under exactly similar conditions before a Swan burner, the light from which was constant for one set of seedlings, and rendered intermittent for the other set by working the flash shutter; in all cases the interrupted light caused the plants to start bending more quickly, and through a greater angle in a given time. As regards the rate the flashes must succeed one another to produce this heliotropic effect, Mr. Romanes found that sparks passed at the rate of fifty in an hour would cause considerable bending in half an hour. It is of interest to note that in no single case was there any green colouring matter produced, the seedlings remaining colourless even when the sparks were passed at the rate of 100 per second continuously during forty-eight hours. Dr. Sanderson writes: Friday, November 17. My dear Romanes,--There was a rather interesting discussion at the R.S. on your paper about the fresh experiments with seedlings. It was objected that there was no evidence that the effects were not due to one-sided drying of the stems of the seedlings, and ---- wanted to know whether sufficient precautions were taken to guard against this. I suppose that he meant heat effects. I said that, under the conditions of this experiment, I could not see how any 'drying effect' could possibly take place. My suggestion is that it would be worth while to add a note, if you think of the impossibility of any effect, excepting a light effect, being concerned. I asked Foster just now, and he agreed with me that it would be useful. I ought to add that it was admitted that the observation was a new one which promised to have very important bearings. I am writing this in great haste. I trust that you are enjoying Costebelle. Very truly yours, T. BURDON SANDERSON.