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.
At this time Mr. Romanes had a very interesting correspondence with the Rev. G. Henslow, on the subject of the direct action of the environment on plant structures.