d.),
emit worm-like masses of mucus, and it would be curious to ascertain
whether wetting the pappus alone would suffice to cause such secretion.
(761/3. See Letter 707.)

LETTER 762. TO G.J. ROMANES. Down, April 18th, 1881.

I am extremely glad of your success with the flashing light. (762/1.
Romanes' paper on the effect of intermittent light on heliotropism was
the "Proc. Royal Soc." Volume LIV., page 333.) If plants are acted on by
light, like some of the lower animals, there is an additional point of
interest, as it seems to me, in your results. Most botanists believe
that light causes a plant to bend to it in as direct a manner as light
affects nitrate of silver. I believe that it merely tells the plant to
which side to bend, and I see indications of this belief prevailing even
with Sachs. Now it might be expected that light would act on a plant in
something the same manner as on the lower animals. As you are at work on
this subject, I will call your attention to another point. Wiesner, of
Vienna (who has lately published a great book on heliotropism) finds
that an intermittent light, say of 20 minutes, produces the same
effect as a continuous light of, say 60 m. (762/2. Wiesner's papers on
heliotropism are in the "Denkschriften" of the Vienna Academy, Volumes
39 and 43.) So that Van Tieghem, in the first part of his book which has
just appeared, remarks, the light during 40 m. out of the 60 m. produced
no effect. I observed an analogous case described in my book. (762/3.
"Power of Movement," page 459.)

Wiesner and Van Tieghem seem to think that this is explained by
calling the whole process "induction," borrowing a term used by some
physico-chemists (of whom I believe Roscoe is one) and implying an
agency which does not produce any effect for some time, and continues
its effect for some time after the cause has ceased. I believe that
photographic paper is an instance. I must ask Leonard (762/4. Mr.
Darwin's son.) whether an interrupted light acts on it in the same
manner as on a plant. At present I must still believe in my explanation
that it is the contrast between light and darkness which excites a
plant.

I have forgotten my main object in writing--viz., to say that I believe
(and have so stated) that seedlings vary much in their sensitiveness
to light; but I did not prove this, for there are many difficulties,
whether the time of incipient curvature or the amount of curvature is
taken as the criterion.