Moreover they vary according to age, and
perhaps from vigour of growth, and there seems inherent variability,
as Strasburger (whom I quote) found with spores. If the curious anomaly
observed by you is due to varying sensitiveness, ought not all the
seedlings to bend if the flashes were at longer intervals of time?
According to my notion of contrast between light and darkness being the
stimulus, I should expect that if flashes were made sufficiently slow
it would be a powerful stimulus, and that you would suddenly arrive at
a period when the result would SUDDENLY become great. On the other hand,
as far as my experience goes, what one expects rarely happens.

LETTER 763. TO JULIUS WIESNER. Down, October 4th, 1881.

I thank you sincerely for your very kind letter, and for the present of
your new work. (763/1. "Das Bewegungsvermogen der Pflanze," 1881. One of
us has given some account of Wiesner's book in the presidential address
to Section D of the British Association, 1891. Wiesner's divergence
from Darwin's views is far-reaching, and includes the main thesis of
the "Power of Movement." See "Life and Letters," III., page 336, for an
interesting letter to Wiesner.) My son Francis, if he had been at home,
would have likewise sent his thanks. I will immediately begin to read
your book, and when I have finished it will write again. But I read
german so very slowly that your book will take me a considerable time,
for I cannot read for more than half an hour each day. I have, also,
been working too hard lately, and with very little success, so that I am
going to leave home for a time and try to forget science.

I quite expect that you will find some gross errors in my work, for you
are a very much more skilful and profound experimentalist than I am.
Although I always am endeavouring to be cautious and to mistrust myself,
yet I know well how apt I am to make blunders. Physiology, both animal
and vegetable, is so difficult a subject, that it seems to me to
progress chiefly by the elimination or correction of ever-recurring
mistakes. I hope that you will not have upset my fundamental notion
that various classes of movement result from the modification of a
universally present movement of circumnutation.

I am very glad that you will again discuss the view of the turgescence
of the cells being the cause of the movement of parts. I adopted De
Vries' views as seeming to me the most probable, but of late I have felt
more doubts