To J. D. Hooker 25 [June 1863]
Down Bromley Kent.
My dear Hooker
I have been observing, pretty carefully, a little fact which has surprised me; & I want to know from you & Oliver1 whether it seems new or odd to you; so just tell me whenever you write: it is very trifling fact so do not think of answering on purpose I have got a plant of Echinocystis lobata to observe the irritability of tendrils described by Asa Gray, & which, of course, is plain enough.2 Having the plant in my study I have been surprised to find that the uppermost part of each branch, (ie the stem between the two uppermost leaves, excluding the growing tip) is constantly and slowly twisting round, making a circle in from 1 to 2 hours: it will sometimes go round 2 or 3 times, & then at same rate untwists & twists in opposite direction. It generally rests half an hour before it retrogrades. The stem does not become permanently twisted. The stem beneath the twisting portion does not move in the least, though not tied. The movement goes on all day & all early night— It has no relation to light for the plant stands in my window & twists from the light just as quickly as towards it.—3
This may be common phenomenon for what I know; but it confounded me quite when I began to observe the irritability of the tendrils.— I do not say it is final cause, but the result is pretty for the plant every 1 or 2 hours sweeps a circle, (according to length of bending shoot & length of tendril) of from 1 foot to 20 inches in diameter, & immediately that the tendril touches any object its sensitiveness causes it immediately to seize it. A clever gardener, my neighbour,4 who saw the plant on my table last night, said “I believe, Sir, the tendrils can see, for wherever I put the plant, it finds out any stick near enough”. I believe the above is the explanation, viz that it sweeps slowly round & round. The tendrils, have some sense, for they do not grasp each other when young.—
Yours affect | C. Darwin
CD describes first observation of gyratory motion of tendrils: explains its adaptive function is to find objects to hold on to.