My Dear Sir

Through the kindness of Dr Gray I have had an opportunity of reading your admirable paper on twining plants, which is so full of admirable observations on vegetable motions.¹ These are all of great interest on account of their physiological import, & I have always believed that they were the result of contractility of cells, analogous to those of the lowest animals.² Not however because this contractility has been actually demonstrated, but because it is the most reasonable to suppose that it exists. The movements of stems, but especially of petioles or tendrils, on the application of an irritation, are so analogous to those of some of the lower animals, that if one were not prepossessed with the idea that the motions of plants & animals could not be alike, we should at once admit their likeness. However I do not propose to discuss the question, but only to mention an experiment which gives a mechanical explanation of the double spiral of tendrils after the free end has become attached. The experiment shows, that if the cells on one side of a tendril were to shorten, & this in a straight line, the tendril (its ends being fixed to moveable supports) would at once assume the form of a double spiral.³

Take a piece of elastic cord (a) 3 inches long & a piece of twine (b) of the same size, 4 or 5 inches long, & tie their two ends together—

Stretch the elastic cord until the two are of the same length & secure them in this condition by fastening the ends, taking care that the two are exactly parallel & in contact. Wind both with fine thread, closely. In this way we have a compound cord contractile (through the elasticity of the india rubber) on one side & passive on the other. Hold the ends of this cord between the fingers, stretch it to its full length, & then allow the two ends to approach each other slowly. The cord at once begins to assume the form of a double spiral exactly reproducing the form of the tendril.

I do not find that the mathematicians can give an analysis of this motion, at any rate did not when asked what it ought to be a priori, & have not after they have seen it. Of course they will make it out, but the problem is not an easy one. I hope you will excuse me for troubling you with this, for although the experiment illustrates the spirals of the tendril & shows how they might be made it does not prove how the actually are made.

With great respect | Sincerely yours | J Wyman

Charles Darwin Esq