My dear Sir!

Permit me to pay you my sincere thanks for the great interest, you show in my researches on the growth of plants. I avow, that the numbers, given by me on pag 260–261 of Arbeiten—Würzburg, the passage quoted by you, may give occasion for the belief, that some leaves may be insensible to the influence of light.¹ This is, however, not my opinion, especially I should not dare assert, that the leaves of the species quoted there (Clematis, Corylus, Inula, Polygonum, Sedum,)² may not be heliotropic. On the contrary, I am convinced, that by repeating my experiments under other circumstances, I should observe heliotropic movements in these cases too. That the leaves, I experimented with, did not show such movements, must be attributed to peculiar circumstances. I suppose that (with the exception of Inula) the very strong epinastical curvations were an impediment to the observation of the heliotropic influence.³ Perhaps the cutting off of the petioles and the isolation of the midribs had an influence, as you suppose, but this influence cannot have been very great, as the epinastical curvations were especially in these cases very manifest.

I wholly agree with you, that Cotyledons and young leaves turn to a lateral light, independent of the movement of their stems; what I myself occasionally observed on this subject is quite conform to your opinion   I do not wish to infer from my experiments another conclusion, than that in isolated petioles and midribs, under the said circumstances, the heliotropism is in some cases so feeble, that it cannot be observed by the method, I employed.⁴

Prof. Asa Gray had the kindness to send me the seeds, you asked him for me, I have sown them as he wrote me to do, and hope to make this summer the experiments on the motion of their tendrils. I am much obliged to you for your kindness, and hope I shall be able to communicate the results of these experiments to you next summer.⁵

On the contraction of roots my observations have but a slow progress; I observed the phenomenon with a large number of plants; it seems to occur very generally. The roots of Iris pallida show it brilliantly, also the roots of the hyacinth, cultivated on waterglasses.⁶

With many thanks for your kind interest in my researches, I remain | dear Sir | Yours sincerely | Hugo de Vries