My dear Sir

I am going to beg a favour of you. I have just read, with great interest & profit, your Essay in Heft 2. 1872 of Arbeiten … Wurzburg¹ I have been observing, for a special purpose, the Cotyledons of a large number of plants, & some young leaves, with the stems of all secured to sticks close beneath. They all grew in pots & were placed close to a North East window, & I was greatly troubled (for I was not attending to Heliotropism) by their all turning to the light, though this was not very bright. From these many cases & from statements in almost every botanical book, I wrote in my notes “that all the cotyledons which I observed turned, like leaves, towards a lateral light”. I was therefore much startled when coming to a passage (p. 261) where you say “Aus diesen Versuchen geht hervor (1) dass in vielen Fallen kein Einfluss des Heliotropismus zu bemerken war,”² I infer therefore that your leaves did not turn to the light. Can the difference between what you so carefully observed, & what I have repeatedly, but only in a few cases, carefully observed, be accounted for by your having cut away the lamina?³

Perhaps you refer exclusively to the heliotropism of the petiole & mid-rib, yet I have often seen the petiole of cotyledons curve towards the light. My plants all grew on their own roots, whilst yours were cut off & stuck in sand; & this perhaps may have made some difference, as it certainly does with the revolving nutation of climbers.⁴

You know so very much more than I do on all these subjects, that I should be extremely obliged if you would tell me whether you think that I err in saying that cotyledons & young leaves turn to a lateral light, independently of the heliotropic movement of their stems; the stems having been secured to sticks.—

But I do not see how I could have erred.

Forgive me for troubling you & believe me my dear Sir | yours sincerely | Charles Darwin

PS. How do your observations progress on the contraction of young stems and radicles? I hope that you will publish soon.— I trust that you received the seeds from Prof. Asa Gray.—⁵