My dear F.

By same post, I wrote to Semper & offered machine.—²

Is this “rendered account” right; if so return it, & I will pay.—³

I have been speculating roughly & trying to get a heap of cases under one sort of rule, but it is horrid not having you to discuss it with.— Partly from the necessity of the case on evolution principles—I am coming to some such conclusion as follows. Firstly In all growing parts there is an emptying of the cells of water on one side & turgescence on the other; the turgescent cells being in most cases retained in enlarged condition by interception of matter or growth: but there may be the emptying of water without subsequent growth, & that this take place, when movement is required for a long period.

Secondly, that light, gravity, or contact with a solid body produces some effect on side illuminated, acted on by gravity or touched; but that it depends on the good of the plant whether there shall be or shall not be increased turgescence & growth on this side.— The plant merely perceives (so to speak) the light, gravity or touch (on a certain part or place, in its circumnutation, as in case of tendril of Ehinocystis which straightens itself in passing over the terminal shoot) & then turgescence & growth is either increased or eliminated through natural selection on this side according to requirements of the species.—⁴

But I daresay there are weighty objections to any such generalised view. You might ask Sachs why roots sh^d be affected by light as they are not naturally exposed to it.—⁵

Do you know (or could you turn conversation with Sachs) whether most or many true subterranean turn either to or from light, if exposed to it; if they all turn to or from it, as this could be no benefit to root, it w^d be a serious objection to above view. One can see why some aerial roots sh^d be anheliotropic—

Again why does mould turn to light: is light of any use to mould, & does it not properly grow exclusively in dark.⁶ So many organs of the same kind are geotropic & apo-geotropic, that I cannot believe that gravity in itself tends to increase or decrease growth on one side. It is just possible with radicles as contact makes them bend from an object; so light may act on them in some manner indirectly, though of not slightest use or importance to them.

Yours affect | C. Darwin

Give me any information which you can—