From Francis Darwin to Horace Darwin 24 November 1875

My dear Jemmy

I will tell you a little stipa news— I am getting more and more to believe that it is something intrinsic in the cells or fibres of the twisting stalk themselves— First of all the fact that a mere shaving of Stipa cut parallel to the long axis of the stalk goes into a spiral in the same way as the whole stalk seems to show it— I cut sections of the anem one which twists and found it to consist of a ring of thick walled tubes or cells *P [diagram of a ring of cells] *Q black dots are the cavities of the cells. The middle of the ring is broken-down empty tissue which can have no effect—so it evidently cannot twist by the central parts contracting more than the periphery. Considering this with the fact that mere shavings of the outside of anemone & like the longitudinal sections of stipa, twist up into corkscrews when drying, I thought that the twisting of Stipa must depend on each cell having a tendency to go into corkscrews. (A stipa does not appear like a corkscrew but like rope) The following experiment shows that a very small amount of corkscrew would make it twist. I wetted a lot stipa till they were straight & then tied them with a cylindrical bundle with several ties of thread. on drying they made a beautiful rope which of course was twisted in the opposite direction to each of its stipa-strands— The mechanism by which it is possible for a cell either to twist on its own axis or to go into a corkscrew depends I believe on what is called the “striation” of its cell wall. If you cut a transverse section of a thick walled cell you see, that it is made up of a number *P ⁠⟨⁠diagram of the alternating light and dark shells of a thick walled cell⁠⟩⁠ *Q of alternate light and dark shells fitting inside each other   These are caused by layers of tissue which alternately contain more & less water–-by drying a section the water is driven out of the wet ones & the striation disappears & vice versa it disappears on extreme wetting. Besides this striation there are two other series of laminæ of alternate densities— One which in transverse sections would be like this *P [diagram of a striation that looks like a spider's web] *Q and another which would be lik this *P [diagram of a striation] *Q a whole cell

These two last systems may play all kind of pranks which I don’t twig yet from descriptions   Sachs draws a cell with these two systems of striation making *P [diagram of a cell] *Q a double spiral— And it this spiral striation which I think I have made out in the cells of anemone—& I believe in stipa but I am waiting for some “Shultz’sSolution” to isolate the cells properly. The two halves into which a pea-pod splits, each roll up into corkscrews on drying, this is due to what you thought must be in Stipa—or something like it

A longitudinal ribbon cut from a half-pod does as well as a whole *P [diagram of a rectangle labeled with points "B", "a", and "c"] *Q arrow shows the direction of contraction

This is a view of the surface of a ribbon (section *P [diagram of a rectangle, the bottom half is black and the top half is white] *Q ) the dark part being more contractile than the white. If you cut a ribbon like (a) parallel with the fibres, it hardly alters at all when it dries or merely goes into a curve; but the the whole big piece B goes into a corkscrew. Here the expansion and contraction depend on the presence or absence of water between the fibres not as in stipa among the molecules of the fibres (cells)

If you cut a bit out like (c) which is perpendic to B as regards the direction of the fibres, it goes into a spiral the opposite to B. The fact that pieces like (a) do bend a little on drying, shows that there is some inter-molecular contraction,

I have made yr clock work most lovelily with a little Indian grass, it kicks awfully when you breathe on it— Amy has gone to Pantlludw to cheer up her mother who felt the loss of M^rs Ford very much who was by far her greatest friend. The whole county seems to grieve for M^rs Ford she was so wonderfully good to all. I go to Pantlludw in 10 days.

Yrs affect^ly | FD

Gurney is at Brighton with his brother John who is seedy—at 8. Marlborough Place, you had better go & look him up

G