My dear Father,

I have had a talk with Sachs about oat & canary grass   he says the chlorophyll is only beginning to be developed when the seedling appears above ground & that the assimilation is extremely small for the first few days— He evidently thought that assimilation was practically not going on for the first few days. If you want to know more accurately I can easily grow some oats, & it would be good practice in section cutting or perhaps in spectroscoping.² But Sachs seemed very confident about the complete unimportance of assimilation. I have borrowed Cieselski & read him.³ Sachs doesn’t consider that there is any puzzle as to how the difference between their results arose. He says the bend up only occurs when the root has previously lost water & & then the cells on the water surface become more turgid & cause the upward bend. I think I only saw a passage about Cieselski in ‘Ablenkung der Wurfeln &’ p 219— I suppose you have seen the place in “Ueber das Wachsthum der Hapt u Nebenw” p 401, where he talks about the fact that the roots must be slightly dry for C’s bend.⁴ I have read most of Cieslski & I dont think there is much to trouble about. Sachs doesn’t think much of him, although some of his experiments were good. One fact of his would be interesting if it were true but Sachs says it is “falsch’— If you cut the extreme tip of a root, it goes on growing pretty well though the punctum vegetationis is destroyed. Cieselski says that while this growth continues & before a new punctum veg^is. is formed that the roots does not care for gravity & will grow in any direction it is placed in⁵  This Sachs says is not true.

Cieselski says if you shave a long sloping bit off a root—& place the root horizontally then if the cut surface is downwards the root will bend down geotropically even quicker than usual, but if the cut surface is uppermost, the root will sometimes bend up before it bends down. This looks like bending towards the cut side.

I have got a most queer looking sleeper—? hygrometrica: it has pinnate leaves like M. Farnesiana & Sachs says it always asleep—he doesn’t understand the movements: he thinks it must be suited for some very exceptional climate or position: it has a great deal of wood & very few leaves & all afternoon in shade they were pressed back agains the stems & the leaflets shut—I will observe it.⁶ Sachs declares he believes that one use of sleeping plants shutting up on being irritated is a protection against hail. There was a summer hailstorm here of only 5 or 10 minutes which did many pounds worth of damage by killing plants, & some mimosas were out in it & were quite uninjured   He thinks the flaccidity of the leaf in the irritated state important as making it yield on being struck. He seemed quite inclined to believe in protection from cold. Also in bloom being protection against dryness. He thinks blo⁠⟨⁠om⁠⟩⁠ must have many functions & said it was a protection against insects.⁷ He had never noticed the fact that when you wet the lower end of a yew stick water appears above & quite agrees with my idea that it is important, he evidently didint much believe in it till he tried it himself & then was pleased with it

I have not got any good Hydrocharis plants so have not begun.⁸ I have been cutting to get exactly central ones. I think Sachs thought I wanted to publish something & that was why he gave me the the Hydrocharis to do, I told him that I would rather do something that hadn’t been finished even tho’ it led to nothing so I have started trying to make out what changes in strength & in elasticity go on as a woody stem loses water

At present the experiments are very simple

A stick or grass stalk fixed horizontally at one end & a weight to take off & put on, & I measured the movement with my mercury microm⁹   The ultimate object is to know what share the tension of the cell walls (not the hydrostatic turgor) takes in causing the stiffness of a stem. One ought to be a first rate mathematician for such a thing. I shall also do some experiments putting fresh & withered shoots into alcohol & seeing what happens. Sachs has done some experiments, & says they both retain their form very nearly, but he doesn’t know exactly. Sachs told me of a real fools experiment he tried, he thought the green colour of cabbage catterpillars must come from chlorophyll & so he tried feeding them on etiolated leaves—but they would not turn white.¹⁰ I think it would be a good thing to have a telescope like Vines used, it would do for any careful observations—they are made in Munich & I could order one like Sachs’   they cost about £10.¹¹ I should think there are at least 50 thermometers in the laboratory   some beauties divided in $\frac{1}{10}$^th Centigrade. The fittings generally are very simple; he has a great many big cylinders of glass & all sorts of zinc covers & boxes for various things—& splendid lots of cork, also silvered wire for fastening things under water, tho’ brass does as well I should think.

My love to everybody | Yr affec son | Frank Darwin