From Daniel Oliver 19 September 1860
19. IX. 60
My dear Sir,
Your observations upon the sensible little Drosera, preferring milk & nitrogenous fluids,—also the effect of Nit. Ama. are very interesting.—1 Are you satisfied, though, that in respect to fluids the density of the drop put upon its leaf has not as much to do with the incurving, &c. as its chemical composition?— Or in the case of the Nit. Ama. that it is not the same result wh. any soluble irritant might occasion,—say common salt,—destitute of Nitrogn. I have experimented on one of our Droseras with fluids at yr suggestion. I tried (not saliva! but) sol. of gum,—Com. gum dissd. in water. (perhaps a little sour!) syrup—, & milk. By far the most marked result has been afforded by the gum drops.2 Had the syrup been denser it might have equalled it, tho‘ the result in its case as yet has been immaterial or next to nothing.
But I am by no means sure whether I can shew that it is a question of density (or Endosmose &c, rather), for I’ve thought it possible the gum sol. might, by virtue of its adhesiveness (?) &c. in some way or other deceive me.3
On Tuesday P.M. (2–2.15) a drop of milk, of syrup, & of gum placed on 3 several leaves of the Drosera. In each case several of the marginal hairs of the rounded extreme of the leaf were patent or erect.
4.45. Gum drop most conspicuously, by far, affected. But 2 glands are left sticking right out. Nearly all the rest are more or less incurved—right into—the drop.— In the milk & syrup not much change noted, perhaps the latter more affd. than the milk.
4.50 a drop of gum placed on another leaf. All the marginal glands perfect. The leaf supported by a slip of glass.— at 6.8 no incurving notable
Wednesday. 8.36. All the glands of the last (4.50) leaf remarkably incurved.— In the old gum-drop no marked further change,—but 1 gland now sticks conspicy. out.
Milk, no very marked change,—proby. a few incline towards the drop.— Syrup no mkd. change.—
I dont know how far you have tried various fluids or whether further experiments are needed to settle the Nitrogenous part of the Curving.
The progress of change is not easy to describe without minute observation or perhaps drawings.— A series of wee photographs might do the work nicely.—
Very faithfully yours | Danl. Oliver Jr
Chas. Darwin, esq.
CD’s observations on preference of Drosera for milk and nitrogenous fluids, and the effect of nitrate of ammonia are interesting. Asks whether CD is satisfied that the effect is not due to density of fluid or to a chemical irritant. His own observations suggest such possibilities.