To Francis Darwin   6 and 7 June [1879]1

June 6th

My dear F.

I have tried your experiment with Cucurbita seeds buried in Peat.— 10 radicles with upper side of apex cauterised & all soon became splendidly geotropic; & 10 on lower side.2 Of the latter 4 slightly geotropic—3 remained quite horizontal & 3 bowed upwards, in opposition to Geotropism, & from the cauterised side of apex.— This latter case very good— I also cut off rectangularly 1 m.m of apex of 6 radicles, & they grew much, but did not in 24 h. become at all geotropic.3

I shall try Peas with tips touched above & below with Caustic.4

C. D.—

If you can bring home some White Mustard seed, perhaps ours is not right species—5

Many thanks for your most interesting letter just received (7th)6

1
The year is established by CD’s mention of experiments using caustic on radicles of peas (see n. 4, below). CD began coating the tips of radicles with lunar caustic (silver nitrate) from July 1878, but most of his experiments using caustic were performed in 1879; CD’s notes on the application of caustic are in DAR 209.5.
2
No letter from Francis mentioning this experiment has been found. Cucurbita is the genus of gourds.
3
CD evidently refers to radicles lying horizontally and to cutting the tips at an angle. Only when the tip was cut perpendicular to the ground was there no relative geotropic movement in either direction.
4
CD’s notes on experiments applying caustic to peas, dated from 8 June 1879, are in DAR 209.5: 143.
5
White mustard is Sinapis alba; CD recorded an experiment using caustic on the tip of the radicle of S. alba on 29 December 1878 (DAR 209.5: 118).

Manuscript Alterations and Comments

0.1 June 6th] added pencil
1.2 & all … geotropic;] interl
1.5 rectangularly] interl
4.1 If … (7th) 5.1] added pencil

Please cite as “DCP-LETT-12091,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-12091