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Darwin Correspondence Project

From Asa Gray   4 August 1862

Cambridge. [Massachusetts]

4th. August, 1862

My Dear Darwin

My pupil, Rothrock, now away in vacation, has sent me a brief abstract of his Observations on Houstonia cærula.1 The following are culled from them.

Long-styled: stigmatic hairs are in length —.04 mm.

Short-styled — " " .023"


wide Long-styled pollen .020. x .017

Short-styled " .036 x .02 :

in the fresh plants, but dry. Distended with water became round, by increase of the shorter axis alone, the long diameter unaltered.

Long-styled becoming  .026

short-styled "    .036.

(In Mitchella, however, the long diameter was shortened as the other enlarged.)

Pollen of Houstonia not wet, seen endwise is quite strongly 3-lobed. In water the reentering angles come out, and so increase the width.

Short-styled had the smaller stigmas, and the largest and best filled anthers.

The above results I overlooked enough to verify substantially. The following I did not. They may be taken as approximately good

A patch of long-styled—a good deal choked by other vegetation, yielded 34 capsules. The 6 largest of them gave 86 seeds,—average of 1413. Another patch of same better placed 6 best capsules gave 123 seeds average 2112.

A patch of short styled: out of 44 capsules, the six best gave gave an average of 1512 each. Out of the 44, 4 were wholly abortive; 8 averaged 2 seeds each. Most of the rest gave about 6 seeds each.

The above was all from wild plants.

Transplanted specimens in the garden.

Long-styled, average no of seeds of best capsules —13

Short-styled —??

Of 13–short-styled pods

11 were wholly sterile

1 had 4 seeds

1 " 8 "

This was from specimens placed far off by themselves, so that they were not likely to be visited by insects which had left long-styled flowers.

Long-styled flowers fertilised artificially by short-styled pollen.: a patch in what proved to be a most unfavorable situation, much dwarfed. But 6 best capsules gave 77 seeds. In flowers of plants covered with fine netting (coarse gauze) a species of Thrips abounded, also a larva of some small beetle—completely dusted with pollen.2 In many flowers a species of Podura.

I have been looking at the flowers of Rhexia Virginica (I suppose you have examined no true Rhexia)3

Style declined to lower side of the flower. Stamens with their anthers also declined. The small subapical pore of, latter facing inwards. A little pressure on the base of the anther causes puffs of pollen to be blown out through the pore.

In one flower only have I observed the style change its position, in that it bent over towards the upper side of the flower;—accidental?4 A clump in water now 4 days: no stigma has been detected with pollen on it.5

As far as I can see, the likeliest way is that an insect approaching the flower from below, and searching the tube of calyx prolonged above the ovary into a cup (where if he finds any nectar he has sharper eyes than I have),—as he knocks his head against the enlarged bases of the anthers, will get puffs of pollen against his body, I should think on the sides of a humble bee &c. or abdomen underneath.— If he approaches the next flower from the front, below, he will brush against the subcapitate stigma.6

No Orchid examined since my last, except Gymnadenia tridentata,—on which I have a few obs.— It is a congener of Platanthera dilatata,—with the discs formed in two large shallow saucers occupying the whole breadth of the stigma; and the pollen pockets most readily, detached from the caudicle, some pulling off at a touch. It must be fertilised by a very fine proboscis.7

Several more Orchids will soon be sent me by sharp-sighted youngster up in Maine,—to whom I have just sent a copy of your book, to stimulate him.

My latest from you is July 14.8 It leaves me in a state of much anxiety for your boy.9 I will hope for a better account in your next.

I looked to-day for seeds of the little Houstonia for you—in vain. I told Rothrock to gather or save some & hope he has, but know not.— I am worth little now for any commission,—and—now that I am to set down to systematic work, shall be worth still less to you.

Ever Yours most cordially | Asa Gray

CD annotations

1.1 My pupil, … from them. 1.2] crossed blue crayon
1.8 Distended … .036. 1.11] closing square bracket, red crayon
1.12 (In … enlarged.)] closing square bracket, red crayon
5.1 A patch … each. 6.3] crossed red crayon
5.1 patch] underl red crayon
5.2 1413] underl red crayon
5.4 2112] underl red crayon
6.2 1512] underl red crayon
7.6 Of 13–short-styled … flowers. 9.11] enclosed in square brackets, red crayon
8.1 Long-styled … 77 seeds. 8.3] ‘6/77 6 — 17 12.8 sq 13 [illeg calculation] ’ ink
8.1 fertilised artificially] underl red crayon
8.1 short-styled] underl red crayon
8.1 pollen.:] ‘Long’ interl red crayon
8.2 6 best capsules gave 77 seeds. 8.3] underl red crayon
9.1 I have … on it. 11.3] crossed red crayon
12.1 the likeliest … stigma. 12.7] crossed red crayon
13.1 No Orchid … to you. 16.4] crossed ink
Top of first page: ‘Rhexia’ ink; ‘Houstonia’ blue crayon; ‘Rubiaceae’ pencil; ‘Rothrock’ pencil


The reference is to Joseph Trimble Rothrock. Gray had informed CD that Houstonia was dimorphic in October 1861, and had promised to look for any differences in the pollen of the two forms the following spring (see Correspondence vol. 9, letter from Asa Gray, 11 October 1861); he sent CD his own observations on differences between the pollen and stigmas of the two forms in his letter of [2 June 1862], and subsequently promised to send further observations by Rothrock (see letter from Asa Gray, 2–3 July 1862). CD included Rothrock’s observations and experimental results in Forms of flowers, pp. 132, 254.
In his letter to CD of 15 July [1862], Gray mentioned that Rothrock found only Thrips in Houstonia.
In the letter to Asa Gray, 16 February [1862], CD asked Gray to help him in his investigation of the possible occurrence of dimorphism in the Melastomataceae, by making observations of the floral anatomy of Rhexia. Gray promised to do so in the summer (see letter to Asa Gray, 16 February [1862], and letter from Asa Gray, 6 March [1862]), but experienced difficulties in obtaining specimens (see letter from Asa Gray, 2–3 July 1862). Meanwhile, CD informed Gray that he was making a number of crosses with a plant of Rhexia glandulosa from the Royal Botanic Gardens, Kew (see letter to Asa Gray, 10–20 June [1862] and n. 10, and letter to Asa Gray, 1 July [1862] and n. 13).
In the letter to Asa Gray, 15 March [1862], CD asked Gray to compare the position of the pistil in young and old Rhexia flowers, noting that in a related genus he had observed that the pistil and stamens changed position over time, and reporting his suspicion that one set of anthers was ‘adapted to pistil in early state, & the other set for it in its later state’.
CD had asked Gray to observe whether Rhexia could be fertilised if insects were excluded (see letter to Asa Gray, 21 April [1862] and n. 13). See also letters to Asa Gray, 10–20 June [1862] and 1 July [1862].
In the letter to Asa Gray, 15 March [1862], CD asked Gray to watch how the anthers and stigma touched bees that visited Rhexia flowers.
Gray had gathered flowers of Gymnadenia tridentata while on holiday in July (see letter from Asa Gray, 29 July 1862); for Gray’s further observations on the species, see the letter from Asa Gray, 18–19 August 1862.
Letter to Asa Gray, 14 July [1862].
Leonard Darwin was suffering from scarlet fever (see letter to Asa Gray, 14 July [1862]).


Gives J. T. Rothrock’s observations on the structure and fertility of the two forms of Houstonia. Mentions his own observations on Rhexia virginica and Gymnadenia tridentata.

Letter details

Letter no.
Asa Gray
Charles Robert Darwin
Sent from
Cambridge Mass.
Source of text
DAR 110 (ser. 2): 67–9
Physical description
6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 3679,” accessed on 29 June 2017,

Also published in The Correspondence of Charles Darwin, vol. 10