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Darwin Correspondence Project

From A. S. Wilson   6 August 1878

124 Bothwell Street. | Glasgow.

6th Aug. 1878.

Charles Darwin Esq.

Dear Sir,

A fortnight or three weeks ago I found while in the island of Arran, the flowers of Erythraea Centaurium, in tolerable abundance.1 They had styles shorter than the stamens, and at first this seemed to be universal, but on examining one of the specimens I had lifted I found it was different from the rest, the style being much longer than the stamens so that the stigma was a good way above them, just as happens in Menyanthes trifoliata.—2 On reaching home I placed the pollen grains from the two forms under the microscope, and found that exactly as in the case of the bog-bean, whose pollen grains I had examined previously, the two forms were perfectly distinct. The pollen-grains from the anthers of the long styled form were smaller, in size and more spherical than those of the short styled form, which were, elliptical or Cassava seed-shaped,— I have not yet had an opportunity of repeating the observations on other specimens— there seems however, little doubt—that the plant is truly dimorphic.3 Since I have troubled you with the above, I may at the same time mention, having found this summer on the top of Ben Lawers three forms of Silene Acaulis,—male, female and perfect hermaphrodite flowers on distinct plants.4 The female flowers resembled those of the female form of the gynodioecious Thymus Serpillum in that the corolla was small, pale, and without markings.—5 According to Axell, Silene Inflata is trimorphic or rather polygamous, but Herman Muller makes no mention of S. acaulis which is described as ‘subdioecious’ in Hooker.—6 Nor is Erythraea, described in Müller’s ‘Befruchtung’, as dimorphic and therefore I have taken the liberty of writing you.7

Perhaps you will pardon me if I refer to one other observation I made, on Saturday last,—in reference to the fertilisation of Schrofularia Nodosa.—8 For a long time I have been trying to find an explanation of the fact that in insect-fertilized flowers,—we usually have proterogynous dichogamy associated with an inconspicuous corolla,— It is difficult to see in what way an inconspicous corolla can be advantageous to a plant dependent on insect visits, for cross fertilisation; And more over another difficulty presents itself, As is well known a bee visiting a plant usually begins with the lower flowers, and goes regularly up, in proterandrous flowers where the infloresence is basifugal were the bee to reverse its course the whole elaborate arrangement for cross fertilisation would be upset for the insect would simply bring pollen from the upper younger, male flowers and deposit it on the stigmas of the older ones low-down—and would finally leave the plant carrying off very little, pollen. In like manner in a proterogynous plant with basifugal developing inflorescence were an insect to begin at the older, lower flowers, which are in the second or male stage, it would simply take the pollen to the younger female flowers higher up the stem, and would finally leave the plant with little or no pollen. In this way the chances of crossing would be greatly lessened. Now what I noticed was this that a wasp visiting Scrofularia Nodosa, alighted on the top flower and went from flower to flower somewhat irregularly, but after completing a downward spiral or two left the plant from the oldest and lowest flower!— It would appear from this that such plants are adapted for fertilization by insects having a different habit, from the bees.—

But about the obscurity of the flowers—animals of prey, being possessed of keener powers of vision scent &c. it seems only fair to suppose that wasps, whose food, consists partly of smaller insects, will haver sharper eyes than a purely vegetable feeding insect like the bee.— Consequently the materials necessary to produce a large coloured corolla, may be more profitably empoyed otherwise in the economy of the plant just as in the case of Cleistogamic flowers.9

I must beg you to excuse me thus trespassing on your time, but my having been led to take an interest in this subject through your labours must be my apology for troubling you with these imperfect & crude observations. | Believe me | Yours Very Respectfully | Alex. S. Wilson.

CD annotations

1.1 A fortnight … dimorphic. 1.13] crossed blue crayon
1.7 and found … seed-shaped,— 1.11] scored ink
1.13 Since … markings.— 1.18] scored red crayon; ‘?’ red crayon
2.1 Perhaps … up, 2.8] crossed blue crayon
Top of letter: ‘Keep’ pencil; ‘for Cross Fertilisation’ blue crayon


Arran is an island in south-west Scotland in the Firth of Clyde. Erythraea is a synonym of Centaurium (the genus of centaury). Erythraea centaurium is a synonym of Centaurium erythraea (common centaury). Its flowers are herkogamic (anthers and stigmas are spatially segregated) and protogynous (female parts mature first) but not heterostyled (Brys and Jacquemyn 2011, p. 922).
Menyanthes trifoliata is common bog-bean.
CD added Wilson’s information on Erythraea centaurium to the preface of Forms of flowers 2d ed., p. vi.
Ben Lawers is a mountain in the southern Highlands of Scotland. Silene acaulis is cushion-pink or moss campion.
Thymus serpyllum is Breckland thyme; CD had described the differences in the corollas of hermaphrodite and female plants of this species in Forms of flowers, pp. 299–300.
Johann Severin Axell had described and figured the three forms of Silene inflata (a synonym of S. vulgaris, bladder campion) in Axell 1869, p. 46. Hermann Müller had, in fact, briefly mentioned Silene acaulis, noting that it was sometimes dioecious, sometimes hermaphrodite and protandrous, that is, with male sexual parts maturing before female ones (H. Müller 1873, p. 190). For Joseph Dalton Hooker’s description of the species as subdioecious, see Hooker 1870, p. 51.
For Müller’s description of flowers of Erythraea centaurium, see H. Müller 1873, p. 333.
Scrophularia nodosa is woodland figwort.
CD had discussed the reduction in the size of the corolla in female plants of gynodioecious species in Forms of flowers, pp. 7, 304–9.


Axell, Severin. 1869. Om anordningarna för de fanerogama växternas befruktning. Stockholm: Iwar Hæggströms Boktryckeri.

Forms of flowers 2d ed.: The different forms of flowers on plants of the same species. 2d edition. By Charles Darwin. London: John Murray. 1880.

Forms of flowers: The different forms of flowers on plants of the same species. By Charles Darwin. London: John Murray. 1877.

Hooker, Joseph Dalton. 1870. The student’s flora of the British Islands. London: Macmillan.

Müller, Hermann. 1873. Die Befruchtung der Blumen durch Insekten und die gegenseitigen Anpassungen beider. Ein Beitrag zur Erkenntniss des ursächlichen Zusammenhanges in der organischen Natur. Leipzig: Wilhelm Engelmann.


Observations on dimorphic and trimorphic plants of Scotland.

On fertilisation of Scrophularia nodosa.

Letter details

Letter no.
Alexander Stephen Wilson
Charles Robert Darwin
Sent from
Source of text
DAR 86: B19–20
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 11641,” accessed on 17 September 2021,