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Darwin Correspondence Project

From Fritz Müller   31 October 1868

Itajahy, Sa Catharina, Brazil.

Octbr. 31. 1868.

My dear Sir

I am very sorry to learn from your letter of August 17th, that your health holds so weak again; but I hope and sincerely wish, that you will soon and fully recover.1

The dimorphic plant, of which I enclosed dried flowers in my letter of June 17th is an Aegiphila (Verbenaceæ).2 It seems to be quite sterile with own-form pollen; I have lately seen numerous bushes on an excursion to the mouth of the Itajahy;3 all those which stood far from others, bore not a single fruit, though they had flowered richly; two bushes growing close together, were loaded with fruits.—

The family of Rubiaceæ continues to furnish me with dimorphic species; to the genera formerly mentioned (Borreria, Suteria, Hedyotis, Lipostoma, Manettia, Coccocypselum)4 I can now add Psychotria (3 or 4 species) and Faramea (These two genera have been named for me in Kew).5 There are now flowering in my forest many bushes of a very beautiful 〈Faramea〉 which is one of the most interesting dimorphic pla〈nts I ever〉 saw.— The difference in the length of the styli an〈d stamens of〉 the two forms is very considerable; in the long-styled form the antheræ are nearly sessile.

pressed plant specimens and excised diagrams6

The two rami of the stigma are long and slender in the short-styled,—short and much broader with the papillæ more crowded together in the long-styled form. The pollen-grains of the long-styled plants are about 118 mm in diameter and have a smooth surface; those of the short-styled plants are much larger (112 mm) and covered with numerous short spines. This structure of the pollen-grains will be, I suppose, a very advantageous contrivance, by preventing the pollen-grains from being blown away by the wind and by causing them to adhere more easily to the hairy body of visiting insects, which will probably touch but slightly the widely projecting anthers of the short-styled plants, while they may rub strongly their proboscis against the enclosed anthers of the long-styled. I must not forget to add, that by a torsion of the long stamens the anthers of the short-styled form turn round so as to face the circumference of the flower. Sometimes the rotation is not perfect, some or all the anthers facing only obliquely towards the circumference. There are even anthers, which do not rotate at all. I think, it may be confidently expected, that in the long-run natural selection will wholly eliminate these imperfectly rotating anthers.—

In one of your letters you told me that all known species of Oxalis were trimorphic or monomorphic; now I have hardly any doubt, that one of our species is dimorphic.7 It is a small creeping plant with bright yellow flowers. I have gathered numerous specimens at several widely distant places, without finding a single short-styled plant.8diagram excised〉The length of the styli in the long-styled and mid styled forms is rather variable, they are often scarcely longer or shorter than the longer stamens; there are even plants, in which the styli reach exactly to the level of the longer stamens. It may be worth mentioning, that this dimorphic plant closely resembles a monomorphic species, in which the styli equal in length the longer stamens and which fertilized 〈itself as〉 I found by experiment, without insect-aid.—

attached specimens

〈    〉 raised seedlings 〈    〉 〈  〉ine white tri-〈morphic〉 Oxalis, which offer a curious instance of embryonic ressemblance. The leaves of the seedlings have the form typical in that genus, from which form the leaves of the adult plant deviate considerably.9

There is another difference, not visible in the dried leaves; those of the seedlings are reddish beneath, those of the adult plant are green on both sides.—

As far as I can judge from wild plants, our several trimorphic Oxalis are quite sterile with own-form pollen or at least with the plant’s own pollen. Two of our species propagate asexually with so extraordinary rapidity, that the descendants of the same mother-plant spread over large areas. I have seen in a large field (many acres) of young sugar-cane, the whole ground covered with the red blossoms of one of these species and all the flowers were of the same form.— Thus also all the white Oxalis growing on my own land are long-styled. Of these two species I have not yet seen a single pod on wild plants.— In my garden, where I have planted close together the three forms, the white Oxalis is now seeding freely.— Of two other trimorphic species (belonging to the section Thamnoxys Endl.)10 the three forms are commonly found mingled with each other and in this case they seed well, while isolated plants appear always to be sterile.—

After reading your paper on the illegitimate offspring of dimorphic and trimorphic plants, it occurred to me, that our sterile Oxalis, of which I formerly told you, might be an illegitimate plant, which had beaten in the struggle for life and exterminated its legitimate parents in consequence of a more luxuriant asexual reproduction;11 for according to Gærtner (Bastarderzeugung pg. 546) “eine der ausgezeichnetsten und allgemeinsten Eigenschaften der Pflanzen bastarde ist die Luxuriation in allen ihren Theilen”,12 and it seems not to be improbable that some illegitimate plants should also possess this luxuriating growth of hybrids.— On this view the more or less complete contabescence of the anthers, the variability of the length of the styli in some plants and their equalling in length the shorter stamens in one of the two forms, as well as the doubleness of the flowers, which I have repeatedly observed, would be explicable.—

As to our Pontederia, I have made lately an excursion to the Itajahy-mirim,13 where it grows abundantly, in order to see whether it was indeed trimorphic; but unfortunately it did not yet flower.14

The Eschscholtziæ, raised from your seeds, are now beginning to flower, so that in my next letter I hope to be able to give you the result of the experiments, which I am about to try.15

With profound respect, believe me, dear Sir | very faithfully yours | Fritz Müller

CD annotations

1.1 I am … anthers.— 3.23] crossed blue crayon
4.2 now I … species, 4.9] scored blue crayon
4.8 this dimorphic … aid.— 4.10] scored red crayon
7.1 As far … sterile.— 7.12] scored blue crayon
7.9 Of two other … sterile.— 7.12] scored red crayon
8.1 After … explicable.— 8.12] ‘Illegitimate’ blue crayon
8.2 our sterile … explicable.— 8.12] scored blue crayon
9.1 As to … try. 10.3] crossed blue crayon
On cover:F. Muller — Wonderful Dimorphism — name of Plant mentioned in letter of Aug 17thpencil, del blue crayon; ‘Much on Oxalis very curious’ pencil; ‘Faramea (paper on by F. Muller (Rubiaceæ)’ pencil, del blue crayon; ‘3’ red crayon, circled red crayon; ‘Faramea & Agriphila [del blue crayon]’ pencil, scored blue crayon ‘For figure’ added blue crayon; ‘Oxalis’ red crayon


See letter to Fritz Müller, 17 August 1868.
Fritz Müller’s letter of 17 June 1868 is incomplete; the section where he refers to the dimorphic plant is missing. Aegiphila is spiritweed.
Müller refers to the Itajahy (now Itajaí Açu) river.
Müller had discussed the genus Lipostoma (now Coccocypselum) in 1866 (see Correspondence vol. 14, letter from Fritz Müller, 1 December 1866 and n. 12). In 1867, Müller had sent specimens of Borreria (now Spermacoce), and discussed his observations of heterostyly in Suteria (now subsumed within Psychotria), Hedyotis, and Manettia (see Correspondence vol. 15, letters from Fritz Müller, 4 March 1867, 2 June 1867, and 17 July 1867).
In Forms of flowers, p. 135, CD noted Müller’s observation of heterostyly in Psychotria, and on pp. 129–31, CD discussed in detail Müller’s later paper on the dimorphic Faramea described in this letter (F. Müller 1869). Müller began corresponding with Joseph Dalton Hooker, the director of the Royal Botanic Gardens, Kew, in 1867, after receiving a letter from Hooker offering to name plants that Müller sent to Kew (letter from Fritz Müller to J. D. Hooker, 7 October 1867, Royal Botanic Gardens, Kew, DC vol. 215, doc. 168; see also Möller ed. 1915–21, 2: 149, letter from Fritz Müller to Hermann Müller, 17 October 1868, for some of the plants identified by Hooker).
For a photograph of the plant specimens, see plate facing p. 822. The excised diagrams were probably of the two different forms of pollen grain (see Forms of Flowers p. 129).
See Correspondence vol. 14, letter to Fritz Müller, [late December 1866 and] 1 January 1867; see also ibid., letter from Fritz Müller, 1 and 3 October 1866 and nn. 2 and 6.
Müller probably refers to Oxalis corniculata (creeping wood sorrel); for more on the evolution of distyly in South American species of Oxalis, see Weller 1992, pp. 253–63.
Müller refers to Oxalis regnellii, which has triangular leaves. Müller later published the results of some of his experiments with this species, without naming it (see F. Müller 1871, p. 75), but CD referred to the species by name when discussing Müller’s experiments in Forms of flowers, p. 212.
Thamnoxys is one of twelve divisions of the genus Oxalis given in Endlicher 1836–42, p. 1172.
Müller had mentioned the sterile Oxalis in his letter of 1 and 3 October 1866 (Correspondence vol. 14). In ‘Illegitimate offspring of dimorphic and trimorphic plants’, p. 431, CD had noted that some illegitimate offspring were ‘profuse flowerers’.
The quotation is from Gärtner 1849, p. 526: ‘One of the most superb and most common characteristics of hybrid plants is the luxurious growth in all their parts’.
Mirim: ‘little’ in the Tupi language of Brazil (see William A. Reid, ‘Review’, Language 20 (1944): 263).
See letter to Fritz Müller, 3 April [1868] and n. 5.
CD had sent Müller seeds of Eschscholzia californica (California poppy) for experiments on self-sterility (see letter to Fritz Müller, 3 April [1868] and nn. 1 and 2).


Writes on various observations and discoveries on dimorphic and trimorphic plants.

Letter details

Letter no.
Johann Friedrich Theodor (Fritz) Müller
Charles Robert Darwin
Sent from
Santa Catharina, Brazil
Source of text
DAR 142: 98, 103
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 6439,” accessed on 28 April 2017,

Also published in The Correspondence of Charles Darwin, vol. 16