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Darwin Correspondence Project

From Hermann Crüger   8 August 1863


8 August | 63.

Dear Sir

Your kind note of 23d May has duly come to hand as also the Journal of Researches & the Paper on Linum for which I beg to tender my best thanks.1 Although my time has been of late much taken up with matters of business, I have at last sight of the researches you recommended me.2 The experiment on Ficus is in progress & I shall report later on it.3 With Melastomas I am not much more advanced but your supposition that insects visit them for nectar also is apparently correct.4 The pollen-seekers are however by far the earliest risers & I think the act of fecundation is performed by them. All Melastomaceae I am acquainted with, or which I have noticed lately, are abundantly visited by insects, so much so that in the forest you often “hear” them in flower before you see them. I am unable to detect any dimorphism, but in the genus Spennera & Nepsera one can distinguish varieties, as in many Species of the arborescent Melastomaceae.

I have found lately a new instance of a plant which to all appearance cannot fertilize itself & which is abundantly provided with a substance attracting insects. This is Narantea, a remarkable Marcgraviacea, & very beautiful creeper, native of our woods. As we cultivate it here in the Garden I have it constantly under observation while it flowers.5 The whole anthers, which separate from the filament, with the included pollen, or perhaps the pollen alone form a viscid extremely sticking mass, soluble with difficulty in water. The stigma reaches maturity only after the anthers. But the pitchers into which the Bracteae of this flower are converted contain a large quantity of syrup, clear as crystal, often 5–6 drops. This is only secreted when the flowers open, the Bracteae are empty up to that moment. Insects are very fond of this syrup & as the opening of the generally pendulous bract is towards the flower, they naturally alight on the latter & proceed from there to drink the syrup. Marcgravia has powdery pollen, & only a few pitchers to a whole umbel of flowers, the pitchers however contain nectar.6

I am curious to examine Ruyschia, but that is a rare plant here. It will perhaps be useless to look amongst these strongly characterized plants for immediate striking varieties, but a Nature which creates itself difficulties in order to overcome them as it were for sport, has been for me always a very sterile idea. The clue that your theory gives to these phenomena is much more satisfactory.7 Have you directed your attention to the genus Mentha? I am afraid that the Mints generally are not easy to make experiments with, but I believe that Salvia also shows sometimes dimorphism.— With regard to sports in cultivated plants here I cannot agree with Schomburgk.8 By neglect dahlias & such degenerate, ie double & fine flowers are no more produced, but otherwise I can see nothing particular. That plants vary here perhaps more than in temperate zones appears to be correct & in this respect I wish to draw your attention to the genus Capsicum. The innumerable varieties of the species, varieties which in their turn become at once true from seed, are something very astonishing. I should think it could be proved there, if one had the time, how quick a variety takes all the characters of a species.9 As generations of plants succeed each other much quicker here, the process of transmutation must go on, caeteris paribus,10 much more rapidly. If I had more time & means at my command I should very much like to experimentalize in these matters but it is impossible under present circumstances to do anything.—

Another subject to which I shall draw your attention, (although it may not be necessary) is that of the periods in which certain cultivated plants begin to vary. It is not my original idea, and is I believe to be found in older writers,11 but I believe I should have been struck with it. You know that for instance the old Caladium bicolor has been cultivated in Europe for nearly a Century now. Yet all the varieties, as far as I can see, are of the last ten or 20 years. Some of them are supposed to be species, but where is the proof.

The most wonderful part of it is however, that here, where the plant has been cultivated for ornament & for superstitious purposes,12 & where it has become wild, the varieties have made their appearance also, & some of the prettiest are to be found in uncultivated spots, where certainly seeds of the introduced ones have not reached. Of course I do not allude to the new Colocasias, & such, but only to the Caladiums. I believe that many more cases will be found in this matter, & not always to be explained by the increased attention people have paid to plants at certain periods.—.

The Catasetums which I am cultivating, begin to show flower stalks now, & in a month or so, I shall study the matter anew.13 Of late I have often observed our larger Orchids Such as Stanhopeas, Gongoras, Coryanthes, & in all of them insects are certainly very busy, but in some the nectar-seeking ones are certainly not those which occasion fecundation. This is particularly the case, as far as my observation goes with Gongora & Coryanthes.14 But then it must be admitted that the insect world of cultivated grounds like this garden is in many respects different from that of the high woods where these orchids prosper.

Besides there are whole tribes of nocturnal insects, which escape our observation, particularly those of the forests. We have here for instance a tribe of nocturnal or vespertini Wasps, which I believe visit flowers like many of their relations. It is extremely likely that many of these visit the stigmas of Orchids, for the sweet fluid found there. I think it is not sufficiently known or kept in mind, that the tissues which compose the stigma of many Orchids, are affected at the critical period in a manner analogous to that in ripening fruits, ie. that part of the cellulose & starch is used up to form sugar, by which process the cells cease at the same time to cohere, & form a sort of pulp, which is very attractive to insects. It is well known that pollen tubes are emitted best artificially in syrup.—

Excuse the length of this letter, I shall report soon on the Catasetum etc, & send you specimens in spirit.15

Very sincerely yours | H Crüger.

CD annotations

2.1 I have found … contain nectar. 2.14] enclosed in square brackets, brown crayon
2.3 This is Narantea … it flowers. 2.5] scored brown crayon
3.1 I am curious … vary. 4.2] crossed pencil
5.1 The most wonderful … ] after opening square bracket, brown crayon
5.1 The most … periods.— 5.8] crossed pencil

CD note:16

Melastomas also *visited by [interl] nectar-seeking insects,—& swarm with insects. | Dichogamy [brown crayon] | Narantea (one of Marcgravaciæ) Bracts so placed to draw insects to flowers. Insects very busy about larger orchids, but not right for


In the letter to Crüger of 25 May [1863], CD indicated that he had sent a copy of Journal of researches (1860), and an off-print copy of ‘Two forms in species of Linum’. For CD’s presentation list for ‘Two forms in species of Linum’, see Correspondence vol.11, Appendix IV.
In a letter to Crüger of 18 March 1863, which has not been found, CD suggested a series of observations that Crüger might make on plants in Trinidad, including Ficus. See letter from Hermann Crüger, 23 April 1863.
Crüger was director of the botanic garden in Trinidad (R. Desmond 1994).
Crüger’s observations on the excretion of a sweet fluid by the bracts of the Marcgraviaceae were reported in Cross and self fertilisation, pp. 404–5.
Like Norantea, Ruyschia was a member of the Marcgraviaceae. Crüger probably refers to CD’s theory that floral structures were adapted to ensure occasional cross-pollination (see, for example, Origin, pp. 96–7, Orchids, p. 1, ‘Two forms in species of Linum’, pp. 82–3 (Collected papers 2: 105). For CD’s comments on floral structures in the Marcgraviaceae, see Cross and self fertilisation, pp. 404–5.
In his letter to Crüger of 25 May [1863], CD had asked whether Crüger could confirm Robert Hermann Schomburgk’s observation that plants such as dahlias and roses when cultivated in the tropics were likely to form sports (Schomburgk 1857, p. 132). CD was seeking information on the question of whether introduced plants from different climates were particularly apt to produce bud-variations (see also Correspondence vol. 10, letter to G. H. K. Thwaites, 29 December [1862]). CD eventually concluded that change of climate was not a necessary factor in the production of bud-variations (Variation 1: 408).
CD omitted the genus Capsicum from his discussion of variation in cultivated plants because botanists were ‘not agreed which kinds ought to rank as species and which as varieties’, and the wild parent-species were unknown (Variation 1: 371).
Caeteris paribus: ‘other things being equal’.
The reference has not been identified. CD took up this point briefly in Variation, under the heading: ‘On the accumulative action of changed conditions of life’ (Variation 2: 261–3). CD argued that while no effect was produced on a species until it had been exposed to cultivation or changed conditions for several generations, once a species had begun to vary, the greater was its tendency to vary further.
No information has been found on the ritualistic use of Caladium bicolor in Trinidad. However, further south, the Amerindians of French Guiana used this species in shamanistic initiations and to propitiate spirits (Grenand et al. 1987, pp. 133, 135).
See letters from Hermann Crüger, 23 February 1863 and n. 6, and 23 April 1863. Crüger reported his observations on Catasetum in his letter of 21 January 1864 (Correspondence vol. 12). See also Crüger 1864, pp. 127–9; ‘Fertilization of orchids’, p. 154 (Collected papers 2: 151); and Orchids, 2d ed., pp. 205–6.
Crüger published his observations on the insect pollination of Catasetum tridentatum, Coryanthes macrantha, Stanhopea grandiflora, and Gongora maculata in Crüger 1864, pp. 127–31. CD cited Crüger’s work in ‘Fertilization of orchids’, pp. 154, 157 (Collected papers 2: 151, 153–4), and Orchids 2d ed., pp. 168, 171, 175, 200.
Crüger sent specimens of Catasetum with his letter of 21 January 1864 (Correspondence vol. 12).
CD’s note is on a separate piece of paper in DAR 161.2: 277.


Thanks for presentation copy of Linum paper [Collected papers 2: 93–105].

Ficus experiments confirm CD’s supposition that insects visit Melastoma for nectar, but HC thinks pollen-seekers fertilise the flowers.

Maranta fertilisation.

Letter details

Letter no.
Hermann Crüger
Charles Robert Darwin
Sent from
Source of text
DAR 161: 277, 277/1
Physical description
4pp †, CD note

Please cite as

Darwin Correspondence Project, “Letter no. 4265,” accessed on 26 August 2019,

Also published in The Correspondence of Charles Darwin, vol. 11