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Darwin Correspondence Project

From Hermann Crüger   23 February 1863

Trinidad

23 Feby 1863.

Dear Sir

In answer to your letter of 25th Jan, which reached me yesterday, I beg to say that I shall be delighted to undertake the examination you wish.1 At this moment little can be done it being the beginning of the dry season, & scarcely any Melastomaceae are in flower, but as soon as I find an opportunity I shall attend to your request. We have a large number of Melastomaceae here some 50–60 Species. As far as I can see they are visited by insects of various classes, & only a few minutes ago I found that a sort of bee or humble-bee was very busy in the flowers of Heteronoma diversifolium De C.2 which we cultivate here for ornament but which is a native & common in certain localities. It appeared to me that the insect was addressing itself principally to the basal appendices of the anthers, but I shall report later on this & other points when I shall have collected a certain number of facts.

I take this opportunity to send you a few remarks which I have had occasion to make in reading your two latest works, particularly the “British & foreign Orchideae”.3 Perhaps you will not find them altogether out of place from an individual who has made vegetable physiology his principal study for some fifteen years or more. That plants are fertilised by insects, is as evident to every body here as well as in Europe, but it is little imagined in Europe on what scale & to what extent it takes place in these latitudes. Besides the agency of butterflies & moths & bees & wasps we have the immense class of ants which are found everywhere visiting the flowers for some purpose or other. Nearly all fig-trees have their Cynips, which is apparently necessary for the fertilisation of these.4 I have never found an insect nor good seeds in Ficus elastica, a native of the old world. Even the higher classes of animals contribute their share in this act, as Humming birds f[or] instance. I am nearly certain that such Plants as Anguria, which have a very viscid & heavy pollen, are principally fertilized by Humming birds.

With regard to Orchids a few facts which I have noted will not be uninteresting to you. We have a few Epidendreae, a Schomburgkia, a Cattleya, & an Epidendron which are hardly ever known to open their flowers, but which nearly always set fruit, which however may drop afterwards. I find on examination, that these are fertilized by ants which creep into the flowers at the base of the sepals. In these cases, as in other Epidendreae the pollinia perform their function in situ, & I believe that it can only explained by the ants carrying the viscosity of the stigma to the pollinia, at least this is the only explanation I can find of the phenomenon.5 The pollinia get pulpy without being removed from their bed, & the pollen tubes grow down over the anterior face of the style into the ovarium. I should like to know if there are any Orchids cultivated at Home which do not open. Here it is entirely confined to native species. In other Orchids Ants are also very active some at the bracts, some at the base at the sepals & petals & in the flowers.

The Catasetum I have examined repeatedly before I got your book, since then I have not succeeded getting good observations, the last season having been very unfavorable. The conclusions I have arrived at are rather different from yours, but I shall examine the question again in reference to your opinions.6 In my opinion Myanthus barbatus & Catasetum tridentatum are distinct species, which assume both three forms, the female & intermediate form being much alike in the two species, but much smaller in My. barbatus. The so called female form of Catasetum tridentatum is in my opinion a fertile hermaphrodite form, the other the male. I believe that the flower of the former is nearly always fertilized by its own pollinia, which I have repeatedly found in a pulpy state & attached to the opening of the stigma which is hardly large enough to receive the pollinia of the male flower. Fecundation takes place in the female long before it reaches that aromatic state of maturity which attracts the noisy crowd of humble bees which surround it later for days, fighting for some substance or other in the flower. The female flower opens very early, when it is still very small & green, & immediately after the ovarium takes a rapid development. The anther although small is fully developed, the pollen apparently sound, there is a caudicle & gland. The pollinia are found soon after at the entrance of the stigmatic cavity as mentioned already, & in that pulpy state which precedes the emission of pollen tubes in these tribes. I believe that here also fecundation is brought on by ants, which are very active about these flowers.

On the other hand I have repeatedly seen that the male & hermaphrodite flowers are visited by the same species (2 or 3) of Humble bees, & I have also seen the gland & pollinia of the male flowers sticking to the back of them. There are in my opinion mechanical obstacles to their fecundating the hermaphrodite flowers.

These are my results before reading your book, since then I have found nothing contradicting them, but I shall cautiously reexamine the matter, & communicate my results if you wish it. I may mention that the intermediate form between the male & hermaphrodite is (here) rare, & that I have no good observations on it.

In conclusion I beg you will command my services in these matters, if f[or] instance you should like to have some materials in Spirit of wine, I shall be most happy to forward whatever I can get together.

I remain dear Sir | sincerely yours | H Crüger

Ch. Darwin Esq

CD annotations

1.1 In answer … facts. 1.12] scored blue crayon
1.11 I shall report … facts. 1.12] ‘Look for punctures.’7 added pencil
2.1 I take … native species. 3.12] crossed blue crayon
2.12 Even … birds. 2.15] double scored ink
3.2 We have … afterwards. 3.4] double scored pencil
3.5 In these … pollinia, 3.8] double scored pencil
3.9 & the pollen … species. 3.12] double scored pencil
3.11 it is entirely confined 3.12] underl pencil
4.1 The Catasetum … book, 4.1] scored blue crayon
Top of letter: ‘Melastoma’ added blue crayon; ‘O’ added red crayon; ‘Melast’ added & del, blue crayon; ‘Catasetum—’ added blue crayon
End of letter: ‘Dr. Hermann Cruger’ added ink

Footnotes

In his letter to Crüger of 25 January [1863], CD asked for information regarding pollination in Melastomataceae. In particular, CD wanted Crüger to observe native species of Melastomataceae to see whether insects punctured the horns of the anthers with their probosces in order to obtain nectar (see also letter to Asa Gray, 19 January [1863]). Crüger was the government botanist and director of the botanic garden in Trinidad in the West Indies (R. Desmond 1994).
The species Heteronoma diversifolium (Arthrostema ciliatum) was named in Candolle and Candolle 1824–73, 3: 122.
Crüger refers to Orchids, which was published in May 1862, and probably also to Origin.
Cynips is a genus of gall-wasps (Grzimek ed. 1975, p. 413).
Crüger published his observations on pollination in Schomburgkia, Cattleya, and Epidendrum, members of the Epidendreae sub-family of orchids, in a paper communicated by CD to the Linnean Society, which was read on 3 March 1864 (Crüger 1864, p. 131). In the paper, Crüger revised his opinion that ants were responsible for pollination in these orchids, and concluded that self-pollination was common in this family. Crüger argued that the case of the Epidendreae undermined CD’s doctrine that nature ‘abhors perpetual self-fertilisation’ (Orchids, p. 359). CD, however, had already reached the conclusion that Crüger’s observations suggested he had underrated the power of tropical orchids to produce seed without insect pollination (see letter to Journal of Horticulture, [17–24 March 1863]). See also letter to John Scott, 24 March [1863]. CD cited Crüger 1864 in ‘Fertilization of orchids’, p. 152 (Collected papers 2: 149).
In Orchids, pp. 236–48, CD argued that Catasetum tridentatum was the male, Monachanthus viridis the female, and Myanthus barbatus the hermaphrodite form of a single species. Crüger’s subsequent observations on Catasetum confirmed CD’s conclusion, and demonstrated that pollination was effected by bees (see Correspondence vol. 12, letter from Hermann Crüger, 21 January 1864; see also Crüger 1864, pp. 127–9, and Orchids, 2d ed., pp. 205–6).
See n. 1, above.

Summary

Will observe fertilisation of melastomads as CD requests.

Observations on fertilisation by ants.

Detailed observations on sexes in Catasetum, which were made before he received Orchids and which differ from CD’s findings.

Letter details

Letter no.
DCP-LETT-4008
From
Hermann Crüger
To
Charles Robert Darwin
Sent from
Trinidad
Source of text
DAR 161: 275
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 4008,” accessed on 17 August 2019, http://www.darwinproject.ac.uk/DCP-LETT-4008

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

letter