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Darwin Correspondence Project

To A. W. Bennett   [before 16 November 1871]1

I am glad you have drawn attention to the difference in the bud-state of the perfect and imperfect flowers; for I remember, many years ago, objecting to Asa Gray that he considered the imperfect flowers (not, I think, in the case of Impatiens) arrested buds, and I maintained that their structure had been specially modified for their functions.2 From observations by myself in 1863, I find I was struck with the small size of the anthers, and the very small quantity of pollen. The grains are of the same diameter as in the perfect flowers, but they appeared to be more unequal in size. I distinctly saw pollen-grains protruding from the grains whilst within the anthers, and penetrating the stigma. I cannot believe that I could have overlooked the facts of the anthers not dehiscing. I do not mention in my notes that the pollen-grains are tied together by threads, as I do in the case of the pollen of the perfect flowers. I speak of the nectary in the cleistogenous flowers as a mere rudiment.3 From the fact of the nectary in the perfect flowers containing nectar, and from the pollen-grains being tied together by threads, I cannot doubt that they are crossed by insects, and I am almost certain that they are frequently visited by humble-bees. The structure of the flowers seems to me so well adapted for crossing, that I expected that the perfect flowers would be sterile without the aid of insects. In this I was quite wrong, as the perfect flowers, when protected, produced pods.4 Eleven such pods from perfect flowers, spontaneously self-fertilized, yielded on an average 3.45 seeds. I carefully brushed away the pollen from some of the perfect flowers, and fertilized them with pollen from a distinct plant, but got only three pods, containing, to my surprise, only 2, 2, and 1 seed. I attributed this poverty at the time to this plant probably requiring repeated doses of pollen, as is certainly sometimes the case.

Footnotes

The date is established by the date that Alfred William Bennett’s paper ‘On the floral structure of Impatiens fulva’ (Bennett 1871) was read. CD’s comments, probably based on a manuscript version of the paper, were included in the published version.
Asa Gray described small, unopened, ‘imperfect’ flowers in ‘Dimorphism in the genitalia of flowers’ (A. Gray 1862d, p. 419); the condition later became known as cleistogamy and plants bearing such flowers were called cleistogenous. In a letter to Gray of 26[–7] November [1862] (Correspondence vol. 10), CD had argued, ‘the state of corolla, of stigma & pollen-grains [in cleistogamic flowers] is different from state of parts in bud; that they are in a condition of special modification’. Gray had replied, ‘In Violets & Impatiens, Lespedeza, &c &c—I believe that the condition is not so much “special modification” as it is arrest of development’ (see Correspondence vol. 10, letter from Asa Gray, 29 December 1862).
CD’s observations on the anthers and pollen of both perfect (CD uses ‘perfect’ to describe open or chasmogamic flowers) and imperfect (cleistogamic) flowers of Impatiens noli-me-tangere (now I. noli-tangere), made on 22 June 1863, are in DAR 111: A41.
Notes made by CD in July 1864 on the production of seed by protected and unprotected flowers of Impatiens noli-me-tangere are in DAR 49: 105.

Summary

Discussed observations made in 1863 of Impatiens pollen and humble-bees.

Letter details

Letter no.
DCP-LETT-8303F
From
Charles Robert Darwin
To
Alfred William Bennett
Source of text
Journal of the Linnean Society (Botany) 13 (1873): 152

Please cite as

Darwin Correspondence Project, “Letter no. 8303F,” accessed on 26 July 2021, https://www.darwinproject.ac.uk/letter/DCP-LETT-8303F.xml

Also published in The Correspondence of Charles Darwin, vol. 24 (Supplement)

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