He found that a wonderfully close parallelism exists between hybridisation (_i.e._ crosses between distinct species), and certain forms of fertilisation among heterostyled plants. So that it is hardly an exaggeration to say that the "illegitimately" reared seedlings are hybrids, although both their parents belong to identically the same species. In a letter to Professor Huxley, given in the second volume of the _Life and Letters_ (p. 384), my father writes as if his researches on heterostyled plants tended to make him believe that sterility is a selected or acquired quality. But in his later publications, _e.g._ in the sixth edition of the _Origin_, he adheres to the belief that sterility is an incidental[289] rather than a selected quality. The result of his work on heterostyled plants is of importance as showing that sterility is no test of specific distinctness, and that it depends on differentiation of the s.e.xual elements which is independent of any racial difference. I imagine that it was his instinctive love of making out a difficulty which to a great extent kept him at work so patiently on the heterostyled plants. But it was the fact that general conclusions of the above character could be drawn from his results which made him think his results worthy of publication.
FOOTNOTES:
[269] The "Genealogy of Animals" (_The Academy_, 1869), reprinted in _Critiques and Addresses_.
[270] An English edition is published by the Clarendon Press, 1890.
[271] Sachs, _Geschichte d. Botanik_, p. 419.
[272] That is to say, flowers possessing both stamens, or male organs, and pistils or female organs.
[273] Christian Conrad Sprengel, born 1750, died 1816.
[274] _Fertilisation of Flowers_ (Eng. Trans.) 1883, p. 3.
[275] _Das entdeckte Geheimniss der Natur im Baue und in der Befruchtung der Blumen._ Berlin, 1793.
[276] The order to which the pea and bean belong.
[277] _Gardeners" Chronicle_, 1857, p. 725. It appears that this paper was a piece of "over-time" work. He wrote to a friend, "that confounded Leguminous paper was done in the afternoon, and the consequence was I had to go to Moor Park for a week."
[278] The sweet pea and everlasting pea belong to the genus Lathyrus.
[279] _Gardeners" Chronicle_, 1858, p. 828.
[280] He published a short paper on the manner of fertilisation of this flower, in the _Gardeners" Chronicle_ 1871, p. 1166.
[281] The woodp.e.c.k.e.r was one of his stock examples of adaptation.
[282] It is a modification of the upper stigma.
[283] This rather obscure statement may be paraphrased thus:--
The machinery is so perfect that the plant can afford to minimise the amount of pollen produced. Where the machinery for pollen distribution is of a cruder sort, for instance where it is carried by the wind, enormous quant.i.ties are produced, _e.g._ in the fir tree.
[284] "Darwin considere, &c.," _Archives des Sciences Physiques et Naturelles_ 3eme periode. Tome vii. 481, 1882.
[285] May 24th, 1862.
[286] June 14th, 1862.
[287] My father"s "Prefatory Notice" to this work is dated February 6th, 1882, and is therefore almost the last of his writings.
[288] See Autobiography, p. 48.
[289] The pollen or fertilising element is in each species adapted to produce a certain change in the egg-cell (or female element), just as a key is adapted to a lock. If a key opens a lock for which it was never intended it is an incidental result. In the same way if the pollen of species of A. proves to be capable of fertilising the egg-cell of species B. we may call it incidental.
CHAPTER XVII.
_Climbing Plants; Power of Movement in Plants; Insectivorous Plants; Kew Index of Plant Names._
My father mentions in his _Autobiography_ (p. 45) that he was led to take up the subject of climbing plants by reading Dr. Gray"s paper, "Note on the Coiling of the Tendrils of Plants."[290] This essay seems to have been read in 1862, but I am only able to guess at the date of the letter in which he asks for a reference to it, so that the precise date of his beginning this work cannot be determined.
In June 1863, he was certainly at work, and wrote to Sir J. D. Hooker for information as to previous publications on the subject, being then in ignorance of Palm"s and H. v. Mohl"s works on climbing plants, both of which were published in 1827.
_C. Darwin to Asa Gray._ Down, August 4 [1863].
My present hobby-horse I owe to you, viz. the tendrils: their irritability is beautiful, as beautiful in all its modifications as anything in Orchids. About the _spontaneous_ movement (independent of touch) of the tendrils and upper internodes, I am rather taken aback by your saying, "is it not well known?" I can find nothing in any book which I have.... The spontaneous movement of the tendrils is independent of the movement of the upper internodes, but both work harmoniously together in sweeping a circle for the tendrils to grasp a stick. So with all climbing plants (without tendrils) as yet examined, the upper internodes go on night and day sweeping a circle in one fixed direction.
It is surprising to watch the Apocyneae with shoots 18 inches long (beyond the supporting stick), steadily searching for something to climb up. When the shoot meets a stick, the motion at that point is arrested, but in the upper part is continued; so that the climbing of all plants yet examined is the simple result of the spontaneous circulatory movement of the upper internodes.[291] Pray tell me whether anything has been published on this subject? I hate publishing what is old; but I shall hardly regret my work if it is old, as it has much amused me....
He soon found that his observations were not entirely novel, and wrote to Hooker: "I have now read two German books, and all I believe that has been written on climbers, and it has stirred me up to find that I have a good deal of new matter. It is strange, but I really think no one has explained simple twining plants. These books have stirred me up, and made me wish for plants specified in them."
He continued his observations on climbing plants during the prolonged illness from which he suffered in the autumn of 1863, and in the following spring. He wrote to Sir J. D. Hooker, apparently in March 1864:--
"The hot-house is such an amus.e.m.e.nt to me, and my amus.e.m.e.nt I owe to you, as my delight is to look at the many odd leaves and plants from Kew.... The only approach to work which I can do is to look at tendrils and climbers, this does not distress my weakened brain. Ask Oliver to look over the enclosed queries (and do you look) and amuse a broken-down brother naturalist by answering any which he can. If you ever lounge through your houses, remember me and climbing plants."
A letter to Dr. Gray, April 9, 1865, has a word or two on the subject.--
"I have began correcting proofs of my paper on Climbing Plants. I suppose I shall be able to send you a copy in four or five weeks. I think it contains a good deal new, and some curious points, but it is so fearfully long, that no one will ever read it. If, however, you do not _skim_ through it, you will be an unnatural parent, for it is your child."
Dr. Gray not only read it but approved of it, to my father"s great satisfaction, as the following extracts show:--
"I was much pleased to get your letter of July 24th. Now that I can do nothing, I maunder over old subjects, and your approbation of my climbing paper gives me _very_ great satisfaction. I made my observations when I could do nothing else and much enjoyed it, but always doubted whether they were worth publishing....
"I received yesterday your article[292] on climbers, and it has pleased me in an extraordinary and even silly manner. You pay me a superb compliment, and as I have just said to my wife, I think my friends must perceive that I like praise, they give me such hearty doses. I always admire your skill in reviews or abstracts, and you have done this article excellently and given the whole essence of my paper.... I have had a letter from a good zoologist in S. Brazil, F. Muller, who has been stirred up to observe climbers, and gives me some curious cases of _branch_-climbers, in which branches are converted into tendrils, and then continue to grow and throw out leaves and new branches, and then lose their tendril character."
The paper on Climbing Plants was republished in 1875, as a separate book. The author had been unable to give his customary amount of care to the style of the original essay, owing to the fact that it was written during a period of continued ill-health, and it was now found to require a great deal of alteration. He wrote to Sir J. D. Hooker (March 3, 1875): "It is lucky for authors in general that they do not require such dreadful work in merely licking what they write into shape." And to Mr.
Murray, in September, he wrote: "The corrections are heavy in _Climbing Plants_, and yet I deliberately went over the MS. and old sheets three times." The book was published in September 1875, an edition of 1500 copies was struck off; the edition sold fairly well, and 500 additional copies were printed in June of the following year.
_The Power of Movement in Plants._ 1880.
The few sentences in the autobiographical chapter give with sufficient clearness the connection between the _Power of Movement_ and the book on Climbing Plants. The central idea of the book is that the movements of plants in relation to light, gravitation, &c., are modifications of a spontaneous tendency to revolve or circ.u.mnutate, which is widely inherent in the growing parts of plants. This conception has not been generally adopted, and has not taken a place among the canons of orthodox physiology. The book has been treated by Professor Sachs with a few words of professorial contempt; and by Professor Wiesner it has been honoured by careful and generously expressed criticism.
Mr. Thiselton Dyer[293] has well said: "Whether this masterly conception of the unity of what has. .h.i.therto seemed a chaos of unrelated phenomena will be sustained, time alone will show. But no one can doubt the importance of what Mr. Darwin has done, in showing that for the future the phenomena of plant movement can and indeed must be studied from a single point of view."
The work was begun in the summer of 1877, after the publication of _Different Forms of Flowers_, and by the autumn his enthusiasm for the subject was thoroughly established, and he wrote to Mr. Dyer: "I am all on fire at the work." At this time he was studying the movements of cotyledons, in which the sleep of plants is to be observed in its simplest form; in the following spring he was trying to discover what useful purpose those sleep-movements could serve, and wrote to Sir Joseph Hooker (March 25th, 1878):--
"I think we have _proved_ that the sleep of plants is to lessen the injury to the leaves from radiation. This has interested me much, and has cost us great labour, as it has been a problem since the time of Linnaeus. But we have killed or badly injured a mult.i.tude of plants.
N.B.--_Oxalis carnosa_ was most valuable, but last night was killed."
The book was published on November 6, 1880, and 1500 copies were disposed of at Mr. Murray"s sale. With regard to it he wrote to Sir J.
D. Hooker (November 23):--