JARS v52n2 - Darwin and Hooker: Azaleas and Rhododendrons, Part II

Darwin and Hooker: Azaleas and Rhododendrons, Part II
Duncan M. Porter
Department of Biology
Virginia Polytechnic Institute & State University
Blacksburg, Virginia

Darwin Correspondence Project
University Library, Cambridge, England

Part I of "Darwin and Hooker: Azaleas and Rhododendrons," appeared in the Winter 1998 issue. This two-part series deals with correspondence between Charles Darwin and Joseph Dalton Hooker regarding the genus Rhododendron.

Crossing

From early in his career, Darwin realized that in order to understand how evolution occurs one must understand the mechanisms of heredity. He realized that this must be done experimentally, and over the years Darwin performed a myriad of crosses with numerous plants, including species of Rhododendron . One can appreciate how important this line of research was to Darwin, when one sees that a number of important papers (1) and three of his books (14, 17, 18) were devoted to this theme.

In 1839, Darwin prepared a series of questions on plant breeding for William Herbert, Dean of Manchester and a leading authority on plant hybridization. One hypothetical question had to do with crossing in Rhododendron (3):

If one flower of a plant were impregnated by the pollen of another species, retaining its normal character, and a second flower, by the pollen of the same species but after it had undergone considerable variation, would the seedlings in the first case be impressed more persistently (that is in a greater number of successive generations) with the character of the plant from which the pollen had been derived in its wild or old when in its new or cultivated state? Or again if two flowers, say of a Rhododendron, were impregnated by the pollen of two Azaleas, of the same variety, but of which two, one had been raised many times successively by seed, and the other had only lately appeared with its new character, has Mr. Herbert reason for prefer[ring] the pollen of the plants of successive generations to produce the desired effect, than that of the other. In short, do species (using the word in its common sense) impress their character more persistently (though perhaps not more on the first generation) than varieties.

These are the same kinds of questions that Darwin asked in On the Origin of Species .

Prior to publishing On the Origin of Species Darwin in 1842 produced a 35-page sketch and in 1844 a 231-page essay of his evolutionary ideas. The latter served Darwin as a template for On the Origin of Species . They were edited and published by his son Francis at the celebration of Darwin's birth centenary and the fiftieth anniversary of the publication of On the Origin of Species . There are several entries on Rhododendron , including the following on crossing (21):

The fact however, of a perfect gradation in the degree of sterility between species, and the circumstance of some species most closely allied...refusing to breed together, whereas other species, widely different, and even belonging to distinct genera, as the...Azalea and Rhododendron...breeding together ought to have caused a doubt whether the sterility did not depend on other causes, distinct from a law, coincident with their creation.

This was in a section of the 1844 essay titled "Differences between 'Races' and 'Species' in Fertility When Crossed." Like most of his contemporaries, Darwin thought that azaleas and rhododendrons were members of different genera.

The manuscript for Natural Selection also contained several observations on crossing in Rhododendron , the first of which was (34):

Everyone has seen the splendid results of the most complicated crosses between the several species of Azalea & Rhododendron: Mr. Gowen [James Robert Gowen of Highcleve, Hampshire, Rhododendron hybridizer] who raised some of the early crosses at Highcleve assures me that some of them yielded numerous self-sown seedlings. I applied to Mr. C. Noble [Charles Noble, nurseryman at Bagshot Park, Surrey] of Bagshot, so well known for the numerous splendid Rhododendrons raised by him, on the degree of fertility of his hybrids, & he has given me the names of several, the offspring of R. arboreum & maximum, & of altaclerense (itself a hybrid from Pontico-catawbiense fertilized by arboreum) & catawbiense, which he says he is sure produces as many seeds as any pure species. He adds that the kind raised in great numbers as stocks for grafting, is a hybrid from R. Ponticum & Catawbiense, & that this "seeds as freely as it is possible to imagine."

This contradicts the general feeling that successive generations of both simple hybrids & hybrids reduced one or two steps towards either parent form, invariably become more & more barren, but with what is known of the history of these several genera of highly cultivated plants, it is scarcely possible to believe in this conclusion...These facts moreover, strengthen my previous remark, that the only fair way of testing, as nature would test, the fertility of hybrids in successive generations, is to have numbers of the same kind growing in the open air, & allowed freely to cross.

In Natural Selection , Darwin includes the footnote: "The following are some cases of bigeneric crosses: Rhododendron & Azalea, Rhododendron & Rhodora, a remarkable cross...” (34). Again these are all members of Rhododendron .

Darwin notes that crossing usually takes place between species with similar "duration of life, period of flowering, adaptation to climate," but he also noted exceptions (34):

Thus the tender Indian Rhododendron arboreum has been crossed with the hardy R. Ponticum, & even with R. Dauricum, which flourishes under the intensely cold climate of Eastern Siberia. In the crosses between Rhododendron & Azalea we see evergreen & deciduous bushes united.

Usually, when three or four species are united, the hybrids are "excessively variable & extremely sterile," Darwin writes but also notes that Rhododendron is an exception, where four or more species may be involved in crosses (34):

In some of the hybrid Rhododendrons, raised by Messers. Standish & Nobel [John Standish and Charles Noble, nurserymen at Bagshot Park, Surrey] no less than six species have been blended together by successive crosses into a single hybrid, namely Rhododendron campanulatum, maximum, Ponticum, purpureum, Catawbiense & arboreum - species coming from the most distant quarters of the world, & having the most different climates; most of these complex Rhododendron hybrids seem to be fairly fertile; & some of them extremely fertile, as has been previously mentioned.

In Natural Selection Darwin concluded his discussions of crossing Rhododendron (34):

In fact there cannot be a doubt that hybrids of the first generation between two species both long cultivated often differ greatly from each other as in hybrids from distinct species of Rhododendron...I saw at Spofforth [North Yorkshire] two hybrids between Rhododendron & Azalea raised by Herbert [William Herbert] from the same seed-capsule, & they differed greatly in appearance.

Crossing in the genus was briefly mentioned in On the Origin of Species (13):

The practical experiments of horticulturists, though not made with scientific precision, deserve some notice. It is notorious in how complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, &c. have been crossed, yet many of these hybrids seed freely...I have taken some pains to ascertain the degree of fertility of some of the complex crosses of Rhododendrons, and I am assured that many of them are perfectly fertile. Mr. C. Noble, for instance, informs me that he raises stocks for grafting from a hybrid between Rhod. Ponticum and Catawbiense, and that this hybrid "seeds as freely as it is possible to imagine."

In Variation Under Domestication , there are two entries on crossing in Rhododendron . In his chapter "On the Direct Action of the Male Element on the Mother Form" Darwin wrote (16):

Mr. J. Anderson Henry [Issac Anderson-Henry, plant breeder of Hay Lodge, Perthshire;] crossed Rhododendron Dalhousiae with the pollen of R. Nuttallii , which is one of the largest-flowered and noblest species of the genus. The largest pod produced by the former species, when fertilised with its own pollen, measured 1 2/8 inch in length and 1½ inch girth; whilst three of the pods which had been fertilised by pollen of R. Nuttallii measured 1⅝ inch in length and no less than 2 inches in girth. Here we see the effect of foreign pollen apparently confined to increasing the size of the ovarium...

Under "Crossing as a Cause of Variability," Darwin stated that, "Herbert [William Herbert], also, has described the offspring from a hybrid rhododendron as being 'as unlike all others in foliage, as if they had been a separate species'" (16).

Darwin's last published comment on crossing in the genus appeared in Cross and Self-fertilisation (17): "Horticulturists do not commonly distinguish between the effects of variability and intercrossing; but I have collected evidence on the natural crossing of varieties of...rhododendron...all of which plants I believe to be self-fertile."

Pollen and Pollination

It is logical to conclude that Darwin's interest in crossing might lead to observations on pollen and pollination, and indeed this is the case. His first notebook entries on these subjects were in his "Questions and Experiments Notebook," perhaps written in 1839 and 1841: "Examine pollen of such flowers as do not seed or seed rarely - Magnolias. Azaleas & plants grown under unfavourable circumstances, as Hyacinths in glasses &c &c" (2). "...At Shrewsbury one branch of Rhod. flowered later - effect of accident? ...Which. Rhododendrum seeds??..." (2).

The following entries in the "Torn Apart Notebook" were made at Maer Hall, Maer, Stafordshire, home of Darwin's in-laws, Josiah and Elizabeth Wedgwood. They record observations Darwin made in June 1841 (2):

Maer June /41/

Rhododendrum - nectary marked by orange freckles on a [circled] upper petal; bees & flies seen directed to it - The Humbles [bumblebees] in crawling out brush over anther & pistil & one I saw impregnate by pollen with which a bee was dusted over, [here is a drawing of a flower] Stamen & pistils curve upwards, so that anthers & stigma lie in fairway to nectary - Is not this so in Kidney Bean. How is it generally - In azalea it is so -

Saw Humble go from great Scarlet Poppy to Rhododendron - from Larkspur to Lupine - two species of Larkspur - two varieties of Cistus speedwell to Rhododendron - anchusa - speedwell Iris - Azalea. Rhododendron. Fraxinella to Anchusa once P [pollen?] on Fraxinella small. Humble alighted on base of filaments & reached nectar again between them, hence quite below stigma & so avoided it. On certain days Humble seen to frequent certain flowers, today early, the great scarlet poppy.

This somewhat impressionistic and stream-of-consciousness entry describes how bumblebees sometimes forage from one species to another rather than visit only one species at a time for nectar and pollen like honeybees. Under the same place and date, Darwin added : "So that, finally, Fraxinella, with respect to nectary is same case as Azalea or Rhododendron."

The following notes are associated with three specimens of Rhododendron preserved in the Darwin Archives at the Cambridge University Library:

1) "Maer. June /41/Rhod. azaloides - utterly barren stamens hybrid described in my notes";

2) "Maer June 1841 Two var. of Rhod. ponticum (a) with long stamens, larger leaves (b) with short stamens & small leaves. Latter with little pollen";

3) "Azalea - Maer June 9/41/Described the shriveled anthers without grains of pollen."

Darwin wrote a number of short articles and notes for the Gardeners' Chronicle . In one published in 1841 titled "Humble-Bees," he described how the bees cut holes in the bases of the corollas of Rhododendron azaleoides in order to steal the nectar (1):

Although I have said that country humble-bees appear to be less cunning than London ones, yet I confess I saw this June, in Staffordshire, some in the act of cutting holes at the base of the corolla of the Rhododendron azaleoides; the greater number entered the mouth of the corolla, as indeed was evident from the quantity of pollen on the stigma, brought by the bees from neighbouring Azaleas - this hybrid not having a single grain of pollen of its own. One bee was seen which entered the mouth of some of the flowers and cut holes in others; this shows that the orifices are made simply to save trouble, and not because the bee cannot extract the nectar from the long tube.

In Natural Selection Darwin included Rhododendron in a list of genera where "true seeds" cannot be collected when different varieties are grown together. That is, the seeds yield hybrid progeny.

Another entry added (34): I found a hybrid Rhododendron which [was] quite destitute of pollen, & which was so seldom visited by Bees, that after long watching the branch for many days I never saw but four Bees visit it: yet on one morning I found from 50 to 100 grains of pollen of Azalea or Rhododendron on the stigmas of these flowers: another day I examined the stigmas of 19 flowers & on 13 of them there was the same pollen.

In a discussion of contabescence (the more or less complete abortion of anthers in a flower), Darwin noted, "...Herbert adduces the N. American Azaleas, which anyone may compare (as I have often done) with the most sterile hybrids, & the anthers will be found to be in exactly the same aborted condition" (34). Darwin also noted that there may be differences in the number of stamens in a flower (34):

Azalea with five stamens & Rhododendron with ten stamens, of which half are of inferior power [that is, using pollen from this half for crossing will yield fewer progeny than using it from the other half] are closely allied genera as everyone may see; & W. Herbert says (Journal of Horticulture Soc. vol. 2, p. 86) he has raised seedlings of A. pontica & Indica frequently producing 7, 8, or 9 stamens. Azalea nudiflora in N. American presents "numberless varieties some of them exhibiting ten or more stamens." Asa Gray Manual, p. 257 2nd Edit.

In a discussion of the corollas of azaleas and other plants perforated by bees, Darwin states (34):

Exotic flowers, I think, are more commonly perforated than endemic species ...In most of these flowers, I have also seen the Humble-bees extracting the nectar without a hole having been cut; & it is evident that they cut the holes in order to save time: it seemed to me that they could visit nearly twice as many perforated, as non-perforated flowers in the same time.

Darwin's last mention of this phenomenon came in Cross and Self-fertilisation (17):

So persistent is the force of habit that when a bee which is visiting perforated flowers comes to one which has not been bitten, it does not go to the mouth, but instantly flies away in search of another bitten flower. Nevertheless, 1 once saw a humble-bee visiting the hybrid Rhododendron azaloides , and it entered the mouths of some flowers and cut holes into the others.

Bud Variation

Darwin's interest in heredity led him to look at both sexual and vegetative reproduction. One aspect of the latter is what he called bud variation, changes of colour or form in plants arising from a flower or leaf bud, changes that often are referred to as sports. Darwin wrote in his 1844 essay, "Occasionally a single bud on a plant assumes at once a new and widely different colour from that of the stock, in ...Azaleas, &c, &c. ..."(21).

In his longest book, Variation under Domestication , Darwin included a chapter on bud variation. In this chapter, Darwin stated (16):

Azalea Indica is well known often to produce by buds new varieties. I have myself seen several cases. A plant of Azalea Indica variegata has been exhibited bearing a truss of flowers of A. Ind. Gledstanesii "as true as could possibly be produced, thus evidencing the origin of that fine variety." On another plant of A. Ind. variegata a perfect flower of A. Ind. lateritia was produced; so that both Gledstanesii and lateritia no doubt originally appeared as sporting branches of A. Ind. variegata .

Pistils and Nectaries

Along with Darwin's interest in pollination went a curiosity about how pollen might be deposited on the stigma of a pistil by an insect. In a series of letters between 1858 and 1862, Darwin shared comments on the relationship between the curved pistils and styles and the nectaries of Rhododendron flowers with J.D. Hooker and Asa Gray, botanist and Fisher Professor of natural history at Harvard University.

Darwin's July 4, 1858, letter to Asa Gray contained the following postscript (8):

In regard to bent pistils & nectaries, I should say that I largely judge of position of nectary, by seeing where Bees suck; according to this, the rule holds in all Leguminosae, in my 6 Fumariaceae, in Polygala - Viola tricolor, Dictamnus Aesculus, Rhododendron - Aquilegia."

When discussing floral style curvature in a letter to Darwin of April 20, 1860, J.D. Hooker wrote (10):

From aequilateral flowers with curved styles nothing can be deduced E.G. Rhododendron, for there is no side more advantageous for passage of insect than another as far as form of Corolla is concerned, the curvature is I think in all flowers?? always towards axis .

Hooker also supplied a drawing of a flower to illustrate its axis and the "five symmetr. nects." Darwin answered on April 26, 1860, "If I remember in Rhododendron honey is secreted by only two petals?? viz the two towards which pistil bends. Insects always know at once where nectar lies, & have not to search for it." (10). The next day he wrote to Hooker: "Rhododendron, Viola tricolor, Horse-chestnut all have pistils more or less bent in same manner, ie in path which bees much brush over in sucking the nectar" (10).

To Hooker again, Darwin wrote on May 15, 1862, (22):

Whenever you have occasion to write, pray tell me whether you have Rhododendron Boothii from Bhootan, with a smallish yellow flower, and pistil bent the wrong way; if so, I would ask Oliver [Daniel Oliver, assistant in the herbarium at the Royal Botanic Gardens, Kew] to look for nectary, for it is an abominable error of Nature that must be corrected. I could hardly believe my eyes when I saw the pistil.

The last phrase of the first sentence shows Darwin's humor, which creeps into much of his correspondence.

Once again, Darwin wrote to Hooker, on May 18, 1862:

Now mind do not answer on purpose, but at some time tell me what the enclosed Sikkim Rd. is; - it has a reflexed pistil like R. Boothii & secretes nectar within the stamens all round the germen [the ovary]! R. Dalhousianum secretes nectar on lower side of flower; so that I do not fear the usual law will hold with respect to bending of pistil & position of nectar.

That is, in this species the bended pistil does not point toward the nectar. Hooker answered on June 19,1862, that, "Your Rhododendron is R. glaucum - it has a very peculiar scent. - This was sent me by you a month ago under some other name."

None of this information on pistils and nectaries in Rhododendron was published by Darwin.

Growth

During his long career as geologist, zoologist, and botanist, Darwin published two books that related his many observations on plant growth, Climbing Plants (1865) and Movement in Plants (1880). Both included descriptions of circumnutation in many species, the phenomenon of the spontaneous revolutions of the apical portions of stems, tendrils, and roots. Darwin's observations on stem movements in Rhododendron were published in Movement in Plants : "13. Azalea Indica ... A bush 21 inches in height was selected for observation, and the cirumnutation of its leading shoot was traced during 26 h 40 m, as shown in Fig. 79" (19). The caption for this figure reads (19):

Azalea Indica: Circumnutation of stem, illuminated from above, traced on horizontal glass, from 9.30 a.m. 9 March to 12.10 p.m. on the 10th. But on the morning of the 10th only four dots were made between 8.30 a.m. and 12.10 p.m., both hours included, so that the circumnutation is not fairly represented in this part of the diagram. Movement of the bead here magnified about 30 times.

Darwin made these measurements in the following ingenious way. A plant in a pot was placed under a horizontal sheet of glass. A small glass filament ¼ to ¾ inches long was glued to the part to be observed, and a minute bead of black sealing wax was attached to the tip of the glass filament. Below this a white card with a small black spot was fixed to a stick driven into the ground. Observations were made at intervals by lining up the bead with the black spot on the card and making an ink dot on the sheet of glass in line with them. The resulting dots were connected in order by straight lines, thus giving a visual track of the movements of the part being observed. Movement in Plants illustrates over 150 such observations.

The last entry on Rhododendron was: "With different plants [that is, species] the complexity, rate, and amount of movement differs much. The stems, for instance, of...Azalea described only a single large ellipse in 24 h; whereas those of the Deutzia made four or five deep zigzags or narrow ellipses in 11½ h..." (19).

Acknowledgments

Permission to quote from Darwin's letters and manuscripts has kindly been granted by George Pember Darwin, the Syndics of the Cambridge University Library, and the Syndics of the Cambridge University Press. Helpful comments on the manuscript were provided by Erik Nilsen, Ruth Douglas, Rhonda Edwards, Wendy Rohrer, and Sarah Porter. All are gratefully acknowledged, as is Lady Cynthia Poston for suggesting that I write on Darwin, Hooker, and Rhododendron .

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