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Darwin Correspondence Project

From Hubert Airy   24 July 1872

13. Eliot Place. Blackheath, S.E.

1872. July 24.

My dear Sir

Your letter of the 15th. gave me much to think about, and I hope you will allow me to write to you further on the several points you raise,—not so much by way of answer as for the sake of clearing my own ideas.1

1. The slight twist required in my mechanism:—what is to represent it in nature?

I can only suppose a slight twist of the bud-axis in the process of development. The supposition is warranted by the frequent (sometimes regular) occurrence of twist in stems and branches;—which are at least founded on bud-axes. The stem of the horse-chestnut is generally twisted to the right (ascending), never to the left. Spanish chestnut is often strongly twisted, to right or to left. The twists of climbing plants are best known to yourself.

It is a suggestive fact that in the elm, beech, and lime, which (according to my view) keep the original two-ranked leaf-order unaltered by any twist, there is, as far as I have yet observed, little or no twist of stem or branch to be seen.

It is curious to see how, when the stem of a tree has a twist, the branches also have the same. In the middle of Greenwich Park there is an old oak, called Queen Elizabeth’s, religiously preserved, naked of bark, and displaying a strongly-marked twist throughout, even to the tips of the small branches.2

Assuming a twist, then, as a probable primary variation from an originally straight bud, I suppose that this twist would be taken advantage of, and perpetuated, by natural selection, in subservience to the close packing of the embryo leaves. The twist is equally required, whether the packing force be a force of contraction of the bud’s boundaries on certain given contents; or a force of expansion of the contents against given boundaries; or a balance between the two.— But I will return to this point further on.

2. It is true that my mechanism fails to produce truly vertical ranks, and I have nothing to add at present to my plea of extenuating circumstances. This figure, (copied from Asa Gray’s, First Lessons in Botany,3 Fig. 142. p. 73. cross-section of stalk of sedge,)

[DIAG]

though only diagrammatic, illustrates the importance of vertical alignment for mutual accommodation. If leaf No. 4 lay to one side of the vertical, it looks as if No. 1 would suffer a very awkward bend on one side of the middle line.

(3. Thank you for pointing out the need of definition of such words as “distichous” & “collateral.”)4

4. I must admit that I attached undue importance to frost as the chief agent in keeping buds small.5 I took it as the representative of cold, and was thinking of the buds in our English winter; but I should have done better to speak more generally of “vicissitudes of climate,” when I was speaking for the whole earth.

5. I must also acknowledge (not without a little regret) that my contrast between grasses and firs, as it stands, is not convincing—6 I ought to go further back to the less-highly-developed Families, in search of the original order. Balfour (Manual of Botany, 1860. p. 570) gives a drawing of one of the Hepaticæ, fig 778. with the note, “Branches covered with imbricated leaves, arranged in a distichous manner.”—7 Flat blades, with marginal gemmation, are frequent among algæ, are they not?8

6. I have no more to say at present about the change of leaf-order that is found in Spanish chestnut and so many other trees.

7. Thank you much for your suggestion of internal pressure among the nascent leaves, combined with small size of bud, as effectual in condensing the leaves— I had a feeling of this state of pressure, but I had not put it in words.

Neither of these two forces alone, unresisted by the other, would have the effect in question: it is the result of a balance between them; not forgetting the subsidiary balance which I insisted upon in my paper, between the advantages of shortness and narrowness.—9 And I do not see how all these forces together could avoid the non-existent 4-ranked, 7-ranked, 9-ranked orders, except by having to operate upon an original 2-ranked order. I quite agree with you that I shall do well to insist more distinctly on the successive stages of stability assumed by my mechanism in condensation, and especially on the explanation which is thus given of the non-appearance of such orders as the above-named.10

8. You mentioned the position of the cotyledons in the seed, as supporting the view that the original order was two-ranked, and I may add that the arrangement of the cotyledons in the Monocots as compared with that in the Dicots gives a striking illustration of the divergence of the alternate and the whorled types from an earlier two-ranked but irregular order. In the Monocotyledonous embryo, the advantage of lateral economy has prevailed:— in the Dicotnous. embryo, the advantage of longitudinal economy has prevailed, compressing the first leaves into the crucial order.11

Lastly, let me thank you for your kind warning against attempting more than one subject at a time. I feel that the one I have taken up is much more than I can do justice to—one needs to examine the whole vegetable kingdom, past and present!

Believe me, my dear Sir, with great respect, yours very sincerely | Hubert Airy

Footnotes

CD’s letter has not been found, but it was a response to the letter from Hubert Airy, [before 15] July 1872. See also letter from Hubert Airy, 16 July 1872.
The oak in Greenwich Park, London, was hollow; its interior was large enough for Queen Elizabeth to use as a place for refreshments, and later, when fitted with a door, it served as a lock-up for those who broke the park rules. The tree died in the nineteenth century but was held upright by the ivy that grew around it. See Webster 1971, p. 7. It was brought down in 1991 when heavy rain washed away the surrounding soil.
A. Gray 1857.
CD had put a pencil ‘X’ in the margin against Airy’s statement that frost was the agent that brought about smaller buds; see letter from Hubert Airy, [before 15] July 1872.
John Hutton Balfour’s drawing showed the branches of Jungermannia tamarisci, a liverwort whose leaves overlap like roof-tiles in two vertical ranks along the branch (see Balfour 1860, p. 570).
Marine algae, or seaweeds, and fresh-water algae asexually produce buds or gemmules on the edges of their blades. These buds break off and produce new individuals (W. H. Harvey 1849, p. 67).
See section 7 of the letter from Hubert Airy, [before 15] July 1872. See also Airy 1873, pp. 177–8.
In his published paper, Airy stated that the action of forces not only gave rise to successive orders ‘of successive maxima of stability’ but also necessarily excluded others (see Airy 1873, p. 178).
See letter from Hubert Airy, 16 July 1872 and n. 2. Airy’s arguments concerning the cotyledons in monocotyledons and dicotyledons were the basis of his conclusion that all leaf orders derived from a two-ranked arrangement; he held that in monocotyledons lateral forces produced an alternate order of leaves (giving rise to spiral orders) and in dicotyledons vertical forces resulted in a crucial order (leaves at right angles to one another), which in turn produced whorled orders (Airy 1873, p. 179).

Bibliography

Airy, Hubert. 1873. On leaf-arrangement. Abstract. Communicated by Charles Darwin. [Read 27 February 1873.] Proceedings of the Royal Society of London 21 (1872–3): 176–9.

Balfour, John Hutton. 1860. A manual of botany: being an introduction to the study of the structure, physiology and classification of plants. Edinburgh: A. and C. Black.

Webster, Angus Duncan. 1971. Greenwich Park: its history and associations. London: Conway Maritime Press.

Summary

Responds to CD’s comments on his MS on phyllotaxy.

The initial variation required by his theory would be a slight twist of the bud-axis; believes the frequent twisting of stems and branches renders such a variation possible.

Admits he placed too much emphasis on the importance of frost. He should have spoken more generally of "vicissitudes of climate".

Letter details

Letter no.
DCP-LETT-8426
From
Hubert Airy
To
Charles Robert Darwin
Sent from
Blackheath
Source of text
DAR 159: 20
Physical description
11pp

Please cite as

Darwin Correspondence Project, “Letter no. 8426,” accessed on 24 September 2020, https://www.darwinproject.ac.uk/letter/DCP-LETT-8426.xml

Also published in The Correspondence of Charles Darwin, vol. 20

letter