My dear Sir

Your letter of the 15^th. 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,) though only diagrammatic, illustrates the importance of vertical alignment for mutual accommodation. If leaf N^o. 4 lay to one side of the vertical, it looks as if N^o. 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 Dicot^nous. 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