My dear Sir

A process of self-eviction from Eliot Place has contributed to delay my writing to thank you for the notice you kindly sent me of M^r. Meehan’s observations in regard to the office of bud-scales.¹

I confess to feeling combative on the point, but must not trouble you with my notions beyond remarking that the hardihood of those naked ash-buds was not proved, for it does not appear that they were allowed to grow to maturity and put forth leaves and become useful members of twig-society, and it is open to conjecture that the effects of early exposure would have been seen in premature decay of the grown leaves.—² Again, the unclothed ear of “corn” (which I take to be Indian corn, maize,) was altogether abnormal, being formed “on the end of a male panicle,” and its hardihood does not appear to have been tested by experimental sowing.³

Turning from these rare and inconclusive instances to my favourite oak-buds with their imbricate armour of scale over scale, and watching how these armour-plates are all thrown aside, to right and left, by the swelling leaves in spring, and hang or fall, dry brown stipules, no longer needed, on either side of the leaf-stalk they belong to,—I find myself unable to believe that the office of those scales is not protection of the bud-leaves from all outward dangers, from cold and heat, birds’ beaks, beetles’ and caterpillars’ jaws, and such chance violence as the chafing of twig against twig in the winds of winter. They are on duty a whole year, within a day or two,—from spring to spring.

While speaking of buds, let me thank you for your suggestion (July 25^th.)⁴ of extensive bud-dissection under a simple microscope as important for the study of Phyllotaxy— I have practised it lately, and ten or eleven years ago, though often without the lens.— Perhaps the enclosed pages, extracted from some old notes, will interest you, in spite of their doubts and blunders. I will ask permission to take them away again when I have the honour of calling upon you, as they have place among consecutive pages.⁵

Perhaps you remember what I regard as the fundamental principle of my theory,—that the operation of the causes which have determined the complex orders of Phyllotaxy is to be looked for, not in the mature twig but in the bud.—⁶ I must now draw the distinction more precisely, and say that the operation of the causes in question is to be looked for, not in the mature twig with long internodes and distant leaves, but in the bud, in the bulb, in the radical rosette, in the involucre, in the receptacle, in the catkin, in the cone, in the spike,—in short wherever the leaves or scales or bracts, with their axillary shoots, are crowded into close contact.

The radical rosette is well exemplified in the different species of Plantain which struggle successfully, by the aid of condensed leaf-order, against the difficulties of existence on the parched and padded gravel of Blackheath. A section at the base of the leaves of Plantago Coronopus, in comparison with P. Major, is a capital study.⁷

The receptacle of the sunflower exhibits the highest degree of condensation that I have yet seen— Taking a seed near the margin to start from, those in contact with it are the 34^th. on one side, the 55^th. on the other, the 89^th. near the 34^th., and even the 144^th between the 55^th. and the 89^th.. (I mean the 34^th. &c. in order of growth from the starting-point.)— The bald head of a dandelion, when the seeds have all gone to the winds, offers a similar study on a smaller scale, with less condensation. These examples are useful in teaching us to pay attention to the spirals and not to look for vertical ranks, for the rank that has a vertical (or rather radial) look near the centre of the receptacle, is seen to curve away into a decided spiral near the margin.

I have not yet felt able,—and now I can only make a first attempt,—to give an answer to your question of July 15^th.,⁸— would not the mutual pressure of the nascent leaves within the bud be different towards the extremity and base of the small growing axis, so as to account for the different Phyllotaxy on the upper and lower parts of the same shoot?

It seems to me that as long as the growth of the bud-axis bears the same proportion to the growth of the embryo leaves, in different parts of the bud, there will be no difference of pressure in different parts, and therefore no cause for difference of arrangement;—

Supposing the order to be $\frac{1}{2}$, and supposing the lower part of the bud-axis to grow in a certain proportion to the growth of the lower leaves, while the upper part grows in exactly the same proportion to the diminished growth of the upper leaves, then I see no reason for expecting a condensed order at one end more than at the other: if all the leaves grow faster than the axis which carries them, and the axis gets a twist, then the whole bud will get the same change of order; but if the axis remains straight, growing as fast as the leaves, then the bud will keep the order $\frac{1}{2}$ throughout.

But if we suppose the growth of the bud-axis to be checked in its upper part,—the upper leaves continuing to grow,—while the lower part of the axis grows in proportion to the growth of the lower leaves; then there will be greater mutual pressure among the upper leaves than among the lower, and the need will arise of a twist in the upper part of the axis, to allow the upper embryo-leaves to fall into a more economical order and fulfil their development in spite of the temporary check to the development of the axis from which they spring.— And this is exactly what I suppose to be the case with the buds of the Spanish Chestnut,—not a contraction of the individual bud in the course of its single life-time, but a contraction of the top of the bud-axis relative to the bottom, taking effect in the course of ages from generation to generation, showing itself in the individual as a stunting of the top of the bud-axis without proportionate stunting of the embryo-leaves that spring from that part.— We often see instances of shoots stunted at their tips by a cold season, and it seems reasonable to think that in process of time the same agency may have produced a like effect on the tips of the spanish chestnut buds.⁹

In these remarks I have assumed that the embryo leaf is organically tied, rooted, to a certain point on the bud-axis, and is not free to shift up or down or to either side without a corresponding growth or stunt or twist of the part of the axis from which it springs. If this assumption is incorrect, the reasoning fails. The opposite assumption would be, that the embryo-leaves enjoy complete and independent mobility; and what this would lead to, I have not yet considered.

I had hoped by this time to have brought my arguments into better order, and to have made my bits of illustrative mechanism more fit to exhibit to you; but I find that it will need a long and patient collection of facts for the former object, and a long course of trial and error for the latter; so I must be content to show you what I have, if I may; and now make bold to remind you of the invitation you kindly gave me to pay you a visit some time in the early autumn— May I come one day next week?— Or will you kindly tell me when my call will be least inconvenient to you?— Do not let me come if your health forbids, but let me wait for another opportunity—¹⁰

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