WCP2337

[1]

Coneysthorpe,

Malton, Yorks.

28 July, 1876.

My dear Wallace

I can hardly say that I have ever speculated on the purport of the odours of leaves, but I have (at your instance) rummaged in my notes & my memory for such evidence as I possess on that head, & will lay some of it before you.

Every structure, every secretion, of a plant, is (before all) beneficial to the plant itself. That is, I suppose, an incontrovertible axiom. Odoriferous glands — especially those imbedded in the leaf — act as a protection against leaf-cutting ants, and (to some extent) also against caterpillars. I can remember no instance of seeing insects attracted to a plant — to aid in its fertilisation, or for any other purpose — by their presence. The glands on which some insects feed are (so far as I know) always exposed, either in the shape of cups on the petioles, involucres, &c.; or of hairs with dilated & hollow bases, and of sessile or stalked cysts, on the leaves, petioles, pedicels, &c.; and the secretion is either tasteless or slightly sweet, but inodorous — to our senses, at least.

Trees with aromatic leaves abound in the plains of equatorial America. Of Those which have the aromatic (and often resinous) secretion ^imbedded in distinct cysts ^include are all Myrtaceae, Myrsineae, Samydeae and many Euphorbiaceae, Compositae, etc. The leaves of very few of these are, when growing, [words 'very rarely'[?] rubbed out] ever touched by leaf-cutting ants. ^{In the few cases} Where, however, ^where the secretion is slightly but pleasantly bitter, and wholesome, as in the Orange, the leaves are quite to their taste. At a farmhouse on the Trombetas I was shown orange-trees which had been entirely denuded in a single night by Saúba ants. Various expedients are resorted to by the inhabitants of Saúba-infested lands to protect their fruit-trees, such as a small moat, kept constantly filled with water, around each tree; or wrapping the base of the trunk with cotton kept soaked with andiroba oil; &c.

(Note. Leaf-cutters in the vicinity of man work chiefly by night, taught doubtless by painful experience of his vicious propensity to interfere with their operations. But in the depths of the forest I have often caught them at work, some up a tree cutting off leaves & even slender young branches, others on the ground sawing them up & carrying them off. I once saw a Sauba who reminded one of Hogarth's picture of the Election, where a man is sawing off a sign-board on which he is seated; for the ant cut off the point of a branch, with 3 or 4 leaves, on which he was standing, and came swirling to the ground with it, but very gently, and unhurt. When at San Carlos, I one day went [2] into the forest to gather a Securidaca (woody Polygaleous twiner) I had seen coming into flower a few weeks before. I found it in full flower, but the ^little tree on which it grew — a Phyllanthus, with slightly milky and quite innocuous juice — had been taken possession of by a horde of ants, and I had to wait until they had stripped it of every leaf before I could pull down my Securidaca, which they had left quite untouched. ¹The Securidada was probably preserved by its drastic properties from sharing the fate of the Phyllanthus.

Many odoriferous leaves seem destitute of special oil-glands, and their essential oil probably exists in nearly every cell, along with the chlorophyl [sic] (as I have found it in several aromatic Hepatics). Many Lauraceae & Burseraceae (Amyrideae of Lindley) are in this case. The latter are eminently resiniferous, and yield the best native pitch (the brea branca) of the Amazon Valley. I have never seen their leaves mutilated by ants, and I think never by caterpillars. Oil-glands indeed exist in many plants where they are either so deeply imbedded or so minute as only to be detected by closer scrutiny. Their presence was denied in the Nutmegs (see Lindley, &c.) until I found them in all the American species, and one species has them so conspicuous that I have called it Myristica punctata.

In nearly all these plants, however, when the essential oil has been wholly or in part dissipated by drying, the leaf-cutters find the leaves apt material for their purpose —whatever that may be. They once fell on some of my dried specimens, and first cut up a Croton — a genus I had never seen them touch in the living state. It reminded me of our cows in England, which cautiously avoid the fresh foliage of Buttercups, but eat it readily when made into hay. The acrid principle in these and many other plants, odorous and inodorous, is known to be highly volatile. (Comp.[are] also the root of Manihot &c.)

Where aromatic plants most abound is in the dry — often nearly treeless — mountainous parts of southern Europe and western Asia, especially in the sierras of Spain. When I was with Dufour² at St. Sever, in April, 1846, he received a large parcel of plants recently gathered in the Sierra Guadarama by Professor Graells³ of Madrid. A very large proportion were aromatic, and many of them Labiates.

I cannot make out that plants with scented leaves abound more in the tropics than in mid-Europe; nor does there seem to be a larger proportion of them in any zone of the equatorial Andes than in the Amazonian plain; although, as hill-plants are often gregarious, and those of hot plains very rarely so, odoriferous plants may seem more prevalent in the high Andes than on the Amazon.

⁴Plants growing nearest eternal snow in the Andes are however (so far as I have observed them) all scentless; but some acquire an aroma in drying, as, for example, the thick roots of the Valerians that abound there.

⁵The Aromatic plants grow of ⁱⁿ the Andes up to perhaps 13000 feet, and consist chiefly of Composites, Myrtles, Labiates and Verbenas. I know a hill-side, at about 9000 feet, which at this time of year is one mass of odoriferous foliage and flowers, chiefly of a Labiate undershrub (Gardoquia fasciculata B[en]th.[am]). Another slope of far [3] wider extent is much gayer with varied colour, mainly of the blue flowers of Dalea Mutisii H[umboldt] B[onpland] K.[unth] — a Papilionaceous shrub, allied to the Indigos — and of the red-purple foxglove-like flowers of Lamourouxia virgata H[umboldt] B[onpland] K.[unth] (which is parasitic on the roots of the Dalea), mingled with the yellow flowers of the Quitenian Broom (Genista Quitensis L.) ; and ^of many other herbs and shrubs with flowers of various shades of colour; but aromatic plants are almost unrepresented except by scattered bushes of a Salvia and a Eupatorium.

Analogous contrasts are common enough in our own country. "I know a bank whereon the wild thyme grows", viz. in Ganthorpe Moor, a mile away from where I am writing, on which, when I was a boy, I used to find in the autumn the still more richly-scented Spiranthes autumnalis. But other slopes in the same moor are clad with Heather, Blue-bells, Veronica officinalis, and other scentless plants.

In those parts of the Peruvian and Quitenian Andes I have explored, I have not found odoriferous plants more abundant than in some parts of England and the Pyrenees; yet they are quite as much so as in the Amazonian plain, and often belong to the same Natural Orders. Now leaf-cutting ants are unknown in the Andes; whence I infer that, although the presence of a pungent smell and taste may be protective to leaves in hot forests where such ants do exist, it has not been acquired originally with a view to provide the requisite protection.

I much doubt the correctness of Mr. Belt's theory that the ants which inhabit leaf-sacs protect the leaves from leaf-cutting ants; for the leaves of such plants are almost invariably thin and dry, whereas the Saúba always selects leaves that are more or less coriaceous, and if it really wanted the sacciferious leaves I fancy it would make short work of their ^frail inhabitants. Besides, there are numbers of Melastomes, allied to Tococa and Myrmidone, which the Saúba never touches, although they have no protective (?) sacs; but it cuts up readily the coriaceous leaves of other Melastomes, such as various Bellucias, Henriettas, &c.

Rich[ar]^d Spruce. [signature]