An introduction to physiological and systematical botany/Chapter 10

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According to the Errata, the "see p. 56" on page 91 should be "see p. 65".



The sap in its passage through the leaves and bark becomes quite a new fluid, possessing the peculiar flavour and qualities of the plant, and not only yielding woody matter for the increase of the vegetable body, but furnishing various secreted substances, more or less numerous and different among themselves. These accordingly are chiefly found in the bark; and the vessels containing them often prove upon dissection very large and conspicuous, as the turpentine-cells of the Fir tribe. In herbaceous plants, whose stems are only of annual duration, the perennial roots frequently contain these fluids in the most perfect state, nor are they, in such, confined to the bark, but deposited throughout the substance or wood of the root, as in Rhubarb, Rheum palmatum, Linn. fil. Fasc. t. 4, and Gentian, Gentiana lutea and purpurea, Ger. emac. 432, f. 1, 2. In the wood of the Fir indeed copious depositions of turpentine are made, and in that of every tree more or less of a gummy, resinous or saccharine matter is found. Such must be formed by branches of those returning vessels that deposit the new alburnum. These juices appear to be matured, or brought to greater perfection, in layers of wood or bark that have no longer any principal share in the circulation of the sap.

The most distinct secretions of vegetables require to be enumerated under several different heads.

Gum or mucilage, a viscid substance of little flavour or smell, soluble in water, is very general. When superabundant it exudes from many trees in the form of large drops or lumps, as in Plum, Cherry, and Peach-trees, and different species of Mimosa or Sensitive plants, one of which yields the Gum Arabic, others the Gum Senegal, &c.

Resin is a substance soluble in spirits, and much more various in different plants than the preceding, as the Turpentine of the Fir and Juniper, the Red Gum of New South Wales, produced by one or more species of Eucalyptus, Bot. of N. Holl. t. 13, and the fragrant Yellow Gum of the same country, see White's Voyage, 235, which exudes spontaneously from the Xanthorrhœa Hastile. Most vegetable exudations partake of a nature between these two, being partly soluble in water, partly in spirits, and are therefore called Gum-resins. The milky juice of the Fig, Spurge, &c., which Dr. Darwin has shown, and which every body may see, to be quite distinct from the sap, is, like animal milk, an emulsion, or combination of a watery fluid with oil or resin. Accordingly, when suffered to evaporate in the air, such fluids become resins or gum-resins, as the Gum Euphorbium. In the Celandine, Chelidonium majus, Engl. Bot. t. 1581, and some plants allied to it, the emulsion is orange-coloured.

The more refined and volatile secretions of a resinous nature are called Essential Oils, and are often highly aromatic and odoriferous. One of the most exquisite of these is afforded by the Cinnamon bark. They exist in the highest perfection in the perfumed effluvia of flowers, some of which, capable of combination with spirituous fluids, are obtainable by distillation, as that of the Lavender and Rose; while the essential oil of the Jasmine is best procured by immersing the flowers in expressed oil which imbibes and retains their fragrance. Such Expressed or Gross Oils, as they are called, to distinguish them from essential oils obtained by distillation, are chiefly found in the seeds of plants. In the pulp of the Olive indeed they occur in the form of an emulsion, mixed with watery and bitter fluids, from which the oil easily separates by its superior lightness. These expressed oils are not soluble in spirits or water, though by certain intermediate substances they may be rendered capable of uniting with both.

The Bitter secretion of many plants does not seem exactly to accord with any of the foregoing. Some facts would seem to prove it of a resinous nature, but it is often perfectly soluble in water. Remarkable instances of this secretion are in the Cinchona officinalis or Peruvian bark, Lambert Cinchona, t. 1, and every species, more or less, of Gentian.

Acid secretions are well known to be very general in plants. Formerly one uniform vegetable or acetous acid was supposed common to all plants, but the refinements of modern chemistry have detected in some a peculiar kind, as the Oxalic acid, obtained from Oxalis or Wood Sorrel, and several others. The astringent principle should seem to be a sort of acid, of which there are many different forms or kinds, and among them the tanning principle of the Oak, Willow, &c.

On the other hand, two kinds of Alkali are furnished by vegetables, of which the most general is the Vegetable Alkali, properly so called, known by the name of Salt of Tartar, or Salt of Wormwood, or more correctly by the Arabic term Kali. The Fossil Alkali, or Soda, is most remarkable in certain succulent plants that grow near the sea, belonging to the genera Chenopodium, Salsola, &c. When these plants are cultivated in a common soil, they secrete Soda as copiously, provided their health be good, as in their natural maritime places of growth.

Sugar, more or less pure, is very generally found in plants. It is not only the seasoning of most eatable fruits, but abounds in various roots, as the Carrot, Beet and Parsnip, and in many plants of the grass or cane kind besides the famous Sugar Cane, Saccharum officinarum. There is great reason to suppose Sugar not so properly an original secretion, as the result of a chemical change in secretions already formed, either of an acid or mucilaginous nature, or possibly a mixture of both. In ripening fruits this change is most striking, and takes place very speedily, seeming to be greatly promoted by heat and light. By the action of frost, as Dr. Darwin observes, a different change is wrought in the mucilage of the vegetable body, and it becomes starch.

A fine red liquor is afforded by some plants, as the Bloody Dock or Rumex sanguineus, Engl. Bot. t. 1533, the Red Cabbage and Red Beet, which appears only to mark a variety in all these plants, and not to constitute a specific difference. It is however perpetuated by seed.

It is curious to observe, not only the various secretions of different plants, or families of plants, by which they differ from each other in taste, smell, qualities and medical virtues, but also their great number, and striking difference, frequently in the same plant. Of this the Peach-tree offers a familiar example. The gum of this tree is mild and mucilaginous. The bark, leaves and flowers abound with a bitter secretion of a purgative and rather dangerous quality, than which nothing can be more distinct from the gum. The fruit is replete, not only with acid, mucilage and sugar, but with its own peculiar aromatic and highly volatile secretion, elaborated within itself, on which its fine flavour depends. How far are we still from understanding the whole anatomy of the vegetable body, which can create and keep separate such distinct and discordant substances!

Nothing is more astonishing than the secretion of flinty earth by plants, which though never suspected till within a few years, appears to me well ascertained. A substance is found in the hollow stem of the Bamboo, (Arundo Bambos of Linnæus, Nastos of Theophrastus), called Tabaxir or Tabasheer, which is supposed in the East Indies (probably because it is rare and difficult of acquisition, like the imaginary stone in the head of a toad) to be endowed with extraordinary virtues. Some of it, brought to England, underwent a chemical examination, and proved, as nearly as possible, pure flint. See Dr. Russell's and Mr. Macie's papers on the subject in the Phil. Trans. for 1790 and 1791. It is even found occasionally in the Bamboo cultivated in our hot-houses. But we need not search exotic plants for flinty earth. I have already, in speaking of the Cuticle, chapter 3d, alluded to the discoveries of Mr. Davy, Professor of Chemistry at the Royal Institution, on this subject. That able chemist has detected pure flint in the cuticle of various plants of the family of Grasses, in the Cane (a kind of Palm) and in the Rough Horsetail, Equisetum hyemale, Engl. Bot. t. 915. In the latter it is very copious, and so disposed as to make a natural file, which renders this plant useful in various manufactures, for even brass cannot resist its action. Common wheat straw, when burnt, is found to contain a portion of flinty earth in the form of a most exquisite powder, and this accounts for the utility of burnt straw in giving the last polish to marble. How great is the contrast between this production, if it be a secretion, of the tender vegetable frame, and those exhalations which constitute the perfume of flowers! One is among the most permanent substances in Nature, an ingredient in the primæval mountains of the globe; the other the invisible untangible breath of a moment!

The odour of plants is unquestionably of a resinous nature, a volatile essential oil, and several phænomena attending it well deserve our attentive consideration. Its general nature is evinced by its ready union with spirits or oil, not with water; yet the moisture of the atmosphere seems, in many instances, powerfully to favour its diffusion. This I apprehend to arise more from the favourable action of such moisture upon the health and vigour of the plant itself, thus occasionally promoting its odorous secretions, than from the fitness of the atmosphere, so circumstanced, to convey them. Both causes however may operate. A number of flowers which have no scent in the course of the day, smell powerfully in an evening, whether the air be moist or dry, or whether they happen to be exposed to it or not. This is the property of some which Linnæus has elegantly called flores tristes, melancholy flowers, belonging to various tribes as discordant as possible, agreeing only in their nocturnal fragrance, which is peculiar, very similar and exquisitely delicious in all of them, and in the pale yellowish, greenish, or brownish tint of their flowers. Among these are Mesembryanthemum noctiflorum, Dill. Elth. t. 206, Pelargonium triste, Cornut. Canad. 110, and several species akin to it, Hesperis tristis, Curt. Mag. t. 730, Cheiranthus tristis, t. 729, Daphne pontica, Andrews's Repos. t. 73, Crassula odoratissima, t. 26, and many others[1]. A few more, greatly resembling these in the green hue of their blossoms, exhale, in the evening chiefly, a most powerful lemon-like scent, as Epidendrum ensifolium, Sm. Spicil. t. 24, and Chloranthus inconspicuus, Phil. Trans, for 1787. t. 14, great favourites of the Chinese, who seem peculiarly fond of this scent. There are other instances of odorous and aromatic secretions, similar among themselves, produced by very different plants, as Camphor. The sweet smell of new hay is found not only in Anthoxanthum odoratum, Engl. Bot. t. 647, and some other grasses, but in Woodruff or Asperula odorata, t. 755, Melilot or Trifolium officinale, t. 1340, and all the varieties, improperly deemed species, of Orchis militaris, t. 16, plants widely different from each other in botanical characters, as well as in colour and every particular except smell. Their odour has one peculiarity, that it is not at all perceptible while the plants are growing, nor till they begin to dry. It proceeds from their whole herbage, and should seem to escape from the orifices of its containing cells, only when the surrounding vessels, by growing less turgid, withdraw their pressure from such orifices. When this scent of new hay is vehement, it becomes the flavour of bitter almonds. The taste of syrup of capillaire, given by an infusion of Orange flowers, is found in the herbage of Gaultheria procumbens, Andr. Repos. t. 116, and Spiræa Ulmaria, Engl. Bot. t. 960, two very different plants.

Some of the above examples show an evident analogy between the smell and colours of flowers, nor are they all that might be pointed out. A variety of the Chrysanthemum indicum with orange-coloured flowers has been lately procured from China by Lady Hume. These faintly agree in scent, as they do in colour, with the Wall-flower, Cheiranthus Cheiri; whereas the common purple variety of the same Chrysanthemum has a totally different and much stronger odour.

There is, of course, still more analogy between the smell of plants in general and their impression on the palate, insomuch that we are frequently unable to discriminate between the two. The taste is commonly more permanent than the smell, but now and then less so. The root of the Arum maculatum, Engl. Bot. t. 1298, for instance, has, when fresh, a most acrid taste and irritating quality, totally lost by drying, when the root becomes simply farinaceous, tasteless and inert; so that well might learned physicians contrive the "Compound Powder of Arum," to excuse the continuance of its use in medicine, unless they had always prescribed the recent plant. Many curious remarks are to be found in Grew relative to the tastes of plants, and their different modes of affecting our organs. Anatomy of Plants, p. 279–292.

To all the foregoing secretions of vegetables may be added those on which their various colours depend. We can but imperfectly account for the green so universal in their herbage, but we may gratefully acknowledge the beneficence of the Creator in clothing the earth with a colour the most pleasing and the least fatiguing to our eyes. We may be dazzled with the brilliancy of a flower-garden, but we repose at leisure on the verdure of a grove or meadow. Of all greens the most delicate and beautiful perhaps is displayed by several umbelliferous plants under our hedges in the spring.

Some of Nature's richest tints and most elegant combinations of colour are reserved for the petals of flowers, the most transient of created beings; and even during the short existence of the parts they decorate, the colours themselves are often undergoing remarkable variations. In the pretty little weed called Scorpion-grass, Myosotis scorpioides, Engl. Bot. t. 480, and several of its natural order, the flower-buds are of the most delicate rose-colour, which turns to a bright blue as they open. Many yellow flowers under the influence of light become white. Numbers of red, purple or blue ones are liable, from some unknown cause in the plant to which they belong, to vary to white. Such varieties are sometimes propagated by seed, and are almost invariably permanent if the plants be propagated by roots, cuttings or grafting. Plants of an acid or astringent nature often become very red in their foliage by the action of light, as in Rumex, Polygonum, Epilobium and Berberis; and it is remarkable that American plants in general, as well as such European ones as are particularly related to them, are distinguished for assuming various rich tints in their foliage of red, yellow, white or even blue, at the decline of the year, witness the Guelder-rose, the Cornel, the Vine, the Sumach, the Azalea pontica, Curt Mag. t. 433, and others. Fruits for the most part incline to a red colour, apparently from the acid they contain. I have been assured by a first-rate chemist that the colouring principle of the Raspberry is a fine blue, turned red by the acid in the fruit. The juices of some Fungi, as Boletus bovinus and Agaricus deliciosus, Sowerb. Fungi, t. 202, change almost instantaneously on exposure to the air, from yellow to dark blue or green.

These are a few hints only on a subject which opens a wide field of inquiry, and which, in professedly chemical works, is carried to a greater length than I have thought necessary in a physiological one. See Thomson's Chemistry, v. 4, and Willdenow's Principles of Botany, 229. We must ever keep in mind, as we explore it, that our anatomical instruments are not more inadequate to dissect the organs of a scarcely distinguishable insect, than our experiments are to investigate the fine chemistry of Nature, over which the living principle presides.

Before we take leave of the secreted fluids of vegetables, a few more remarks upon their direct utility to the plants themselves may not be superfluous. Malpighi first suggested that these secretions might nourish the plant, and our latest inquiries confirm the suggestion. Du Hamel compares them to the blood of animals, and so does Darwin. But the analogy seems more plain between the sap, as being nearly uniform in all plants, and the animal blood, as in that particular they accord, while the secreted fluids are so very various. Mr. Knight's theory confirms this analogy, at the same time that it establishes the opinion of Malpighi. The sap returning from the leaf, where it has been acted upon by the air and light, forming new wood, is clearly the cause of the increase of the vegetable body. But it is not so clear how the resinous, gummy or other secretions, laid aside, as it were, in vessels, out of the great line of circulation, can directly minister to the growth of the tree. I conceive they may be in this respect analogous to animal fat, a reservoir of nourishment whenever its ordinary supplies are interrupted, as in the winter, or in seasons of great drought, or of unusual cold. In such circumstances the mucilaginous or saccharine secretions especially, perhaps the most general of all, may be absorbed into the vegetable constitution; just as fat is into the animal one, during the existence of any disease that interrupts the ordinary supplies of food, or interferes with its due appropriation. It is well known that such animals as sleep through the winter, grow fat in the autumn and awake very lean in the spring. Perhaps the more recent layers of wood in a Plum or Cherry-tree, if they could be accurately examined, might be found to contain a greater proportion of mucilage at the end of Autumn than in the early spring. If these substances do not nourish the plant, they seem to be of no use to it, whatever secondary purposes they may answer in the schemes of Providence. The direct end, with respect to the plant, of the finer secreted fluids of its fruit can very well be perceived, as tempting the appetite of animals, and occasioning, through their means, the dispersion of the seeds; and the perfume of flowers may attract insects, and so promote the fertilization of the seed, as will be explained hereafter.

After what has been said we need not waste much time in considering the hypothesis, advanced by some philosophers, that the sap-vessels are veins and the returning vessels arteries. This is so far correct, that, as the chyle prepared by the digestive organs, poured into the veins and mixed with the blood is, through the medium of the heart, sent into the lungs to be acted upon by the air; so the nutrimental juices of plants, taken up from the earth, which has been called their stomach, are carried by the sap-vessels into the leaves, for similar purposes already mentioned. The improved sap, like the vivid arterial blood, then proceeds to nourish and invigorate the whole frame. I very much doubt, however, if those who suggested the above hypothesis, could have given so satisfactory an explanation of it.

That the secretions of plants are wonderfully constant appears from the operation of grafting. This consists in uniting the branches of two or more separate trees, as Dr. Hope's willows, see p. 60, and a whole row of Lime-trees in the garden of New College, Oxford, whose branches thus make a network. This is called grafting by approach. A more common practice, called budding, or inoculating, is to insert a bud of one tree, accompanied by a portion of its bark, into the bark of another, and the tree which is thus engrafted upon is called the stock. By this mode different kinds of fruits, as apples, pears, plums, &c., each of which is only a variety accidentally raised from seed, but no further perpetuated in the same manner, are multiplied, buds of the kind wanted to be propagated being engrafted on so many stocks of a wild nature. The mechanical part of this practice is detailed in Du Hamel, Miller, and most gardening books. It is of primary importance that the liber, or young bark, of the bud, and that of the stock, should be accurately united by their edges. The air and wet must of course be excluded.

It is requisite for the success of this operation that the plants should be nearly akin. Thus the Chionanthus virginica, Fringe-tree, succeeds well on the Common Ash, Fraxinus excelsior by which means it is propagated in our gardens. Varieties of the same species succeed best of all; but apples and pears, two different species of the same genus, may be grafted on one stock. The story of a Black Rose being produced by grafting a common rose, it is not worth inquiring which, on a black currant stock, is, as far as I can learn, without any foundation, and is indeed at the first sight absurd. I have known the experiment tried to no purpose. The rose vulgarly reported to be so produced is merely a dark Double Velvet Rose, a variety, as we presume, of Rosa centifolia. Another report of the same kind has been raised concerning the Maltese Oranges, whose red juice has been attributed to their being budded on a Pomegranate stock, of which I have never been able to obtain the smallest confirmation.

Heat can scarcely be denominated a secretion, and yet is undoubtedly a production, of the vegetable as well as animal body, though in a much lower degree in the former than the latter. The heat of plants is evinced by the more speedy melting of snow when in contact with their leaves or stems, compared with what is lodged upon dead substances, provided the preceding frost has been sufficiently permanent to cool those substances thoroughly. Mr. Hunter appears to have detected this heat by a thermometer applied in frosty weather to the internal parts of vegetables newly opened. It is evident that a certain appropriate portion of heat is a necessary stimulus to the constitution of every plant, without which its living principle is destroyed. Most tropical plants are as effectually killed by a freezing degree of cold, as by a boiling heat, and have nearly the same appearance, which is exemplified every autumn in the Garden Nasturtium, Tropæolum majus. The vegetables of cold climates, on the contrary, support a much greater degree of cold without injury, at least while in a torpid state; for when their buds begin to expand they become vastly more sensible, as is but too frequently experienced in the fickle spring of our climate. Nor is this owing, as vulgarly supposed, merely to the greater power of the cold to penetrate through their opening buds. It must penetrate equally through them in the course of long and severe winter frosts, which are never known to injure them. The extremely pernicious effects therefore of cold on opening buds can only be attributed to the increased susceptibility of the vital principle, after it has been revived by the warmth of spring.

The vegetation of most plants may be accelerated by artificial heat, which is called forcing them, and others may, by the same means, be kept in tolerable health, under a colder sky than is natural to them. But many alpine plants, naturally buried for months under a deep snow, are not only extremely impatient of sharp frosts, but will not bear the least portion of artificial heat. The pretty Primula marginata, Curt. Mag. t. 191, if brought into a room with a fire when beginning to blossom, never opens another bud; while the American Cowslip, Dodecatheon Meadia, t. 12, one of the most hardy of plants with respect to cold, bears forcing admirably well.

Mr. Knight very satisfactorily shows, Phil. Trans. for 1801, 343, that plants acquire habits with regard to heat which prove their vitality, and that a forced Peach-tree will in the following season expand its buds prematurely in the open air, so as to expose them to inevitable destruction. See p. 65[errata 1]. A thousand parallel instances may be observed, by the sagacious gardener, of plants retaining the habits of their native climates, which very often proves one of the greatest impediments to their successful cultivation.

The most remarkable account that has fallen in my way concerning the production of heat in plants, is that given by Lamarck in his Flore Françoise, v. 3. 538, of the Common Arum maculatum, Engl. Bot. t. 1298, (the white-veined variety), the flower of which, at a certain period of its growth, he asserts to be, for a few hours, "so hot as to seem burning." The learned M. Senebier of Geneva, examining into this fact, discovered that the heat began when the sheath was about to open, and the cylindrical body within just peeping forth; and that it was perceptible from about three or four o'clock in the afternoon till eleven or twelve at night. Its greatest degree was seven of Reaumur's scale above the heat of the air, which at the time of his observation was about fourteen or fifteen of that thermometer. Such is the account with which I have been favoured by Dr. Bostock of Liverpool, from a letter of M. Senebier[2], dated Nov. 28. 1796, to M. De la Rive. I have not hitherto been successful in observing the phænomenon in question, which however is well worthy of attention, and may probably not be confined to this species of Arum.

  1. These flowers afford the poet a new image, which is introduced into the following imitation of Martial, and offered here solely for its novelty.

    Go mingle Arabia's gums
    With the spices all India yields.
    Go crop each young flower as it blooms.
    Go ransack the gardens and fields.

    Let Pæstum's all-flowery groves
    Their roses profusely bestow.
    Go catch the light zephyr that roves
    Where the wild thyme and marjoram grow.

    Let every pale night-scented flower,
    Sad emblem of passion forlorn,
    Resign it's appropriate hour,
    To enhance the rich breath of the morn.

    All that art or that nature can find,
    Not half so delightful would prove,
    Nor their sweets all together combined,
    Half so sweet as the breath of my love.

  2. It is now published in his Physiologie Végétale, v. 3. 314, where nevertheless this ingenious philosopher has declared his opinion to be rather against the existence of a spontaneous heat in vegetables, and he explains even the above striking phænomenon upon chemical principles, which seem to me very inadequate.

  1. Original: 56 was amended to 65