The milk sap, when exposed to the air, coagulates into a tenacious viscid solid. The white juice is generally acrid, or narcotic, or both, and for the most part extremely poisonous, though exhibiting strong contrasts even in nearly allied species. In the order Euphorbiaceæ or Spurgeworts, comprising nearly 1,500 species, a large proportion are hurtful; but there is a gradation from mere stimulants to the most formidable poisons. This order furnishes the Ethiope and the native Brazilian with poison for their arrows. It contains the Manchineel, and Excœcaria Agallocha, the most poisonous of plants; even the smoke from the burning branches of the Excœcaria affects the eyes with insufferable pain. The white juice of the Fig, one of the Morad order, is violently poisonous; in many, as in the common fig, it is acrid and irritating. The Antiaris toxicaria, the celebrated Upas-tree of Java, which is of the Artocarpeæ or Bread-fruit order, owes its virulence to its milky juice, which contains strychnia, the most fatal of drugs. Dangerous and acrid as these orders are, the Bread-fruit, abounding in starch, supplies the inhabitants of the East Indian islands with excellent food; the milky juice of the Cow-trees, chiefly of the Bread-fruit and Fig orders, furnishes a wholesome beverage to the South Americans; and the Manihot or Cassava, a poisonous spurgewort when raw, yields when roasted nutritious food to whole nations, the heat driving off the dangerous principle. Caoutchouc, a most harmless substance, is the solid produce of many of the most acrid and virulent juices of plants belonging to the preceding orders; the poison is probably left in the liquid. The chemico-vital power is strikingly illustrated by the number of safe and excellent fruits produced by trees full of the most deleterious juices, whether milky or not. Some of the finest fruits in the Indian Archipelago are products of eminently dangerous species of the Sapindaceæ or Soapworts. The acrid juices of the leaves and branches, are so much diluted with water in the fruits, that they become innocuous, or they may be changed into sugar, as in the common fig. Nothing can surpass the virulence of the juice of the Upas-tree, yet its nuts are eaten with impunity, and the pulpy contents of the fruit of the Strychnos nux vomica is food for birds. The leaves and berries of the potato are so strongly narcotic, that an extract from them is intermediate in power between that from deadly nightshade and hemlock, yet the potato itself, like the cassava, is rendered wholesome by being boiled or roasted.
The alkaloids are alkaline substances formed in the bark and milky juices of plants, always combined with an acid during the life of the plant. The chemical structure of this class of substances is very much alike, and chemists have succeeded in forming many of them synthetically; they all contain azote, and have a great affinity for acids. The bark of the different species of Cinchoneæ, especially the Cinchona cordifolia and C. Condaminea, yield three alkaloids—namely, cinchonine, quinine, and cusconine—they are all formed of carbon, hydrogen, and azote in the same proportions; but the first has one atom of oxygen in addition, the second has two atoms in addition, and the third has three; so that in these alkaloids the carbon, hydrogen, and azote combine to form an organic radical, which is oxidized in three different degrees. Six of the alkaloids have been obtained from opium, which is the solid portion of the milk juice of the poppy; of these, morphine seems to be the narcotic principle; and the orange-coloured milk sap of the Chelidonium, a very poisonous and acrid plant of the poppy order, has furnished chelidonine. The Colchicum order, containing the meadow saffron or autumnal crocus, and Veratrum album or white hellebore, as well as many other plants, yield alkaloids, all of which are medicinal or poisonous, according to the dose.
There is scarcely a people, however savage, that has not discovered some exciting narcotic. Opium is almost universally smoked or eaten among Eastern nations; and bhang, a strong narcotic, obtained from the leaves of Indian hemp, is in equally universal use among the Brazilian savages and Hottentots, but especially among the Malays, who are excited to madness when they smoke it too freely. The same intoxicating effect is produced by a strong liquor prepared from the Datura sanguinea, a species of stramonium; and its congener tobacco, now all but a necessary of life among civilized mankind, was smoked by the natives of the American continent, before the arrival of the Europeans, as a relief from hunger.
Coffee has been long in use on account of its stimulating principle caffeine, which is now discovered to be the same with theine, the latter, however, being less exciting, unless the tea plant grows in a very hot climate. In countries where nature furnishes few narcotic principles, wine, beer, and spirits supply their place, especially in the far north, where animal heat is rapidly carried off by the cold, and carbon must be furnished to satisfy the all-devouring oxygen which we draw in at every breath.
Caffeine, the highly azotized principle of coffee, obtained from tea leaves and coffee beans, is one of the substances known as neutral crystallisable principles. Similar substances are found in asparagus, pepper, almonds, the bark on the roots of the apple, pear, plum, and cherry trees, as well as in the bark of the willow. The two last are especially analogous, and contain no azote, as the others do.
The colouring matter of flowers is a fluid contained in cells, situated immediately under the skin, which itself is perfectly transparent and colourless. The whiteness of the white Camellia, rose, lily, and other flowers, is supposed to be owing to the total reflection of light from the cells immediately below the skin, which are either full of air, or of a colourless liquid. The predominating colours are yellow, red, and blue, with the various intermediate tints. Sometimes these colours are converted one into another in the petal after fertilization, at which period the colours are brightest. The chemical nature of these liquids, the cause of their variety, and their definite arrangement in one and the same petal, do not seem as yet to be ascertained.
The parts of plants that are not green inhale oxygen from the atmosphere, and exhale carbonic acid gas exactly like animals. During the chemical combinations of the oxygen with the carbon derived from the nutriment to form the carbonic acid gas, heat is necessarily evolved, especially in the flower, the point of maximum heat varying with its expansion. The blossoms of the Aroideæ, or Arums, are remarkable for the evolution of heat. According to Saussure, a blossom of the common Arum maculatum consumes five times its volume of oxygen in twenty-four hours previous to its evolution of fruit, so it is not wonderful that the chemical combination of such a quantity of oxygen should produce a strong development of specific temperature. By M. Dutrochet’s observations, the heat evolved by the Arum maculatum has a maximum in the day and a minimum in the night, and he found that it exceeded the heat of the surrounding air by between 25° and 27°. The heat of the Colocasia odorata, another Arad, was determined by several observers to be even 50° above the warmth of the air. The heat evolved by germinating seeds when in a heap is not from fermentation; it is owing to their consumption of oxygen and expiration of carbonic acid gas. The temperature of all vegetating parts of plants, the roots, leaves, young juicy shoots, &c., is far superior to that of their flowers. It arises from the nutritive process, and has a maximum at noon, and a minimum at midnight, like that of the flower. The growth of plants is most vigorous at noon; consequently there is then a greater evolution of heat.
Water in small quantities is secreted night and morning from the points of the leaves of many plants, probably to relieve them from a superabundance of liquid, which evaporation is insufficient to carry off. The arums are remarkable for the quantity they eject. It falls in drops from the points of the leaves. About half a pint is given out every night by the enormous leaves of the Caladium distillatorium, a species of Arad. In that plant, and in the Colocasia, the water flows in canals along each rib into a general duct, which runs along the border of the leaf, and terminates in an orifice upon the surface.
Since electricity is developed by chemical action in unorganized matter, it may be inferred that it is also developed within the vegetable cell where so many organic compounds are formed; but it is probably given off from the points of the leaves or by evaporation from their surfaces. Professor Fleming ascertained by actual experiment, that the sap of a leaf, and its surface, are in different electric states; he also found that the surface of the spongioles of the roots of plants and the ascending sap have opposite electricities. Both of the preceding cases the Professor ascribes, in part at least, to organic changes which take place during vegetation. Slight currents of electricity were obtained from the petioles of flowers, but fruits and tubers give powerful electrical currents due to the reaction of different vegetable juices upon one another. The tuberose is said to emit scintillations and dart small sparks of light in a hot electric evening, and gardeners have long been aware that mushroom spawn is most prolific in stormy weather.
The irritability of the tissues of plants which renders them liable to be acted upon by external causes, has occupied the attention of many celebrated botanists. From experiments by Professor Ferdinand Cohn and his pupil M. Krabsch upon the irritability of the stamens in the florets on the discs of composite flowers, more especially the Centaureas, they have come to the conclusion that susceptibility to the excitement of light, as well as to that of mechanical and probably electrical impulse, is possessed by all young vigorous tissues, and upon comparing the phenomena of these with those of animal irritability, they further conclude that the faculty of responding to external irritation by internal movements and change of form, belongs to cells, and holds good in the vegetable as in the animal kingdom. To be irritable, to change its normal form as a vessel of excitation, and to revert to the normal form after a while by its internal elasticity, are characteristics of the living cell. In plants these properties are met with only when the vital processes are in full activity, and therefore are particularly noticed during the period of flowering, when the processes are at the maximum. And it may be remarked that the stamens, in which irritability is most frequently noticed, are the only organs in which an elevation of temperature measurable by the thermometer occurs, although a certain degree of heat is generated in all plant cells by the chemical process going on within them. It is to be supposed that irritable properties belong to all parts of plants, but that they exist in an intensified degree, and for a certain epoch, in those parts where their results arrest attention, as in the stamens of the Centaurea, berberry, cactus, Cistus, nettle, &c., and in the anthers of the Stylideæ, the leaves of Dionæa muscipula, and many others, all of which are more or less affected by the external action of mechanical force and electricity; for it is scarcely possible that plants should not be under the influence of atmospheric electricity, since every shower of rain forms a perfect conductor between the clouds and the earth. The motion does not always immediately follow the excitement; plants often require to be rudely shaken before the movement begins. M. Hofmeister has observed that all young shoots and leaves become curved by mechanical shaking.