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It has been just stated, that we have derived from botanical science a philosophical language which enables us to describe the structure and habits of any plant, with a luminous brevity and an unerring perspicuity; but we are moreover indebted to botany for another service no less important to the successful investigation of the Materia Medica,—that of throwing into well defined groups, those plants which possess obvious natural affinities, and which will be found at the same time to present certain medicinal analogies; indeed, as a general rule, we may admit the axiom, “Quæ genere conveniunt, virtute conveniunt.”^[85]

The Umbelliferæ which grow on dry ground are aromatic, whilst the aquatic species are among the most deadly poisons. The Cruciform plants are aromatic and acrid in their nature, containing essential oils, (hence the peculiar smell of cabbage-water, &c.) which are obtainable by distillation; and Linnæus asserts that “among all the Leguminous or Papilionaceous tribe there is no deleterious plant to be found:” this however is not exactly true. Some of the individuals in these natural orders, although very nearly related, do nevertheless possess various, and even opposite qualities; in the leguminous tribe above mentioned, which is as consistent as any one we possess, we have the Cytisus Laburnum, the seeds of which are violently emetic, and those of Lathyrus Sativus, which have been supposed at Florence to soften the bones and cause death.

In the subdivision of a genus there is often a remarkable difference in the properties of the species; there are, for instance, Solanums, Lettuces, Cucumbers, and Mushrooms, both esculent and poisonous. The Digitalis or Foxglove, and the Verbascum, or common Mullein of our fields, are included in the same Natural family, and yet the one is as active, as the other is mild in its effects; the plants of the natural family of Contortæ abound with a highly acrid milky juice, but Dr. Afzelius met with a shrub of this order at Sierra Leone, the milk of whose fruit was so sweet, as well as copious, as to be used instead of cream for tea; this is certainly what no one could have guessed from analogy. The same individual will vary from culture or other circumstances, as much as any two plants which have no botanic affinity; the Chamomile, Anthemis Nobilis, with which we are well acquainted, may have its whole disk changed by cultivation, to ligulate white florets, destitute of medicinal properties. But, what is more embarrassing, the different parts of the same plant have often very different powers; a fact which is beautifully exemplified in the Podophyllum Peltatum, or May Apple, the leaves of which are poisonous, the root powerfully cathartic, and the fruit agreeably esculent; so the leaves of the Jatropa Manihot are employed as a common esculent, while its root secretes a most virulent poison; but we need not seek further for an example than the fruit of the Lemon, the juice of which is acid, its seeds bitter, and its rind aromatic; in some instances it happens that the energy of a plant is concentrated in one particular part, and that all the rest is absolutely inert; thus, the root of the Convolvulus Scammonia, is the only portion of that plant which possesses any medicinal quality;^[86] and the tree which yields the drastic Camboge, presents at the same time an esculent fruit, which is eaten by the natives with as much impunity as the orange; yet, notwithstanding all these difficulties, botany is capable of furnishing us with analogies which will lead to important conclusions with respect to the medicinal properties of different vegetables.

The system of Linnæus, although in a great degree artificial, corresponds in a surprising manner with the natural properties of plants; thus a plant whose calyx is a double valved glume, with three stamina, two pistils, and one naked seed, bears seeds of a farinaceous and nutritious quality; a flower with twelve, or more stamina, all of which are inserted in the internal side of the calyx, will furnish a wholesome fruit; whereas a plant whose flower has five stamina, one pistil, one petal, and whose fruit is of the berry kind, may at once be pronounced as poisonous.

It is also in a great degree true that the sensible qualities of plants, such as colour, taste, and smell, have an intimate relation to their properties, and may often lead by analogy to an indication of their powers; we have an example of this in the dark and gloomy aspect of the Luridæ, which is indicative of their narcotic and very dangerous qualities, as Datura, Hyoscyamus, Atropa, and Nicotiana. Colour is certainly in many cases a test of activity; the deepest coloured flowers of the Digitalis, for example, are the most active, and when the leaves of powerful plants lose their green hue, we may conclude that a corresponding deterioration has taken place with respect to their virtues; but Linnæus ascribed too much importance to such an indication, and his aphorisms are unsupported by facts; for instance, he says “Color pallidus insipidum, viridis crudum, luteus amarum, ruber acidum, albus dulce, niger ingratum, indicat.”^[87] A peculiar heavy odour, which is well known, but is with difficulty defined, is a sure indication of narcotic properties. Bitterness, when not extreme, denotes a tonic quality, which will stimulate the stomach and intestines, and promote the process of digestion. When the bitterness is more intense and pungent,^[88] as in Aloes, Colocynth, &c. we may infer that such substances will produce a more active effect upon the primæ viæ, and that catharsis will follow their administration.

Botanical, like human physiognomy, may frequently afford an insight into character, but it is very often a fallacious index. With regard to the indications of Smell and Taste, it may be observed that in the examination of an unknown substance we instinctively apply to these senses for information respecting its properties. It is certainly reasonable to suppose, that those bodies which produce upon the organs of taste a sensible, astringent, or pungent effect, may occasion an impression, corresponding in degree upon the stomach or intestines, which are but an extension of the same structure. But what numerous exceptions are there to such a law? nay, some of the most poisonous substances affect in a very slight degree the organs of taste, especially those that belong to the mineral kingdom, as Arsenious Acid, Oxyd of Antimony, Calomel, &c.; yet some of these are, perhaps, but apparent exceptions, depending upon the degree of solubility which they possess, in consequence of which their energies are not developed until they have traversed a considerable portion of mucous surface. Nor ought it to be forgotten, that cultivation and artificial habits may have blunted the natural susceptibility of our organs, and in some instances changed and depraved their functions: certain qualities for instance are so strongly connected with each other by the chain of association, that by presenting only one to the mind, the other links follow in succession.^[89] It has been remarked, that persons in social life, are more affected by vegetable odours, while the Savage smells better the putrid and fœtid exhalations of animal bodies:^[90] thus the people of Kamskatcha, did not smell the perfume of a vegetable Essence (Aqua Melissæ,) but they discovered by their olfactory sense, a rotten fish, or a stranded whale at a considerable distance.^[91] There is no sense more under the dominion of imagination, or more liable to be perverted by education, than those of taste and smell; we are also liable to form unjust prejudices from the indications of colour; for particular colours, from the influence of hidden associations, are not unfrequently the exciting cause of agreeable or unpleasant impressions. I have met with a person who regards green food, if it be of an animal nature, with unconquerable aversion and disgust, indeed an idea of unwholesomeness has not unfrequently been attached to this colour, without the least foundation of truth; the bones of the Gar fish, or Sea Needle, (Esox Helone,) have been deemed unwholesome from the circumstance of their turning green on being boiled, although not a single instance can be adduced in which that fish ever occasioned any harm. I have met with persons who have been made violently sick from eating the green part of the oyster;^[92] an effect which can have no other cause than that of unjust prejudice; these examples are sufficient to shew, with what caution such indications respecting the medicinal qualities of bodies are to be received.