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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.