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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.