The pubescence of plants varies greatly in degree according to differences of soil or exposure; several kinds, as Mentha hirsuta, t. 447, 448, naturally hairy, being occasionally found smooth, but if transplanted they soon resume their proper habit. Yet the direction of the hairs or bristles proves a very sure means of distinguishing species, especially in the genus Mentha, the hairs about whose calyx and flower-stalk point differently in different species, and I have found it the only infallible distinction between one Mint and another. See Trans. of Linn. Soc. v. 5. 171. The accurate Dr. Roth has lately applied the same test to the species of Myosotis, which all botanists before him had either confounded under M. scorpioides, Engl. Bot. t. 480, or else separated upon vague principles. Some species of Galium are admirably characterized by the bristles of their leaves, or of parts of their leaves, being hooked backward or forward. We therefore accept the 272d maxim of Linnæus's Philosophia Botanica with that limitation which he himself has allowed in his commentary
