1074. The odours of plants depend upon volatile oils, which are continually flying off, but in different degrees. Some odours are not perceptible till the parts of the plant are rubbed; some are most apparent in a recent state, and others in a dried state : in the latter case, the oil has been combined with too much water. In some of the cruciferï¾µ, and other orders, the oil is so rapidly dispersed in the day-time, as to render its odour imperceptible, while during night it is eminently odoriferous; as in the case of the night-smelling stock, the rocket, and several other plants. In general, plants in hot countries are more fragrant than those in cold countries; but their fragrance is so much dispersed by the heat, as to be imperceptible in the day-time.