1254. The organs of plants are so delicately constituted that they can only take the solid food requisite for their nourishment in the form of gas, or when held in solution in water. Thus carbon, which is a solid substance, can only be imbibed by plants in the form of carbonic acid gas, and the saline and earthy particles which are required must be dissolved in water. Water itself, when decomposed, supplies the plants with hydrogen, and a part of their oxygen, the rest being obtained from the air. Their nitrogen is principally supplied in the form of ammonia, in which state it is found abundantly in the excrements of animals. But a considerable proportion of all the elements required for the food of plants, excepting the earthy and saline substances, is procured from the air.
