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16 Excess of stimulus affects vegetables in the same manner as it does animals, destroys them. Willdenow, in his Principles of Botany, when alluding to Boletus, says, "these plants require a very small quantity of oxygen to promote their growth, and, therefore, as soon as they are brought into the open air they decay. This is soon proved by the well known observation that rooms, or repositories, which are fusty or mouldy, are freed from the inconvenience by the admission of air."

We have now endeavoured to trace the analogy between the two kingdoms, so far as they can, with propriety, be compared. Linnæus suffered the brilliancy of his imagination to get the better of his judgment, when he stated that heat was the heart, and the earth the stomach of plants. In his Philosophia Botanica he says, "stones grow, vegetables grow and live, animals grow, live, and feel." The want of locomotion in plants is a very distinguishing characteristic.

The quality of the juices secreted by vegetables varies very much, e. g. in the Euphorhia it is extremely acrid, so much as to produce a caustic effect when applied to the skin. in others it it very bland, as the juice of the Acacia Vera, or gum arabic of the shops. The Saccharina or Sugar-Maple, furnishes to the North Americans an article well known under the name of Sugar, but which is generally obtained from the Sugar Cane or Succhnrum Officinarum of Linnæus. Manna is procured from a species of Ash termed Fraxinus Ornus: and many other vegetable products might be enumerated, were it not that they would form too distended a list for present consideration.

Plants, according to the places of which they are natives, require different degrees of heat. Elasticity is common to vegetables; and it is a well known fact, that wood expands in wet weather, and contracts in dry.

Carbon, hydrogen, and oxygen, constitute the ultimate elements of vegetables, although in some plants there is a fourth element, viz. nitrogen, which exists in plants belonging to the natural orders, Cruciatæ and Fungi.

The proximate principles of vegetables, together with their chemical history, will be comprehended in the Sixth Lecture.