Page:Frost (1827) Some account of the science of botany.pdf/17

Rh, in which Nature seems to take unto herself those elements and constituent principles, which, for a while, she had lent to a superior agent as the means and instruments of the spirit of life."

Plants are furnished with apparatus or organs for the purpose of carrying on the functions of transpiration, secretion, and absorption; and it has been proposed, as a distinguishing characteristic between animals and vegetables, that the former are nourished by their internal, and the latter by their external surfaces. How far such a criterion is to be taken is another matter, and we must be careful how we lay down axioms that are liable to dispute. The spiral tubes, of which the vegetable texture is constituted, are pourtrayed in the drawing behind me, shewing the magniﬁed section of a leaf of the aloe; the various juices circulating therein undergo chemical changes, according to the purposes for which they are destined. The effect of light is very remarkable:—celery (upium graveolens), sea-kale (crambe maritima) are instances of it; for, were they exposed to it, the former would be so bitter, and the latter so acrid, that they could not be eaten. Common tansy, well known for its bitterness and pungency, has been eaten with impunity when excluded from the action of light. Linnæus compared the leaves of plants to the lungs of animals, as the organs by which exhalation and transpiration are carried on in vegetables; and in the animal kingdom the air is taken in by the lungs and suffers a chemical change, and does not a similar effect take place on atmospheric air? Do they not liberate a quantity of pure oxygen, and carbonic acid gas?

Strip off all the leaves of any plant and it will die, or even if the process of transpiration be impeded by dirt, or carbonaceous matter on their surfaces, remark how soon the vegetable denotes the obstruction of its necessary office.

There are vessels which convey the juices to various parts of the plant, and others that return it; adducent and reducent vessels, corresponding, as it were, to arteries and veins in animals; deprive a plant of its sap, and you kill it; take away the blood of an animal, and you destroy it.

The various juices of plants are formed from the sap, and are not the different secretions of animals formed from the blood? Vegetables cannot exist without air any more than animals can. Place a plant under the exhausted receiver of an air pump; and it dies; put an animal under similar circumstances, and it expires.