Page:The Works of Aristotle - Vol. 6 - Opuscula (1913).djvu/111

Rh which grows upon solid rock takes a long time to grow; for the air which is enclosed in the stone strives to rise, and when it cannot find a way, owing to the resistance of the stone, it retreats back again and becomes heated, and attracts the residuum of the moisture in the stone upwards, and with this moisture a vapour comes forth accompanied by a resolution of small particles of the stone; and because the sun often acts upon the stone, it assists the moisture in the process of concoction, and as a result a plant is produced. Such plants do not generally grow to any height, unless they are near some soil or moisture. The growth of a plant requires soil, water, and air. A rock plant will grow low, and if it faces the east, it will grow quickly, and slowly if it faces the west. A plant, when water is the predominant element in it, will retain the air and will not allow it to rise, and thus the plant is not nourished. Similarly, when dryness predominates, the natural heat will be diverted into the extremities of the plant and will block up the ducts through which the flow of water passed, and the plant does not receive nourishment.

plant of whatsoever kind needs four things (just as an animal needs four things), namely, a definite seed, a suitable position, and properly attempered water and air. When these four conditions are fulfilled, a plant will grow and increase; but if they do not harmonize, the plant will be correspondingly weakened. A plant which is used for medicinal purposes will be more serviceable and suitable for such purposes if it grows on high mountains; its fruit, however, will be harder to assimilate and will contain less nourishment. Places which are secluded from the sun's rays will not produce much plant life (just as they will not produce much animal life), because the sun makes the day long or short according to the duration of its presence or absence, and it is the sun which draws out the moisture;