1095. Elevation influences the habits of plants in various ways;-by exposing them to the wind; to be watered by a very fresh and pure water from the melting of adjoining snow ; and to be covered in winter by a thick layer of snow, which protects them from severe frosts. Hence many alpine plants become frozen during winter in the plains, and in gardens which are naturally warmer than their native situations. In great elevations, the diminution of the density of the air may also have some influence on vegetation. The rarity of the atmosphere admits a more free passage for the rays of light which, being in consequence more active, ought to produce a more active vegetation. Experience seems to prove this in high mountains; and the same effect is produced in high latitudes by the length of the day. On the other hand, vegetables require to absorb a certain quantity of oxygen gas from the air during the night ; and as they find less of that in the rarefled air of the mountains, they ought to be proportionably feeble and languishing. According to experiments made by Theodore de Saussure, plants which grow best in the high Alps are those which require to absorb least oxygen during the night; and, in this point or view, the shortness or the nights near the poles correspond. These causes, however, are obviously very weak, compared to the powerful action of temperature.
