The Cohesion Theory of Sap Ascent

Abstract

Pressures in the xylem of north temperate trees are very rarely positive. It does happen in certain species, such as Acer, Betula, TVitis,etc., during late winter when roots are active but leaves have not yet unfolded. If at that time the xylem is injured, it bleeds. In certain herbs, xylem pressures become positive at night when transpiration is suppressed. Leaves then guttate from hydathodes which are normally located at the leaf margins. Under humid conditions of the tropical rain forest, positive pressures have been recorded in the xylem of several species even in the presence of leaves. Guttation from leaves was then observed (e.g., von Faber 1915); walking in the forest under such conditions reportedly gave the impression of walking in a drizzle of rain.